Ecto.Schema — Ecto v3.12.5 (original) (raw)

An Ecto schema maps external data into Elixir structs.

The definition of the schema is possible through two main APIs:schema/2 and embedded_schema/1.

schema/2 is typically used to map data from a persisted source, usually a database table, into Elixir structs and vice-versa via the Ecto.Repo module. For this reason, the first argument of schema/2is the source (table) name. Structs defined with schema/2 also contain a __meta__ field with metadata holding the status of the struct, for example, if it has been built, loaded or deleted. Schemas also support associations, through APIs such as has_one/3 and belongs_to/3. Check out the Associations cheatsheet for a reference on the different associations types and their migrations.

On the other hand, embedded_schema/1 is used for defining schemas that are embedded in other schemas or only exist in-memory. For example, you can use such schemas to receive data from a command line interface or a contact form, and validate it, without ever persisting it elsewhere. Such structs do not contain a __meta__ field, as they are never persisted.

Both schemas can be used alongside changesets to filter, cast, and validate data. Besides working as data mappers, embedded_schema/1 and schema/2can also be used together to decouple how the data is represented in your applications from the database.

Example

defmodule User do
  use Ecto.Schema

  schema "users" do
    field :name, :string
    field :age, :integer, default: 0
    field :password, :string, redact: true
    has_many :posts, Post
  end
end

By default, a schema will automatically generate a primary key which is namedid and of type :integer. The field macro defines a field in the schema with given name and type. has_many associates many posts with the user schema. Schemas are regular structs and can be created and manipulated directly using Elixir's struct API:

iex> user = %User{name: "jane"}
iex> %{user | age: 30}

However, most commonly, structs are cast, validated and manipulated with theEcto.Changeset module.

The first argument of schema/2 is the name of database's table, which does not need to correlate to your module name (commonly referred to as the schema/schema name). For example, if you are working with a legacy database, you can reference the table name (legacy_users) when you define your schema (User):

defmodule User do
  use Ecto.Schema

  schema "legacy_users" do
    # ... fields ...
  end
end

Source-based schemas are queryable by default, which means we can pass them to Ecto.Repo modules and also build queries:

MyRepo.all(User)
MyRepo.all(from u in User, where: u.id == 13)

The repository will then run the query against the source/table.

Embedded schemas are defined similarly to source-based schemas. For example, you can use an embedded schema to represent your UI, mapping and validating its inputs, and then you convert such embedded schema to other schemas that are persisted to the database:

defmodule SignUp do
  use Ecto.Schema

  embedded_schema do
    field :name, :string
    field :age, :integer
    field :email, :string
    field :accepts_conditions, :boolean
  end
end

defmodule Profile do
  use Ecto.Schema

  schema "profiles" do
    field :name
    field :age
    belongs_to :account, Account
  end
end

defmodule Account do
  use Ecto.Schema

  schema "accounts" do
    field :email
  end
end

The SignUp schema can be cast and validated with the help of theEcto.Changeset module, and afterwards, you can copy its data to the Profile and Account structs that will be persisted to the database with the help of Ecto.Repo. On the other hand, embedded schemas cannot be queried directly (they are not queryable).

use Ecto.Schema

When you use Ecto.Schema, it will:

We detail those throughout the module documentation.

Redacting fields

A field marked with redact: true will display a value of **redacted**when inspected in changes inside a Ecto.Changeset and be excluded from inspect on the schema unless the schema module is tagged with the option @ecto_derive_inspect_for_redacted_fields false.

Schema attributes

Supported attributes for configuring the defined schema. They must be set after the use Ecto.Schema call and before the schema/2definition.

These attributes are:

The advantage of configuring the schema via those attributes is that they can be set with a macro to configure application wide defaults.

For example, if your database does not support autoincrementing primary keys and requires something like UUID or a RecordID, you can configure and use :binary_id as your primary key type as follows:

# Define a module to be used as base
defmodule MyApp.Schema do
  defmacro __using__(_) do
    quote do
      use Ecto.Schema
      @primary_key {:id, :binary_id, autogenerate: true}
      @foreign_key_type :binary_id
    end
  end
end

# Now use MyApp.Schema to define new schemas
defmodule MyApp.Comment do
  use MyApp.Schema

  schema "comments" do
    belongs_to :post, MyApp.Post
  end
end

Any schemas using MyApp.Schema will get the :id field with type:binary_id as the primary key. We explain what the :binary_id type entails in the next section.

