Ecto v3.1.1 Ecto.Query View Source

Provides the Query DSL.

Queries are used to retrieve and manipulate data from a repository (see Ecto.Repo). Ecto queries come in two flavors: keyword-based and macro-based. Most examples will use the keyword-based syntax, the macro one will be explored in later sections.

Let's see a sample query:

# Imports only from/2 of Ecto.Query
import Ecto.Query, only: [from: 2]

# Create a query
query = from u in "users",
          where: u.age > 18,
          select: u.name

# Send the query to the repository
Repo.all(query)

In the example above, we are directly querying the "users" table from the database.

Query expressions

Ecto allows a limited set of expressions inside queries. In the query below, for example, we use u.age to access a field, the > comparison operator and the literal 0:

query = from u in "users", where: u.age > 0, select: u.name

You can find the full list of operations in Ecto.Query.API. Besides the operations listed there, the following literals are supported in queries:

  • Integers: 1, 2, 3
  • Floats: 1.0, 2.0, 3.0
  • Booleans: true, false
  • Binaries: <<1, 2, 3>>
  • Strings: "foo bar", ~s(this is a string)
  • Arrays: [1, 2, 3], ~w(interpolate words)

All other types and dynamic values must be passed as a parameter using interpolation as explained below.

Interpolation and casting

External values and Elixir expressions can be injected into a query expression with ^:

def with_minimum(age, height_ft) do
  from u in "users",
    where: u.age > ^age and u.height > ^(height_ft * 3.28),
    select: u.name
end

with_minimum(18, 5.0)

When interpolating values, you may want to explicitly tell Ecto what is the expected type of the value being interpolated:

age = "18"
Repo.all(from u in "users",
          where: u.age > type(^age, :integer),
          select: u.name)

In the example above, Ecto will cast the age to type integer. When a value cannot be cast, Ecto.Query.CastError is raised.

To avoid the repetition of always specifying the types, you may define an Ecto.Schema. In such cases, Ecto will analyze your queries and automatically cast the interpolated "age" when compared to the u.age field, as long as the age field is defined with type :integer in your schema:

age = "18"
Repo.all(from u in User, where: u.age > ^age, select: u.name)

Another advantage of using schemas is that we no longer need to specify the select option in queries, as by default Ecto will retrieve all fields specified in the schema:

age = "18"
Repo.all(from u in User, where: u.age > ^age)

For this reason, we will use schemas on the remaining examples but remember Ecto does not require them in order to write queries.

nil comparison

nil comparison in filters, such as where and having, is forbidden and it will raise an error:

# Raises if age is nil
from u in User, where: u.age == ^age

This is done as a security measure to avoid attacks that attempt to traverse entries with nil columns. To check that value is nil, use is_nil/1 instead:

from u in User, where: is_nil(u.age)

Composition

Ecto queries are composable. For example, the query above can actually be defined in two parts:

# Create a query
query = from u in User, where: u.age > 18

# Extend the query
query = from u in query, select: u.name

Composing queries uses the same syntax as creating a query. The difference is that, instead of passing a schema like User on the right side of in, we passed the query itself.

Any value can be used on the right-side of in as long as it implements the Ecto.Queryable protocol. For now, we know the protocol is implemented for both atoms (like User) and strings (like "users").

In any case, regardless if a schema has been given or not, Ecto queries are always composable thanks to its binding system.

Positional bindings

On the left side of in we specify the query bindings. This is done inside from and join clauses. In the query below u is a binding and u.age is a field access using this binding.

query = from u in User, where: u.age > 18

Bindings are not exposed from the query. When composing queries, you must specify bindings again for each refinement query. For example, to further narrow down the above query, we again need to tell Ecto what bindings to expect:

query = from u in query, select: u.city

Bindings in Ecto are positional, and the names do not have to be consistent between input and refinement queries. For example, the query above could also be written as:

query = from q in query, select: q.city

It would make no difference to Ecto. This is important because it allows developers to compose queries without caring about the bindings used in the initial query.

When using joins, the bindings should be matched in the order they are specified:

# Create a query
query = from p in Post,
          join: c in Comment, where: c.post_id == p.id

# Extend the query
query = from [p, c] in query,
          select: {p.title, c.body}

You are not required to specify all bindings when composing. For example, if we would like to order the results above by post insertion date, we could further extend it as:

query = from q in query, order_by: q.inserted_at

The example above will work if the input query has 1 or 10 bindings. As long as the number of bindings is less than the number of from + joins, Ecto will match only what you have specified. The first binding always matches the source given in from.

Similarly, if you are interested only on the last binding (or the last bindings) in a query, you can use ... to specify "all bindings before" and match on the last one.

For instance, imagine you wrote:

posts_with_comments =
  from p in query, join: c in Comment, where: c.post_id == p.id

And now we want to make sure to return both the post title and the comment body. Although we may not know how many bindings there are in the query, we are sure posts is the first binding and comments are the last one, so we can write:

from [p, ..., c] in posts_with_comments, select: {p.title, c.body}

In other words, ... will include all the binding between the first and the last, which may be no binding at all, one or many.

