Resets a previously set field on a query.

It can reset any query field except the query source (from).

Example

query |> Ecto.Query.exclude(:select)

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

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

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 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, the row that is being kept depends on the ordering of the rows. When an order_by expression is also added to the query, all fields in the distinct expression are automatically referenced order_by too.

distinct also accepts a list of atoms where each atom refers to a field in source.

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])

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

Expressions example

Post
|> distinct(true)
|> order_by([p], [p.category, p.author])

Creates a query.

It can either be a keyword query or a query expression. If it is a keyword query the first argument should be an in expression and the second argument a keyword query where the keys are expression types and the values are expressions.

If it is a query expression the first argument is the original query and the second argument the expression.

Keywords example

from(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 such. However, extending a 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_a))
end

Notice we have created a p variable to represent each item in the query. When the given query has more than one from expression, a variable must be given for each in the order they were bound:

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.

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.

Keywords examples

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

# Group on all fields on the Post schema
from(p in Post, group_by: p, select: p)

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

Expressions example

Post |> group_by([p], p.category) |> select([p], count(p.id))

A 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))

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 or :full. For a keyword query the :join keyword can be changed to: :inner_join, :left_join, :right_join or :full_join.

Currently it is possible to join on an Ecto.Schema (a module), an existing source (a binary representing a table), an association or a fragment. See the examples below.

Keywords examples

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

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

Expressions examples

Comment
  |> join(:inner, [c], p in Post, 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})

Joining with fragments

When you need to join on a complex expression that cannot be expressed via Ecto associations, Ecto supports fragments in joins:

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

This style discouraged due to its complexity.

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)

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")

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)

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 shown in the examples. There can be several order by expressions in a query.

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: :population])

A keyword list can also be interpolated:

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

Expressions example

City |> order_by([c], asc: c.name, desc: c.population)
City |> order_by(asc: :name) # Sorts by the cities name

Preloads the associations into the given struct.

Preloading allows developers to specify associations that are preloaded into the struct. Consider this example:

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 to the given posts.

However, often times, you want posts and comments to be selected and filtered in the same query. For such cases, you can explicitly tell the association to be preloaded into the struct:

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.

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}]

Keep in mind neither format can be nested arbitrarily. For example, the query below is invalid because we cannot preload likes with the join association c.

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

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.

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

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.

The sub-expressions in the query can be wrapped in lists, tuples or maps as shown in the examples. 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.

select also accepts a list of atoms where each atom refers to a field in 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: {c.name, ^to_string(40 + 2), 43})
from(c in City, select: %{n: c.name, answer: 42})

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: take(city, [:name]))

For more information, read the docs for Ecto.Query.API.take/2.

Expressions examples

City |> select([c], c)
City |> select([c], {c.name, c.country})
City |> select([c], %{"name" => c.name})

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"])

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"]])

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

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

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

Expressions example

City |> where([c], c.state == "Sweden")
City |> where(state: "Sweden")