The belongs_to association on MyApp.Comment will also define a :post_id field with :binary_id type that references the :idfield of the MyApp.Post schema.

Primary keys

Ecto supports two ID types, called :id and :binary_id, which are often used as the type for primary keys and associations.

The :id type is used when the primary key is an integer while the:binary_id is used for primary keys in particular binary formats, which may be Ecto.UUID for databases like PostgreSQL and MySQL, or some specific ObjectID or RecordID often imposed by NoSQL databases.

In both cases, both types have their semantics specified by the underlying adapter/database. If you use the :id type with:autogenerate, it means the database will be responsible for auto-generation of the id. This is often the case for primary keys in relational databases which are auto-incremented.

There are two ways to define primary keys in Ecto: using the @primary_keymodule attribute and using primary_key: true as option for field/3 in your schema definition. They are not mutually exclusive and can be used together.

Using @primary_key should be preferred for single field primary keys and sharing primary key definitions between multiple schemas using macros. Setting @primary_key also automatically configures the reference types for has_one and has_many associations.

Ecto also supports composite primary keys, which is where you need to useprimary_key: true for the fields in your schema. This usually goes along with setting @primary_key false to disable generation of additional primary key fields.

Besides :id and :binary_id, which are often used by primary and foreign keys, Ecto provides a huge variety of types to be used by any field.

Types and casting

When defining the schema, types need to be given. Types are split into two categories, primitive types and custom types.

Primitive types

The primitive types are:

Ecto type Elixir type Literal syntax in query
:id integer 1, 2, 3
:binary_id binary <<int, int, int, ...>>
:integer integer 1, 2, 3
:float float 1.0, 2.0, 3.0
:boolean boolean true, false
:string UTF-8 encoded string "hello"
:binary binary <<int, int, int, ...>>
:bitstring bitstring <<_::size>>
{:array, inner_type} list [value, value, value, ...]
:map map
{:map, inner_type} map
:decimal Decimal
:date Date
:time Time
:time_usec Time
:naive_datetime NaiveDateTime
:naive_datetime_usec NaiveDateTime
:utc_datetime DateTime
:utc_datetime_usec DateTime
:duration Duration

Notes:

Custom types

Besides providing primitive types, Ecto allows custom types to be implemented by developers, allowing Ecto behaviour to be extended.

A custom type is a module that implements one of the Ecto.Typeor Ecto.ParameterizedType behaviours. By default, Ecto provides the following custom types:

Custom type Database type Elixir type
Ecto.UUID :uuid (as a binary) string() (as a UUID)
Ecto.Enum :string atom()

Finally, schemas can also have virtual fields by passing thevirtual: true option. These fields are not persisted to the database and can optionally not be type checked by declaring type :any.

The datetime types

Four different datetime primitive types are available:

All of those types are represented by the same timestamp/datetime in the underlying data storage, the difference are in their precision and how the data is loaded into Elixir.

Having different precisions allows developers to choose a type that will be compatible with the database and your project's precision requirements. For example, some older versions of MySQL do not support microseconds in datetime fields.

When choosing what datetime type to work with, keep in mind that Elixir functions like NaiveDateTime.utc_now/0 have a default precision of 6. Casting a value with a precision greater than 0 to a non-usec type will truncate all microseconds and set the precision to 0.

The map type

The map type allows developers to store an Elixir map directly in the database:

# In your migration
create table(:users) do
  add :data, :map
end

# In your schema
field :data, :map

# Now in your code
user = Repo.insert! %User{data: %{"foo" => "bar"}}

Keep in mind that we advise the map keys to be strings or integers instead of atoms. Atoms may be accepted depending on how maps are serialized but the database will always convert atom keys to strings due to security reasons.

In order to support maps, different databases may employ different techniques. For example, PostgreSQL will store those values in jsonb fields, allowing you to just query parts of it. MSSQL, on the other hand, does not yet provide a JSON type, so the value will be stored in a text field.