Named bindings

Another option for flexibly building queries with joins are named bindings. Coming back to the previous example, provided we bind a join to a concrete name:

posts_with_comments =
  from p in query,
    join: c in Comment, as: :comment, where: c.post_id == p.id

We can refer to it by that name using a following form of bindings list:

from [p, comment: c] in posts_with_comments, select: {p.title, c.body}

This approach lets us not worry about keeping track of the position of the bindings when composing the query.

What's more, a name can be assigned to the first binding as well:

from p in Post, as: :post

Only atoms are accepted for binding names. Named binding references are expected to be placed in the tail position of the bindings list.

Bindingless operations

Although bindings are extremely useful when working with joins, they are not necessary when the query has only the from clause. For such cases, Ecto supports a way for building queries without specifying the binding:

from Post,
  where: [category: "fresh and new"],
  order_by: [desc: :published_at],
  select: [:id, :title, :body]

The query above will select all posts with category "fresh and new", order by the most recently published, and return Post structs with only the id, title and body fields set. It is equivalent to:

from p in Post,
  where: p.category == "fresh and new",
  order_by: [desc: p.published_at],
  select: struct(p, [:id, :title, :body])

One advantage of bindingless queries is that they are data-driven and therefore useful for dynamically building queries. For example, the query above could also be written as:

where = [category: "fresh and new"]
order_by = [desc: :published_at]
select = [:id, :title, :body]
from Post, where: ^where, order_by: ^order_by, select: ^select

This feature is very useful when queries need to be built based on some user input, like web search forms, CLIs and so on.

Fragments

If you need an escape hatch, Ecto provides fragments (see Ecto.Query.API.fragment/1) to inject SQL (and non-SQL) fragments into queries.

For example, to get all posts while running the "lower(?)" function in the database where p.title is interpolated in place of ?, one can write:

from p in Post,
  where: is_nil(p.published_at) and
         fragment("lower(?)", p.title) == ^title

Also, most adapters provide direct APIs for queries, like Ecto.Adapters.SQL.query/4, allowing developers to completely bypass Ecto queries.

Macro API

In all examples so far we have used the keywords query syntax to create a query:

import Ecto.Query
from u in "users", where: u.age > 18, select: u.name

Due to the prevalence of the pipe operator in Elixir, Ecto also supports a pipe-based syntax:

"users"
|> where([u], u.age > 18)
|> select([u], u.name)

The keyword-based and pipe-based examples are equivalent. The downside of using macros is that the binding must be specified for every operation. However, since keyword-based and pipe-based examples are equivalent, the bindingless syntax also works for macros:

"users"
|> where([u], u.age > 18)
|> select([:name])

Such allows developers to write queries using bindings only in more complex query expressions.

This module documents each of those macros, providing examples in both the keywords query and pipe expression formats.

Query Prefix

It is possible to set a prefix for the queries. For Postgres users, this will specify the schema where the table is located, while for MySQL users this will specify the database where the table is located. When no prefix is set, Postgres queries are assumed to be in the public schema, while MySQL queries are assumed to be in the database set in the config for the repo.

The query prefix may be set either for the whole query or on each individual from and join expression. If a prefix is not given to a from or a join, the prefix of the schema given to the from or join is used. The query prefix is used only if none of the above are declared.

Let's see some examples. To see the query prefix globally, the simplest mechanism is to pass an option to the repository operation:

results = Repo.all(query, prefix: "accounts")

You may also set the prefix for the whole query by setting the prefix field:

results =
  query # May be User or an Ecto.Query itself
  |> Ecto.Queryable.to_query
  |> Map.put(:prefix, "accounts")
  |> Repo.all()

Setting the prefix in the query changes the default prefix of all from and join expressions. You can override the query prefix by either setting the @schema_prefix in your schema definitions or by passing the prefix option:

from u in User,
  prefix: "accounts",
  join: p in assoc(u, :posts),
  prefix: "public"

Overall, here is the prefix lookup precedence:

  1. The :prefix option given to from/join has the highest precedence
  2. Then it falls back to the @schema_prefix attribute declared in the schema given to from/join
  3. Then it falls back to the query prefix

The prefixes set in the query will be preserved when loading data.

Link to this section Summary

Functions

A distinct query expression.

Builds a dynamic query expression.

An except (set difference) query expression.

An except (set difference) query expression.

Resets a previously set field on a query.

Restricts the query to return the first result ordered by primary key.

Creates a query.

A group by query expression.

Returns true if query has binding with a given name, otherwise false.

An AND having query expression.

An intersect query expression.

An intersect query expression.

Restricts the query to return the last result ordered by primary key.

A limit query expression.

A lock query expression.

An offset query expression.

An OR having query expression.

An OR where query expression.

An order by query expression.

Preloads the associations into the result set.

Reverses the ordering of the query.

A select query expression.

Mergeable select query expression.

Converts a query into a subquery.

A union query expression.

A union all query expression.

An update query expression.

An AND where query expression.

Defines windows which can be used with Ecto.Query.WindowAPI.

