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)

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.

Keywords examples

# Returns the list of different categories in the Post model
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])

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: p.published_at != nil
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: p.published_at != nil and o.published_at != nil
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 model 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.

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 model
from(p in Post,
  group_by: p,
  select: p)

Expressions example

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

A having query expression.

Like where, having filters rows from the model, 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 model 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 an existing model, an existing source (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.Model.OptimisticLock module.

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 direction can be specified in a keyword list as shown in the examples. There can be several order by expressions in a query.

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])

Expressions example

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

Atom values

For simplicity, order_by also allows the fields to be given as atoms. In such cases, the field always applies to the source given in from (i.e. the first binding). For example, the two expressions below are equivalent:

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

A keyword list can also be interpolated:

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

Preloads the associations into the given model.

Preloading allows developers to specify associations that are preloaded into the model. 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 model:

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 and their associated 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 model and any transformations that should be performed on the fields. Any expression that is accepted in a query can be a select field.

There can only be one select expression in a query, if the select expression is omitted, the query will by default select the full model.

The sub-expressions in the query can be wrapped in lists, tuples or maps as shown in the examples. A full model can also be selected. Note that map keys can only be atoms, binaries, integers or floats otherwise an Ecto.Query.CompileError exception is raised at compile-time.

Keywords examples

from(c in City, select: c) # selects the entire model
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})

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")