For maps to work in such databases, Ecto will need a JSON library. By default Ecto will use Jasonwhich needs to be added to your deps in mix.exs:

{:jason, "~> 1.0"}

You can however configure the adapter to use another library. For example, if using Postgres:

config :postgrex, :json_library, YourLibraryOfChoice

Or if using MySQL:

config :myxql, :json_library, YourLibraryOfChoice

If changing the JSON library, remember to recompile the adapter afterwards by cleaning the current build:

mix deps.clean --build postgrex

Casting

When directly manipulating the struct, it is the responsibility of the developer to ensure the field values have the proper type. For example, you can create a user struct with an invalid value for age:

iex> user = %User{age: "0"}
iex> user.age
"0"

However, if you attempt to persist the struct above, an error will be raised since Ecto validates the types when sending them to the adapter/database.

Therefore, when working with and manipulating external data, it is recommended to use Ecto.Changeset's that are able to filter and properly cast external data:

changeset = Ecto.Changeset.cast(%User{}, %{"age" => "0"}, [:age])
user = Repo.insert!(changeset)

You can use Ecto schemas and changesets to cast and validate any kind of data, regardless if the data will be persisted to an Ecto repository or not.

Reflection

Any schema module will generate the __schema__ function that can be used for runtime introspection of the schema:

Furthermore, both __struct__ and __changeset__ functions are defined so structs and changeset functionalities are available.

Working with typespecs

Generating typespecs for schemas is out of the scope of Ecto.Schema.

In order to be able to use types such as User.t(), t/0 has to be defined manually:

defmodule User do
  use Ecto.Schema

  @type t :: %__MODULE__{
    name: String.t(),
    age: non_neg_integer()
  }

  # ... schema ...
end

Defining the type of each field is not mandatory, but it is preferable.

Summary

Functions

Indicates a one-to-one or many-to-one association with another schema.

Defines an embedded schema with the given field definitions.

Indicates an embedding of many schemas.

Indicates an embedding of many schemas.

Indicates an embedding of a schema.

Indicates an embedding of a schema.

Defines a field on the schema with given name and type.

Indicates a one-to-many association with another schema.

Indicates a one-to-one association with another schema.

Indicates a many-to-many association with another schema.

Defines a schema struct with a source name and field definitions.

Generates :inserted_at and :updated_at timestamp fields.

Types

@type embedded_schema() :: %{optional(atom()) => any(), struct: atom()}

@type embeds_many(t) :: [t]

@type t() :: schema() | embedded_schema()

Functions

Indicates a one-to-one or many-to-one association with another schema.

The current schema belongs to zero or one records of the other schema. The other schema often has a has_one or a has_many field with the reverse association.

You should use belongs_to in the table that contains the foreign key. Imagine a company <-> employee relationship. If the employee contains the company_id in the underlying database table, we say the employee belongs to company.

In fact, when you invoke this macro, a field with the name of foreign key is automatically defined in the schema for you.

Options

Examples

defmodule Comment do
  use Ecto.Schema

  schema "comments" do
    belongs_to :post, Post
  end
end

# The post can come preloaded on the comment record
[comment] = Repo.all(from(c in Comment, where: c.id == 42, preload: :post))
comment.post #=> %Post{...}

If you need custom options on the underlying field, you can define the field explicitly and then pass define_field: false to belongs_to:

defmodule Comment do
  use Ecto.Schema

  schema "comments" do
    field :post_id, :integer, ... # custom options
    belongs_to :post, Post, define_field: false
  end
end

If using EctoSQL, the comments table should have a post_id column that references the posts table. In your migrations, this can be done as:

add :post_id,
    references(:posts, on_delete: :delete_all),
    null: false

See the Associations cheatsheet for more examples.

Polymorphic associations

One common use case for belongs to associations is to handle polymorphism. For example, imagine you have defined a Comment schema and you wish to use it for commenting on both tasks and posts.

Some abstractions would force you to define some sort of polymorphic association with two fields in your database:

* commentable_type
* commentable_id

The problem with this approach is that it breaks references in the database. You can't use foreign keys and it is very inefficient, both in terms of query time and storage.

In Ecto, we have three ways to solve this issue. The simplest is to define multiple fields in the Comment schema, one for each association:

* task_id
* post_id

Unless you have dozens of columns, this is simpler for the developer, more DB friendly and more efficient in all aspects.

Alternatively, because Ecto does not tie a schema to a given table, we can use separate tables for each association. Let's start over and define a new Comment schema:

defmodule Comment do
  use Ecto.Schema

  schema "abstract table: comments" do
    # This will be used by associations on each "concrete" table
    field :assoc_id, :integer
  end
end

Notice we have changed the table name to "abstract table: comments". You can choose whatever name you want, the point here is that this particular table will never exist.