Link to this section Types

Link to this type

t() View Source
t() :: %Ecto.Query{
  aliases: term(),
  assocs: term(),
  combinations: term(),
  distinct: term(),
  from: term(),
  group_bys: term(),
  havings: term(),
  joins: term(),
  limit: term(),
  lock: term(),
  offset: term(),
  order_bys: term(),
  prefix: term(),
  preloads: term(),
  select: term(),
  sources: term(),
  updates: term(),
  wheres: term(),
  windows: term()
}

Link to this section Functions

Link to this macro

distinct(query, binding \\ [], expr) View Source (macro)

A distinct query expression.

When true, only keeps distinct values from the resulting select expression.

If supported by your database, you can also pass query expressions to distinct and it will generate a query with DISTINCT ON. In such cases, distinct accepts exactly the same expressions as order_by and any distinct expression will be automatically prepended to the order_by expressions in case there is any order_by expression.

Keywords examples

# Returns the list of different categories in the Post schema
from(p in Post, distinct: true, select: p.category)

# If your database supports DISTINCT ON(),
# you can pass expressions to distinct too
from(p in Post,
   distinct: p.category,
   order_by: [p.date])

# The DISTINCT ON() also supports ordering similar to ORDER BY.
from(p in Post,
   distinct: [desc: p.category],
   order_by: [p.date])

# Using atoms
from(p in Post, distinct: :category, order_by: :date)

Expressions example

Post
|> distinct(true)
|> order_by([p], [p.category, p.author])
Link to this macro

dynamic(binding \\ [], expr) View Source (macro)

Builds a dynamic query expression.

Dynamic query expressions allows developers to build queries expression bit by bit so they are later interpolated in a query.

Examples

For example, imagine you have a set of conditions you want to build your query on:

conditions = false

conditions =
  if params["is_public"] do
    dynamic([p], p.is_public or ^conditions)
  else
    conditions
  end

conditions =
  if params["allow_reviewers"] do
    dynamic([p, a], a.reviewer == true or ^conditions)
  else
    conditions
  end

from query, where: ^conditions

In the example above, we were able to build the query expressions bit by bit, using different bindings, and later interpolate it all at once inside the query.

A dynamic expression can always be interpolated inside another dynamic expression and into the constructs described below.

where, having and a join's `on'

The dynamic macro can be interpolated at the root of a where, having or a join's on.

For example, assuming the conditions variable defined in the previous section, the following is forbidden because it is not at the root of a where:

from q in query, where: q.some_condition and ^conditions

Fortunately that's easily solvable by simply rewriting it to:

conditions = dynamic([q], q.some_condition and ^conditions)
from query, where: ^conditions

Updates

Dynamic is also supported as each field in an update, for example:

update_to = dynamic([p], p.sum / p.count)
from query, update: [set: [average: ^update_to]]
Link to this macro

except(query, other_query) View Source (macro)

An except (set difference) query expression.

Takes the difference of the result sets of multiple queries. The select of each query must be exactly the same, with the same types in the same order.

Except expression returns only unique rows as if each query returned distinct results. This may cause performance penalty. If you need just to take the difference of multiple result sets without removing duplicate rows consider using except_all/2.

Note that the operations order_by, limit and offset of the current query apply to the result of the set difference.

Keywords example

supplier_query = from s in Supplier, select: s.city
from c in Customer, select: c.city, except: ^supplier_query

Expressions example

supplier_query = Supplier |> select([s], s.city)
Customer |> select([c], c.city) |> except(^supplier_query)
Link to this macro

except_all(query, other_query) View Source (macro)

An except (set difference) query expression.

Takes the difference of the result sets of multiple queries. The select of each query must be exactly the same, with the same types in the same order.

Note that the operations order_by, limit and offset of the current query apply to the result of the set difference.

Keywords example

supplier_query = from s in Supplier, select: s.city
from c in Customer, select: c.city, except_all: ^supplier_query

Expressions example

supplier_query = Supplier |> select([s], s.city)
Customer |> select([c], c.city) |> except_all(^supplier_query)

Resets a previously set field on a query.

It can reset many fields except the query source (from). When excluding a :join, it will remove all types of joins. If you prefer to remove a single type of join, please see paragraph below.

Examples

Ecto.Query.exclude(query, :join)
Ecto.Query.exclude(query, :where)
Ecto.Query.exclude(query, :order_by)
Ecto.Query.exclude(query, :group_by)
Ecto.Query.exclude(query, :having)
Ecto.Query.exclude(query, :distinct)
Ecto.Query.exclude(query, :select)
Ecto.Query.exclude(query, :combinations)
Ecto.Query.exclude(query, :limit)
Ecto.Query.exclude(query, :offset)
Ecto.Query.exclude(query, :lock)
Ecto.Query.exclude(query, :preload)

You can also remove specific joins as well such as left_join and inner_join:

Ecto.Query.exclude(query, :inner_join)
Ecto.Query.exclude(query, :cross_join)
Ecto.Query.exclude(query, :left_join)
Ecto.Query.exclude(query, :right_join)
Ecto.Query.exclude(query, :full_join)
Ecto.Query.exclude(query, :inner_lateral_join)
Ecto.Query.exclude(query, :left_lateral_join)

However, keep in mind that if a join is removed and its bindings were referenced elsewhere, the bindings won't be removed, leading to a query that won't compile.