Now in your Post and Task schemas:

defmodule Post do
  use Ecto.Schema

  schema "posts" do
    has_many :comments, {"posts_comments", Comment}, foreign_key: :assoc_id
  end
end

defmodule Task do
  use Ecto.Schema

  schema "tasks" do
    has_many :comments, {"tasks_comments", Comment}, foreign_key: :assoc_id
  end
end

Now each association uses its own specific table, "posts_comments" and "tasks_comments", which must be created on migrations. The advantage of this approach is that we never store unrelated data together, also ensuring we keep database references fast and correct.

When using this technique, the only limitation is that you cannot build comments directly. For example, the command below

Repo.insert!(%Comment{})

will attempt to use the abstract table. Instead, one should use

Repo.insert!(build_assoc(post, :comments))

leveraging the Ecto.build_assoc/3 function. You can also use Ecto.assoc/2 or pass a tuple in the query syntax to easily retrieve associated comments to a given post or task:

# Fetch all comments associated with the given task
Repo.all(Ecto.assoc(task, :comments))

Or all comments in a given table:

Repo.all from(c in {"posts_comments", Comment}), ...)

The third and final option is to use many_to_many/3 to define the relationships between the resources. In this case, the comments table won't have the foreign key, instead there is an intermediary table responsible for associating the entries:

defmodule Comment do
  use Ecto.Schema
  schema "comments" do
    # ...
  end
end

In your posts and tasks:

defmodule Post do
  use Ecto.Schema

  schema "posts" do
    many_to_many :comments, Comment, join_through: "posts_comments"
  end
end

defmodule Task do
  use Ecto.Schema

  schema "tasks" do
    many_to_many :comments, Comment, join_through: "tasks_comments"
  end
end

See many_to_many/3 for more information on this particular approach.

Defines an embedded schema with the given field definitions.

An embedded schema is either embedded into another schema or kept exclusively in memory. For this reason, an embedded schema does not require a source name and it does not include a metadata field.

Embedded schemas by default set the primary key type to :binary_id but such can be configured with the@primary_key attribute.

belongs_to/3 associations may be defined inside of embedded schemas. However, any association nested inside of an embedded schema won't be persisted to the database when calling Ecto.Repo.insert/2 or Ecto.Repo.update/2.

Indicates an embedding of many schemas.

The current schema has zero or more records of the other schema embedded inside of it. Embeds have all the things regular schemas have.

It is recommended to declare your embeds_many/3 field with type :mapin your migrations, instead of using {:array, :map}. Ecto can work with both maps and arrays as the container for embeds (and in most databases maps are represented as JSON which allows Ecto to choose what works best).

The embedded may or may not have a primary key. Ecto uses the primary keys to detect if an embed is being updated or not. If a primary key is not present and you still want the list of embeds to be updated, :on_replacemust be set to :delete, forcing all current embeds to be deleted and replaced by new ones whenever a new list of embeds is set.

For encoding and decoding of embeds, please read the docs forembeds_one/3.

Options

Examples

defmodule Order do
  use Ecto.Schema

  schema "orders" do
    embeds_many :items, Item
  end
end

defmodule Item do
  use Ecto.Schema

  embedded_schema do
    field :title
  end
end

# The items are loaded with the order
order = Repo.get!(Order, 42)
order.items #=> [%Item{...}, ...]

Adding and removal of embeds can only be done via the Ecto.ChangesetAPI so Ecto can properly track the embed life-cycle:

# Order has no items
order = Repo.get!(Order, 42)
order.items
# => []

items  = [%Item{title: "Soap"}]

# Generate a changeset
changeset = Ecto.Changeset.change(order)

# Put a one or more new items
changeset = Ecto.Changeset.put_embed(changeset, :items, items)

# Update the order and fetch items
items = Repo.update!(changeset).items

# Items are generated with a unique identification
items
# => [%Item{id: "20a97d94-f79b-4e63-a875-85deed7719b7", title: "Soap"}]

Updating of embeds must be done using a changeset for each changed embed.

# Order has an existing items
order = Repo.get!(Order, 42)
order.items
# => [%Item{id: "20a97d94-f79b-4e63-a875-85deed7719b7", title: "Soap"}]

# Generate a changeset
changeset = Ecto.Changeset.change(order)

# Put the updated item as a changeset
current_item = List.first(order.items)
item_changeset = Ecto.Changeset.change(current_item, title: "Mujju's Soap")
order_changeset = Ecto.Changeset.put_embed(changeset, :items, [item_changeset])

# Update the order and fetch items
items = Repo.update!(order_changeset).items

# Item has the updated title
items
# => [%Item{id: "20a97d94-f79b-4e63-a875-85deed7719b7", title: "Mujju's Soap"}]

Inline embedded schema

The schema module can be defined inline in the parent schema in simple cases:

defmodule Parent do
  use Ecto.Schema

  schema "parents" do
    field :name, :string

    embeds_many :children, Child do
      field :name, :string
      field :age,  :integer
    end
  end
end

Primary keys are automatically set up for embedded schemas as well, defaulting to {:id, :binary_id, autogenerate: true}. You can customize it by passing a :primary_key option with the same arguments as @primary_key (see the Schema attributessection for more info).

Defining embedded schema in such a way will define a Parent.Child module with the appropriate struct. In order to properly cast the embedded schema. When casting the inline-defined embedded schemas you need to use the :withoption of cast_embed/3 to provide the proper function to do the casting. For example:

def changeset(schema, params) do
  schema
  |> cast(params, [:name])
  |> cast_embed(:children, with: &child_changeset/2)
end

defp child_changeset(schema, params) do
  schema
  |> cast(params, [:name, :age])
end

Indicates an embedding of many schemas.

For options and examples see documentation of embeds_many/3.

Indicates an embedding of a schema.

The current schema has zero or one records of the other schema embedded inside of it. It uses a field similar to the :map type for storage, but allows embeds to have all the things regular schema can.

You must declare your embeds_one/3 field with type :map at the database level.

The embedded may or may not have a primary key. Ecto uses the primary keys to detect if an embed is being updated or not. If a primary key is not present,:on_replace should be set to either :update or :delete if there is a desire to either update or delete the current embed when a new one is set.

Options

Examples

defmodule Order do
  use Ecto.Schema

  schema "orders" do
    embeds_one :item, Item
  end
end

defmodule Item do
  use Ecto.Schema

  embedded_schema do
    field :title
  end
end

# The item is loaded with the order
order = Repo.get!(Order, 42)
order.item #=> %Item{...}

Adding and removal of embeds can only be done via the Ecto.ChangesetAPI so Ecto can properly track the embed life-cycle:

order = Repo.get!(Order, 42)
item  = %Item{title: "Soap"}

# Generate a changeset
changeset = Ecto.Changeset.change(order)

# Put a new embed to the changeset
changeset = Ecto.Changeset.put_embed(changeset, :item, item)

# Update the order, and fetch the item
item = Repo.update!(changeset).item

# Item is generated with a unique identification
item
# => %Item{id: "20a97d94-f79b-4e63-a875-85deed7719b7", title: "Soap"}

Inline embedded schema

The schema module can be defined inline in the parent schema in simple cases:

defmodule Parent do
  use Ecto.Schema

  schema "parents" do
    field :name, :string

    embeds_one :child, Child do
      field :name, :string
      field :age,  :integer
    end
  end
end

Options should be passed before the do block like this:

embeds_one :child, Child, on_replace: :delete, primary_key: false do
  field :name, :string
  field :age,  :integer
end

Defining embedded schema in such a way will define a Parent.Child module with the appropriate struct. In order to properly cast the embedded schema. When casting the inline-defined embedded schemas you need to use the :withoption of Ecto.Changeset.cast_embed/3 to provide the proper function to do the casting. For example:

def changeset(schema, params) do
  schema
  |> cast(params, [:name])
  |> cast_embed(:child, with: &child_changeset/2)
end

defp child_changeset(schema, params) do
  schema
  |> cast(params, [:name, :age])
end

Encoding and decoding

Because many databases do not support direct encoding and decoding of embeds, it is often emulated by Ecto by using specific encoding and decoding rules.

For example, PostgreSQL will store embeds on top of JSONB columns, which means types in embedded schemas won't go through the usual dump->DB->load cycle but rather encode->DB->decode->cast. This means that, when using embedded schemas with databases like PG or MySQL, make sure all of your types can be JSON encoded/decoded correctly. Ecto provides this guarantee for all built-in types.