Link to this function

first(queryable, order_by \\ nil) View Source

Restricts the query to return the first result ordered by primary key.

The query will be automatically ordered by the primary key unless order_by is given or order_by is set in the query. Limit is always set to 1.

Examples

Post |> first |> Repo.one
query |> first(:inserted_at) |> Repo.one
Link to this macro

from(expr, kw \\ []) View Source (macro)

Creates a query.

It can either be a keyword query or a query expression.

If it is a keyword query the first argument must be either an in expression, or a value that implements the Ecto.Queryable protocol. If the query needs a reference to the data source in any other part of the expression, then an in must be used to create a reference variable. The second argument should be a keyword query where the keys are expression types and the values are expressions.

If it is a query expression the first argument must be a value that implements the Ecto.Queryable protocol and the second argument the expression.

Keywords example

from(c in City, select: c)

Expressions example

City |> select([c], c)

Examples

def paginate(query, page, size) do
  from query,
    limit: ^size,
    offset: ^((page-1) * size)
end

The example above does not use in because limit and offset do not require a reference to the data source. However, extending the query with a where expression would require the use of in:

def published(query) do
  from p in query, where: not(is_nil(p.published_at))
end

Notice we have created a p variable to reference the query's original data source. This assumes that the original query only had one source. When the given query has more than one source, positonal or named bindings may be used to access the additional sources.

def published_multi(query) do
  from [p,o] in query,
  where: not(is_nil(p.published_at)) and not(is_nil(o.published_at))
end

Note the variables p and o can be named whatever you like as they have no importance in the query sent to the database.

Link to this macro

group_by(query, binding \\ [], expr) View Source (macro)

A group by query expression.

Groups together rows from the schema that have the same values in the given fields. Using group_by "groups" the query giving it different semantics in the select expression. If a query is grouped, only fields that were referenced in the group_by can be used in the select or if the field is given as an argument to an aggregate function.

group_by also accepts a list of atoms where each atom refers to a field in source. For more complicated queries you can access fields directly instead of atoms.

Keywords examples

# Returns the number of posts in each category
from(p in Post,
  group_by: p.category,
  select: {p.category, count(p.id)})

# Using atoms
from(p in Post, group_by: :category, select: {p.category, count(p.id)})

# Using direct fields access
from(p in Post,
  join: c in assoc(p, :category)
  group_by: [p.id, c.name])

Expressions example

Post |> group_by([p], p.category) |> select([p], count(p.id))
Link to this function

has_named_binding?(queryable, key) View Source

Returns true if query has binding with a given name, otherwise false.

For more information on named bindings see "Named bindings" in this module doc.

Link to this macro

having(query, binding \\ [], expr) View Source (macro)

An AND having query expression.

Like where, having filters rows from the schema, but after the grouping is performed giving it the same semantics as select for a grouped query (see group_by/3). having groups the query even if the query has no group_by expression.

Keywords example

# Returns the number of posts in each category where the
# average number of comments is above ten
from(p in Post,
  group_by: p.category,
  having: avg(p.num_comments) > 10,
  select: {p.category, count(p.id)})

Expressions example

Post
|> group_by([p], p.category)
|> having([p], avg(p.num_comments) > 10)
|> select([p], count(p.id))
Link to this macro

intersect(query, other_query) View Source (macro)

An intersect query expression.

Takes the overlap of the result sets of multiple queries. The select of each query must be exactly the same, with the same types in the same order.

Intersect expression returns only unique rows as if each query returned distinct results. This may cause performance penalty. If you need just to take the intersection of multiple result sets without removing duplicate rows consider using intersect_all/2.

Note that the operations order_by, limit and offset of the current query apply to the result of the set difference.

Keywords example

supplier_query = from s in Supplier, select: s.city
from c in Customer, select: c.city, intersect: ^supplier_query

Expressions example

supplier_query = Supplier |> select([s], s.city)
Customer |> select([c], c.city) |> intersect(^supplier_query)
Link to this macro

intersect_all(query, other_query) View Source (macro)

An intersect query expression.

Takes the overlap of the result sets of multiple queries. The select of each query must be exactly the same, with the same types in the same order.

Note that the operations order_by, limit and offset of the current query apply to the result of the set difference.

Keywords example

supplier_query = from s in Supplier, select: s.city
from c in Customer, select: c.city, intersect_all: ^supplier_query

Expressions example

supplier_query = Supplier |> select([s], s.city)
Customer |> select([c], c.city) |> intersect_all(^supplier_query)
Link to this macro

join(query, qual, binding \\ [], expr, opts \\ []) View Source (macro)

A join query expression.

Receives a source that is to be joined to the query and a condition for the join. The join condition can be any expression that evaluates to a boolean value. The join is by default an inner join, the qualifier can be changed by giving the atoms: :inner, :left, :right, :cross, :full, :inner_lateral or :left_lateral. For a keyword query the :join keyword can be changed to: :inner_join, :left_join, :right_join, :cross_join, :full_join, :inner_lateral_join or :left_lateral_join.