When decoding, if a key exists in the database not defined in the schema, it'll be ignored. If a field exists in the schema that's not in the database, it's value will be nil.

Indicates an embedding of a schema.

For options and examples see documentation of embeds_one/3.

Defines a field on the schema with given name and type.

The field name will be used as is to read and write to the database by all of the built-in adapters unless overridden with the :sourceoption.

Options

Indicates a one-to-many association with another schema.

The current schema has zero or more records of the other schema. The other schema often has a belongs_to field with the reverse association.

Options

Examples

defmodule Post do
  use Ecto.Schema
  schema "posts" do
    has_many :comments, Comment
  end
end

# Get all comments for a given post
post = Repo.get(Post, 42)
comments = Repo.all assoc(post, :comments)

# The comments can come preloaded on the post struct
[post] = Repo.all(from(p in Post, where: p.id == 42, preload: :comments))
post.comments #=> [%Comment{...}, ...]

If using EctoSQL, the foreign key should be defined in the comments table, as shown in belongs_to/3 examples. You may also see the Associations cheatsheetfor more examples.

has_many can be used to define hierarchical relationships within a single schema, for example threaded comments.

defmodule Comment do
  use Ecto.Schema
  schema "comments" do
    field :content, :string
    field :parent_id, :integer
    belongs_to :parent, Comment, foreign_key: :parent_id, references: :id, define_field: false
    has_many :children, Comment, foreign_key: :parent_id, references: :id
  end
end

Filtering associations

It is possible to specify a :where option that will filter the records returned by the association. Querying, joining or preloading the association will use the given conditions as shown next:

defmodule Post do
  use Ecto.Schema

  schema "posts" do
    has_many :public_comments, Comment,
      where: [public: true]
  end
end

The :where option expects a keyword list where the key is an atom representing the field and the value is either:

Note the values above are distinctly different from the values you would pass to where when building a query. For example, if you attempt to build a query such as

from Post, where: [id: nil]

it will emit an error. This is because queries can be built dynamically, and therefore passing nil can lead to security errors. However, the:where values for an association are given at compile-time, which is less dynamic and cannot leverage the full power of Ecto queries, which explains why they have different APIs.

Important! Please use this feature only when strictly necessary, otherwise it is very easy to end-up with large schemas with dozens of different associations polluting your schema and affecting your application performance. For instance, if you are using associations only for different querying purposes, then it is preferable to build and compose queries. For instance, instead of having two associations, one for comments and another for deleted comments, you might have a single comments association and filter it instead:

posts
|> Ecto.assoc(:comments)
|> Comment.deleted()

Or when preloading:

from posts, preload: [comments: ^Comment.deleted()]

has_many/has_one :through

Ecto also supports defining associations in terms of other associations via the :through option. Let's see an example:

defmodule Post do
  use Ecto.Schema

  schema "posts" do
    has_many :comments, Comment
    has_one :permalink, Permalink

    # In the has_many :through example below, the `:comments`
    # in the list [:comments, :author] refers to the
    # `has_many :comments` in the Post own schema and the
    # `:author` refers to the `belongs_to :author` of the
    # Comment's schema (the module below).
    # (see the description below for more details)
    has_many :comments_authors, through: [:comments, :author]

    # Specify the association with custom source
    has_many :tags, {"posts_tags", Tag}
  end
end

defmodule Comment do
  use Ecto.Schema

  schema "comments" do
    belongs_to :author, Author
    belongs_to :post, Post
    has_one :post_permalink, through: [:post, :permalink]
  end
end

In the example above, we have defined a has_many :through association named :comments_authors. A :through association always expects a list and the first element of the list must be a previously defined association in the current module. For example, :comments_authors first points to:comments in the same module (Post), which then points to :author in the next schema, Comment.

This :through association will return all authors for all comments that belongs to that post:

# Get all comments authors for a given post
post = Repo.get(Post, 42)
authors = Repo.all assoc(post, :comments_authors)

:through associations can also be preloaded. In such cases, not only the :through association is preloaded but all intermediate steps are preloaded too:

[post] = Repo.all(from(p in Post, where: p.id == 42, preload: :comments_authors))
post.comments_authors #=> [%Author{...}, ...]

# The comments for each post will be preloaded too
post.comments #=> [%Comment{...}, ...]

# And the author for each comment too
hd(post.comments).author #=> %Author{...}

When the :through association is expected to return one or zero items,has_one :through should be used instead, as in the example at the beginning of this section:

# How we defined the association above in Comments
has_one :post_permalink, through: [:post, :permalink]

# Get a preloaded comment
[comment] = Repo.all(Comment) |> Repo.preload(:post_permalink)
comment.post_permalink #=> %Permalink{...}

If possible, Ecto will avoid traversing intermediate associations in queries. For example, in the example above, Comment has a post_idcolumn (defined by belongs_to :post) and it is expected forPermalink to have the same. Therefore, when preloading the permalinks, Ecto may avoid traversing the "posts" table altogether. Of course, this assumes your database guarantees those references are valid, which can be done by defining foreign key constraints and references your database (often done via EctoSQL migrations).

Note :through associations are read-only. For example, you cannot useEcto.Changeset.cast_assoc/3 to modify through associations.

Indicates a one-to-one association with another schema.

The current schema has zero or one records of the other schema. The other schema often has a belongs_to field with the reverse association.

Options

Examples

defmodule Post do
  use Ecto.Schema

  schema "posts" do
    has_one :permalink, Permalink

    # Specify the association with custom source
    has_one :category, {"posts_categories", Category}
  end
end

# The permalink can come preloaded on the post struct
[post] = Repo.all(from(p in Post, where: p.id == 42, preload: :permalink))
post.permalink #=> %Permalink{...}

If using EctoSQL, a foreign key must be defined in the permalinks and categories tables, as shown in belongs_to/3examples. You may also see the Associations cheatsheetfor more examples.

Indicates a many-to-many association with another schema.

The association happens through a join schema or source, containing foreign keys to the associated schemas. For example, the association below:

# from MyApp.Post
many_to_many :tags, MyApp.Tag, join_through: "posts_tags"

is backed by relational databases through a join table as follows:

[Post] <-> [posts_tags] <-> [Tag]
  id   <--   post_id
              tag_id    -->  id

More information on the migration for creating such a schema is shown below.

Options

Using Ecto.assoc/2

One of the benefits of using many_to_many is that Ecto will avoid loading the intermediate whenever possible, making your queries more efficient. For this reason, developers should not refer to the join table of many_to_many in queries. The join table is accessible in few occasions, such as in Ecto.assoc/2. For example, if you do this:

post
|> Ecto.assoc(:tags)
|> where([t, _pt, p], p.public == t.public)

It may not work as expected because the posts_tags table may not be included in the query. You can address this problem in multiple ways. One option is to use ...:

post
|> Ecto.assoc(:tags)
|> where([t, ..., p], p.public == t.public)

Another and preferred option is to rewrite to an explicit join, which leaves out the intermediate bindings as they are resolved only later on:

# keyword syntax
from t in Tag,
  join: p in assoc(t, :post), on: p.id == ^post.id

# pipe syntax
Tag
|> join(:inner, [t], p in assoc(t, :post), on: p.id == ^post.id)

If you need to access the join table, then you likely want to usehas_many/3 with the :through option instead.

Removing data

If you attempt to remove associated many_to_many data, Ecto will always remove data from the join schema and never from the target associations be it by setting :on_replace to :delete, :on_deleteto :delete_all or by using changeset functions such asEcto.Changeset.put_assoc/3. For example, if a Post has a many to many relationship with Tag, setting :on_delete to :delete_all will only delete entries from the "posts_tags" table in case Post is deleted.

Migration

How your migration should be structured depends on the value you pass in :join_through. If :join_through is simply a string, representing a table, you may define a table without primary keys and you must not include any further columns, as those values won't be set by Ecto:

create table(:posts_tags, primary_key: false) do
  add :post_id, references(:posts, on_delete: :delete_all), null: false
  add :tag_id, references(:tags, on_delete: :delete_all), null: false
end

However, if your :join_through is a schema, like MyApp.PostTag, your join table may be structured as any other table in your codebase, including timestamps:

create table(:posts_tags) do
  add :post_id, references(:posts, on_delete: :delete_all), null: false
  add :tag_id, references(:tags, on_delete: :delete_all), null: false
  timestamps()
end

Because :join_through contains a schema, in such cases, autogenerated values and primary keys will be automatically handled by Ecto.