Currently it is possible to join on:

  • an Ecto.Schema, such as p in Post
  • an interpolated Ecto query with zero or more where clauses, such as c in ^(from "posts", where: [public: true])
  • an association, such as c in assoc(post, :comments)
  • a subquery, such as c in subquery(another_query)
  • a query fragment, such as c in fragment("SOME COMPLEX QUERY"), see "Joining with fragments" below.

Options

Each join accepts the following options:

  • :on - a query expression or keyword list to filter the join
  • :as - a named binding for the join
  • :prefix - the prefix to be used for the join when issuing a database query
  • :hints - a string or a list of strings to be used as database hints

In the keyword query syntax, those options must be given immediately after the join. In the expression syntax, the options are given as the fifth argument.

Keywords examples

from c in Comment,
  join: p in Post,
  on: p.id == c.post_id,
  select: {p.title, c.text}

from p in Post,
  left_join: c in assoc(p, :comments),
  select: {p, c}

Keywords can also be given or interpolated as part of on:

from c in Comment,
  join: p in Post,
  on: [id: c.post_id],
  select: {p.title, c.text}

Any key in on will apply to the currently joined expression.

It is also possible to interpolate an Ecto query on the right side of in. For example, the query above can also be written as:

posts = Post
from c in Comment,
  join: p in ^posts,
  on: [id: c.post_id],
  select: {p.title, c.text}

The above is specially useful to dynamically join on existing queries, for example, to dynamically choose a source, or by choosing between public posts or posts that have been recently published:

posts =
  if params["drafts"] do
    from p in Post, where: [drafts: true]
  else
    from p in Post, where: [public: true]
  end

from c in Comment,
  join: p in ^posts, on: [id: c.post_id],
  select: {p.title, c.text}

Only simple queries with where expressions can be interpolated in join.

Expressions examples

Comment
|> join(:inner, [c], p in Post, on: c.post_id == p.id)
|> select([c, p], {p.title, c.text})

Post
|> join(:left, [p], c in assoc(p, :comments))
|> select([p, c], {p, c})

Post
|> join(:left, [p], c in Comment, on: c.post_id == p.id and c.is_visible == true)
|> select([p, c], {p, c})

Joining with fragments

When you need to join on a complex query, Ecto supports fragments in joins:

Comment
|> join(:inner, [c], p in fragment("SOME COMPLEX QUERY", c.id, ^some_param))

Although using fragments in joins is discouraged in favor of Ecto Query syntax, they are necessary when writing lateral joins as lateral joins require a subquery that refer to previous bindings:

Game
|> join(:inner_lateral, [g], gs in fragment("SELECT * FROM games_sold AS gs WHERE gs.game_id = ? ORDER BY gs.sold_on LIMIT 2", g.id))
|> select([g, gs], {g.name, gs.sold_on})

Note that the join does not automatically wrap the fragment in parentheses, since some expressions require parens and others require no parens. Therefore, in cases such as common table expressions, you will have to explicitly wrap the fragment content in parens.

Hints

from and join also support index hints, as found in databases such as MySQL and MSSQL.

For example, a developer using MySQL may write:

from p in Post,
  join: c in Comment,
  hints: ["USE INDEX FOO", "USE INDEX BAR"],
  where: p.id == c.post_id,
  select: c

Keep in mind you want to use hints rarely, so don't forget to read the database disclaimers about such functionality.

Link to this function

last(queryable, order_by \\ nil) View Source

Restricts the query to return the last result ordered by primary key.

The query ordering will be automatically reversed, with ASC columns becoming DESC columns (and vice-versa) and limit is set to 1. If there is no ordering, the query will be automatically ordered decreasingly by primary key.

Examples

Post |> last |> Repo.one
query |> last(:inserted_at) |> Repo.one
Link to this macro

limit(query, binding \\ [], expr) View Source (macro)

A limit query expression.

Limits the number of rows returned from the result. Can be any expression but has to evaluate to an integer value and it can't include any field.

If limit is given twice, it overrides the previous value.

Keywords example

from(u in User, where: u.id == ^current_user, limit: 1)

Expressions example

User |> where([u], u.id == ^current_user) |> limit(1)
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lock(query, expr) View Source (macro)

A lock query expression.

Provides support for row-level pessimistic locking using SELECT ... FOR UPDATE or other, database-specific, locking clauses. expr can be any expression but has to evaluate to a boolean value or to a string and it can't include any fields.

If lock is used more than once, the last one used takes precedence.

Ecto also supports optimistic locking but not through queries. For more information on optimistic locking, have a look at the Ecto.Changeset.optimistic_lock/3 function

Keywords example

from(u in User, where: u.id == ^current_user, lock: "FOR SHARE NOWAIT")

Expressions example

User |> where(u.id == ^current_user) |> lock("FOR SHARE NOWAIT")
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offset(query, binding \\ [], expr) View Source (macro)

An offset query expression.

Offsets the number of rows selected from the result. Can be any expression but it must evaluate to an integer value and it can't include any field.

If offset is given twice, it overrides the previous value.

Keywords example

# Get all posts on page 4
from(p in Post, limit: 10, offset: 30)

Expressions example

Post |> limit(10) |> offset(30)
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or_having(query, binding \\ [], expr) View Source (macro)

An OR having query expression.

Like having but combines with the previous expression by using OR. or_having behaves for having the same way or_where behaves for where.

Keywords example

# Augment a previous group_by with a having condition.
from(p in query, or_having: avg(p.num_comments) > 10)

Expressions example

# Augment a previous group_by with a having condition.
Post |> or_having([p], avg(p.num_comments) > 10)
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or_where(query, binding \\ [], expr) View Source (macro)

An OR where query expression.

Behaves exactly the same as where except it combines with any previous expression by using an OR. All expressions have to evaluate to a boolean value.

or_where also accepts a keyword list where each key is a field to be compared with the given value. Each key-value pair will be combined using AND, exactly as in where.

Keywords example

from(c in City, where: [country: "Sweden"], or_where: [country: "Brazil"])

If interpolating keyword lists, the keyword list entries are combined using ANDs and joined to any existing expression with an OR:

filters = [country: "USA", name: "New York"]
from(c in City, where: [country: "Sweden"], or_where: ^filters)

is equivalent to:

from c in City, where: (c.country == "Sweden") or
                       (c.country == "USA" and c.name == "New York")

The behaviour above is by design to keep the changes between where and or_where minimal. Plus, if you have a keyword list and you would like each pair to be combined using or, it can be easily done with Enum.reduce/3:

filters = [country: "USA", is_tax_exempt: true]
Enum.reduce(filters, City, fn {key, value}, query ->
  from q in query, or_where: field(q, ^key) == ^value
end)

which will be equivalent to:

from c in City, or_where: (c.country == "USA"), or_where: c.is_tax_exempt == true

Expressions example

City |> where([c], c.country == "Sweden") |> or_where([c], c.country == "Brazil")
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order_by(query, binding \\ [], expr) View Source (macro)

An order by query expression.

Orders the fields based on one or more fields. It accepts a single field or a list of fields. The default direction is ascending (:asc) and can be customized in a keyword list as one of the following:

  • :asc
  • :asc_nulls_last
  • :asc_nulls_first
  • :desc
  • :desc_nulls_last
  • :desc_nulls_first

The *_nulls_first and *_nulls_last variants are not supported by all databases. While all databases default to ascending order, the choice of "nulls first" or "nulls last" is specific to each database implementation.

order_by may be invoked or listed in a query many times. New expressions are always appended to the previous ones.

order_by also accepts a list of atoms where each atom refers to a field in source or a keyword list where the direction is given as key and the field to order as value.

Keywords examples

from(c in City, order_by: c.name, order_by: c.population)
from(c in City, order_by: [c.name, c.population])
from(c in City, order_by: [asc: c.name, desc: c.population])

from(c in City, order_by: [:name, :population])
from(c in City, order_by: [asc: :name, desc_nulls_first: :population])

A keyword list can also be interpolated:

values = [asc: :name, desc_nulls_first: :population]
from(c in City, order_by: ^values)

A fragment can also be used:

from c in City, order_by: [
  # a deterministic shuffled order
  fragment("? % ? DESC", c.id, ^modulus),
  desc: c.id,
]

Expressions example

City |> order_by([c], asc: c.name, desc: c.population)
City |> order_by(asc: :name) # Sorts by the cities name
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preload(query, bindings \\ [], expr) View Source (macro)

Preloads the associations into the result set.

Imagine you have an schema Post with a has_many :comments association and you execute the following query:

Repo.all from p in Post, preload: [:comments]

The example above will fetch all posts from the database and then do a separate query returning all comments associated with the given posts. The comments are then processed and associated to each returned post under the comments field.

Often times, you may want posts and comments to be selected and filtered in the same query. For such cases, you can explicitly tell an existing join to be preloaded into the result set:

Repo.all from p in Post,
           join: c in assoc(p, :comments),
           where: c.published_at > p.updated_at,
           preload: [comments: c]

In the example above, instead of issuing a separate query to fetch comments, Ecto will fetch posts and comments in a single query and then do a separate pass associating each comment to its parent post. Therefore, instead of returning number_of_posts * number_of_comments results, like a join would, it returns only posts with the comments fields properly filled in.

Nested associations can also be preloaded in both formats:

Repo.all from p in Post,
           preload: [comments: :likes]

Repo.all from p in Post,
           join: c in assoc(p, :comments),
           join: l in assoc(c, :likes),
           where: l.inserted_at > c.updated_at,
           preload: [comments: {c, likes: l}]

Preload queries

Preload also allows queries to be given, allowing you to filter or customize how the preloads are fetched:

comments_query = from c in Comment, order_by: c.published_at
Repo.all from p in Post, preload: [comments: ^comments_query]

The example above will issue two queries, one for loading posts and then another for loading the comments associated with the posts. Comments will be ordered by published_at.

Note: keep in mind operations like limit and offset in the preload query will affect the whole result set and not each association. For example, the query below:

comments_query = from c in Comment, order_by: c.popularity, limit: 5
Repo.all from p in Post, preload: [comments: ^comments_query]

won't bring the top of comments per post. Rather, it will only bring the 5 top comments across all posts.