Preload Order

The :preload_order option may be used to return the preloaded structs in a deterministic order. It accepts either a compile-time keyword list/list or an MFA tuple, such as {Mod, fun, []}. The MFA tuple will be used to generate the order_by expression at runtime.

When specifying a compile-time keyword list/list, the ordering applies to the association's table and not the join table. Ordering by the join table can be achieved by specifying an MFA tuple that utilizes Ecto.Query.dynamic/2.

For example, say we have an association Assoc being joined through the tablejoin_through. The default preload query generated by Ecto is roughly:

from a in Assoc, join: jt in "join_through", on: ...

If :preload_order is given as [asc: :field] then the preload query will be changed to the following:

from a in Assoc, join: jt in "join_through", on: ..., order_by: [asc: a.field]

Similarly, any compile-time keyword list/list will have its fields interpreted as belonging to the association's table. To order by a field from the join table, an MFA tuple can be specified that utilizes Ecto.Query.dynamic/2.

For example, if :preload_order is given as {Mod, fun, []}, corresponding to the following function:

defmodule Mod do
  def fun() do
    [desc: dynamic([assoc, join], join.field)]
  end
end

then the preload query will be changed to the following:

from a in Assoc, join: jt in "join_through", on: ..., order_by: [desc: jt.field]

Note the ordering of the bindings. The join table always comes last.

Examples

defmodule Post do
  use Ecto.Schema
  schema "posts" do
    many_to_many :tags, Tag, join_through: "posts_tags"
  end
end

# Let's create a post and a tag
post = Repo.insert!(%Post{})
tag = Repo.insert!(%Tag{name: "introduction"})

# We can associate at any time post and tags together using changesets
post
|> Repo.preload(:tags) # Load existing data
|> Ecto.Changeset.change() # Build the changeset
|> Ecto.Changeset.put_assoc(:tags, [tag]) # Set the association
|> Repo.update!

# In a later moment, we may get all tags for a given post
post = Repo.get(Post, 42)
tags = Repo.all(assoc(post, :tags))

# The tags may also be preloaded on the post struct for reading
[post] = Repo.all(from(p in Post, where: p.id == 42, preload: :tags))
post.tags #=> [%Tag{...}, ...]

Join Schema Example

You may prefer to use a join schema to handle many_to_many associations. The decoupled nature of Ecto allows us to create a "join" struct whichbelongs_to both sides of the many to many association.

In our example, a User has and belongs to many Organizations:

defmodule MyApp.Repo.Migrations.CreateUserOrganization do
  use Ecto.Migration

  def change do
    create table(:users_organizations) do
      add :user_id, references(:users)
      add :organization_id, references(:organizations)

      timestamps()
    end
  end
end

defmodule UserOrganization do
  use Ecto.Schema

  @primary_key false
  schema "users_organizations" do
    belongs_to :user, User
    belongs_to :organization, Organization
    timestamps() # Added bonus, a join schema will also allow you to set timestamps
  end

  def changeset(struct, params \\ %{}) do
    struct
    |> Ecto.Changeset.cast(params, [:user_id, :organization_id])
    |> Ecto.Changeset.validate_required([:user_id, :organization_id])
    # Maybe do some counter caching here!
  end
end

defmodule User do
  use Ecto.Schema

  schema "users" do
    many_to_many :organizations, Organization, join_through: UserOrganization
  end
end

defmodule Organization do
  use Ecto.Schema

  schema "organizations" do
    many_to_many :users, User, join_through: UserOrganization
  end
end

To create the association, pass in the IDs of an existing User andOrganization to UserOrganization.changeset/2:

changeset = UserOrganization.changeset(%UserOrganization{}, %{user_id: id, organization_id: id})

case Repo.insert(changeset) do
  {:ok, assoc} -> # Assoc was created!
  {:error, changeset} -> # Handle the error
end

Defines a schema struct with a source name and field definitions.

An additional field called __meta__ is added to the struct for storing internal Ecto state. This field always has a Ecto.Schema.Metadata struct as value and can be manipulated with the Ecto.put_meta/2 function.

Generates :inserted_at and :updated_at timestamp fields.

The fields generated by this macro will automatically be set to the current time when inserting and updating values in a repository.

Options

All options can be pre-configured by setting @timestamps_opts.