Preload functions

Preload also allows functions to be given. In such cases, the function receives the IDs of the parent association and it must return the associated data. Ecto then will map this data and sort it by the relationship key:

comment_preloader = fn post_ids -> fetch_comments_by_post_ids(post_ids) end
Repo.all from p in Post, preload: [comments: ^comment_preloader]

This is useful when the whole dataset was already loaded or must be explicitly fetched from elsewhere. The IDs received by the preloading function and the result returned depends on the association type:

  • For has_many and belongs_to - the function receives the IDs of the parent association and it must return a list of maps or structs with the associated entries. The associated map/struct must contain the "foreign_key" field. For example, if a post has many comments, when preloading the comments with a custom function, the function will receive a list of "post_ids" as argument and it must return maps or structs representing the comments. The maps/structs must include the :post_id field

  • For has_many :through - it behaves similarly to a regular has_many but note that the IDs received are the ones from the closest parent and not the furthest one. Imagine for example a post has many comments and each comment has an author. Therefore, a post may have many comments_authors, written as has_many :comments_authors, through: [:comments, :author]. When preloading authors with a custom function via :comments_authors, the function will receive the IDs of the comments and not of the posts. That's because through associations are still loaded step by step

  • For many_to_many - the function receives the IDs of the parent association and it must return a tuple with the parent id as first element and the association map or struct as second. For example, if a post has many tags, when preloading the tags with a custom function, the function will receive a list of "post_ids" as argument and it must return a tuple in the format of {post_id, tag}

Keywords example

# Returns all posts, their associated comments, and the associated
# likes for those comments.
from(p in Post,
  preload: [:comments, comments: :likes],
  select: p)

Expressions examples

Post |> preload(:comments) |> select([p], p)

Post
|> join(:left, [p], c in assoc(p, :comments))
|> preload([p, c], [:user, comments: c])
|> select([p], p)

Reverses the ordering of the query.

ASC columns become DESC columns (and vice-versa). If the query has no order_bys, it orders by the inverse of the primary key.

Examples

query |> reverse_order |> Repo.one
Post |> order(asc: :id) |> reverse_order == Post |> order(desc: :id)
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select(query, binding \\ [], expr) View Source (macro)

A select query expression.

Selects which fields will be selected from the schema and any transformations that should be performed on the fields. Any expression that is accepted in a query can be a select field.

Select also allows each expression to be wrapped in lists, tuples or maps as shown in the examples below. A full schema can also be selected.

There can only be one select expression in a query, if the select expression is omitted, the query will by default select the full schema. If select is given more than once, an error is raised. Use exclude/2 if you would like to remove a previous select for overriding or see select_merge/3 for a limited version of select that is composable and can be called multiple times.

select also accepts a list of atoms where each atom refers to a field in the source to be selected.

Keywords examples

from(c in City, select: c) # returns the schema as a struct
from(c in City, select: {c.name, c.population})
from(c in City, select: [c.name, c.county])
from(c in City, select: %{n: c.name, answer: 42})
from(c in City, select: %{c | alternative_name: c.name})
from(c in City, select: %Data{name: c.name})

It is also possible to select a struct and limit the returned fields at the same time:

from(City, select: [:name])

The syntax above is equivalent to:

from(city in City, select: struct(city, [:name]))

You can also write:

from(city in City, select: map(city, [:name]))

If you want a map with only the selected fields to be returned. For more information, read the docs for Ecto.Query.API.struct/2 and Ecto.Query.API.map/2.

Expressions examples

City |> select([c], c)
City |> select([c], {c.name, c.country})
City |> select([c], %{"name" => c.name})
City |> select([:name])
City |> select([c], struct(c, [:name]))
City |> select([c], map(c, [:name]))
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select_merge(query, binding \\ [], expr) View Source (macro)

Mergeable select query expression.

This macro is similar to select/3 except it may be specified multiple times as long as every entry is a map. This is useful for merging and composing selects. For example:

query = from p in Post, select: %{}

query =
  if include_title? do
    from p in query, select_merge: %{title: p.title}
  else
    query
  end

query =
  if include_visits? do
    from p in query, select_merge: %{visits: p.visits}
  else
    query
  end

In the example above, the query is built little by little by merging into a final map. If both conditions above are true, the final query would be equivalent to:

from p in Post, select: %{title: p.title, visits: p.visits}

If :select_merge is called and there is no value selected previously, it will default to the source, p in the example above.

The argument given to :select_merge must always be a map. The value being merged on must be a struct or a map. If it is a struct, the fields merged later on must be part of the struct, otherwise an error is raised.

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subquery(query, opts \\ []) View Source

Converts a query into a subquery.

If a subquery is given, returns the subquery itself. If any other value is given, it is converted to a query via Ecto.Queryable and wrapped in the Ecto.SubQuery struct.

Subqueries are currently only supported in the from and join fields.

Examples

# Get the average salary of the top 10 highest salaries
query = from Employee, order_by: [desc: :salary], limit: 10
from e in subquery(query), select: avg(e.salary)

A prefix can be specified for a subquery, similar to standard repo operations:

query = from Employee, order_by: [desc: :salary], limit: 10
from e in subquery(query, prefix: "my_prefix"), select: avg(e.salary)

Although subqueries are not allowed in WHERE expressions, most subqueries in WHERE expression can be rewritten as JOINs. Imagine you want to write this query:

UPDATE posts
  SET sync_started_at = $1
  WHERE id IN (
    SELECT id FROM posts
      WHERE synced = false AND (sync_started_at IS NULL OR sync_started_at < $1)
      LIMIT $2
  )

If you attempt to write it as where: p.id in ^subquery(foo), Ecto won't accept such query. However, the subquery above can be written as a JOIN, which is supported by Ecto. The final Ecto query will look like this:

subset_query = from(p in Post,
  where: p.synced == false and
           (is_nil(p.sync_started_at) or p.sync_started_at < ^min_sync_started_at),
  limit: ^batch_size
)

Repo.update_all(
  from(p in Post, join: s in subquery(subset_query), on: s.id == p.id),
  set: [sync_started_at: NaiveDateTime.utc_now()]
)
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union(query, other_query) View Source (macro)

A union query expression.

Combines result sets of multiple queries. The select of each query must be exactly the same, with the same types in the same order.

Union expression returns only unique rows as if each query returned distinct results. This may cause performance penalty. If you need just to combine multiple result sets without removing duplicate rows consider using union_all/2.

Note that the operations order_by, limit and offset of the current query apply to the result of the union.

Keywords example

supplier_query = from s in Supplier, select: s.city
from c in Customer, select: c.city, union: ^supplier_query

Expressions example

supplier_query = Supplier |> select([s], s.city)
Customer |> select([c], c.city) |> union(^supplier_query)
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union_all(query, other_query) View Source (macro)

A union all query expression.

Combines result sets of multiple queries. The select of each query must be exactly the same, with the same types in the same order.

Note that the operations order_by, limit and offset of the current query apply to the result of the union.

Keywords example

supplier_query = from s in Supplier, select: s.city
from c in Customer, select: c.city, union_all: ^supplier_query

Expressions example

supplier_query = Supplier |> select([s], s.city)
Customer |> select([c], c.city) |> union_all(^supplier_query)
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update(query, binding \\ [], expr) View Source (macro)

An update query expression.

Updates are used to update the filtered entries. In order for updates to be applied, Ecto.Repo.update_all/3 must be invoked.

Keywords example

from(u in User, update: [set: [name: "new name"]])

Expressions example

User |> update([u], set: [name: "new name"])
User |> update(set: [name: "new name"])

Interpolation

new_name = "new name"
from(u in User, update: [set: [name: ^new_name]])

new_name = "new name"
from(u in User, update: [set: [name: fragment("upper(?)", ^new_name)]])

Operators

The update expression in Ecto supports the following operators:

  • set - sets the given field in the table to the given value

    from(u in User, update: [set: [name: "new name"]])

  • inc - increments (or decrements if the value is negative) the given field in the table by the given value

    from(u in User, update: [inc: [accesses: 1]])

  • push - pushes (appends) the given value to the end of the array field

    from(u in User, update: [push: [tags: "cool"]])

  • pull - pulls (removes) the given value from the array field

    from(u in User, update: [pull: [tags: "not cool"]])

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where(query, binding \\ [], expr) View Source (macro)

An AND where query expression.

where expressions are used to filter the result set. If there is more than one where expression, they are combined with an and operator. All where expressions have to evaluate to a boolean value.

where also accepts a keyword list where the field given as key is going to be compared with the given value. The fields will always refer to the source given in from.

Keywords example

from(c in City, where: c.country == "Sweden")
from(c in City, where: [country: "Sweden"])

It is also possible to interpolate the whole keyword list, allowing you to dynamically filter the source:

filters = [country: "Sweden"]
from(c in City, where: ^filters)

Expressions example

City |> where([c], c.country == "Sweden")
City |> where(country: "Sweden")
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windows(query, binding \\ [], expr) View Source (macro)

Defines windows which can be used with Ecto.Query.WindowAPI.

Receives a keyword list where keys are names of the windows and values are a keyword list with window expressions.

Examples

# Compare each employee's salary with the average salary in his or her department
from e in Employee,
  select: {e.depname, e.empno, e.salary, over(avg(e.salary), :department)},
  windows: [department: [partition_by: e.depname]]

In the example above, we get the average salary per department. :department is the window name, partitioned by e.depname and avg/1 is the window function. For more information on windows functions, see Ecto.Query.WindowAPI.

Window expressions

The following keys are allowed when specifying a window.

:partition_by

A list of fields to partition the window by, for example:

windows: [department: [partition_by: e.depname]]

A list of atoms can also be interpolated for dynamic partitioning:

fields = [:depname, :year]
windows: [dynamic_window: [partition_by: ^fields]]

:order_by

A list of fields to order the window by, for example:

windows: [ordered_names: [order_by: e.name]]

It works exactly as the keyword query version of order_by/3.

:frame

A fragment which defines the frame for window functions.

Examples

# compare each employee's salary for each month with his average salary for previous 3 months
from p in Payroll,
  select: {p.empno, p.date, p.salary, over(avg(p.salary), :prev_months)},
  windows: [prev_months: [partition_by: p.empno, order_by: p.date, frame: fragment("ROWS 3 PRECEDING EXCLUDE CURRENT ROW")]]