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 <-> manager relationship. If the company contains the manager_id in the underlying database table, we say the company belongs to manager.

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

Options

• :foreign_key - Sets the foreign key field name, defaults to the name of the association suffixed by _id. For example, belongs_to :company will define foreign key of :company_id

• :references - Sets the key on the other schema to be used for the association, defaults to: :id

• :define_field - When false, does not automatically define a :foreign_key field, implying the user is defining the field manually elsewhere

• :type - Sets the type of automatically defined :foreign_key. Defaults to: :id and can be set per schema via @foreign_key_type

• :on_replace - The action taken on associations when the record is replaced when casting or manipulating parent changeset. May be :raise (default), :mark_as_invalid, :nilify, or :delete. See Ecto.Changeset’s section on related data for more info.

• :defaults - Default values to use when building the association

All other options are forwarded to the underlying foreign key definition and therefore accept the same options as field/3.

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{...}

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))

where build_assoc/3 is defined in Ecto. You can also use assoc/2 in both Ecto and in the query syntax to easily retrieve associated comments to a given post or task:

# Fetch all comments associated to the given task
Repo.all(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 a 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.

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.

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 {:array, :map} and default value of [] at the database level. In fact, Ecto will automatically translate nil values from the database into empty lists for embeds many (this behaviour is specific to embeds_many/3 fields in order to mimic has_many/3.

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

Options

• :on_replace - The action taken on associations when the embed is replaced when casting or manipulating parent changeset. May be :raise (default), :mark_as_invalid, or :delete. See Ecto.Changeset’s section on related data for more info.

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 :name
  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.Changeset API so Ecto can properly track the embed life-cycle:

order = Repo.get!(Order, 42)

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

# Change, put a new one or remove all items
changeset = Ecto.Changeset.put_change(changeset, :items, [])

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

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.

Options

• :on_replace - The action taken on associations when the embed is replaced when casting or manipulating parent changeset. May be :raise (default), :mark_as_invalid, or :delete. See Ecto.Changeset’s section on related data for more info.

Examples

defmodule Order do
  use Ecto.Schema

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

defmodule Item do
  use Ecto.Schema

  # A required field for all embedded documents
  @primary_key {:id, :binary_id, autogenerate: true}
  schema "" do
    field :name
  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.Changeset API so Ecto can properly track the embed life-cycle:

order = Repo.get!(Order, 42)

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

# Change, put a new one or remove an item
changeset = Ecto.Changeset.put_change(changeset, :item, nil)

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

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.

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

Options

• :default - Sets the default value on the schema and the struct. The default value is calculated at compilation time, so don’t use expressions like Ecto.DateTime.utc or Ecto.UUID.generate as they would then be the same for all records

• :autogenerate - Annotates the field to be autogenerated before insertion if value is not set. It will call the autogenerate/0 function in the field’s type.

• :read_after_writes - When true, the field is always read back from the database after insert and updates.

  For relational databases, this means the RETURNING option of those statements is used. For this reason, MySQL does not support this option and will raise an error if a schema is inserted/updated with read after writes fields.

• :virtual - When true, the field is not persisted to the database. Notice virtual fields do not support :autogenerate nor :read_after_writes.

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

• :foreign_key - Sets the foreign key, this should map to a field on the other schema, defaults to the underscored name of the current schema suffixed by _id

• :references - Sets the key on the current schema to be used for the association, defaults to the primary key on the schema

• :through - If this association must be defined in terms of existing associations. Read below for more information

• :on_delete - The action taken on associations when parent record is deleted. May be :nothing (default), :nilify_all and :delete_all. Notice :on_delete may also be set in migrations when creating a reference. If supported, relying on the database via migrations is preferred. :nilify_all and :delete_all will not cascade to child records unless set via database migrations.

• :on_replace - The action taken on associations when the record is replaced when casting or manipulating parent changeset. May be :raise (default), :mark_as_invalid, :nilify, or :delete. See Ecto.Changeset’s section on related data for more info.

• :defaults - Default values to use when building the association

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{...}, ...]

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

:through associations are read-only as they are useful for avoiding repetition, allowing the developer to easily retrieve data that is often seen together but stored across different tables.

: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{...}

Finally, :through can be used with multiple associations (not only 2) and with associations of any kind, including belongs_to and other :through associations. 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
has_one :post_permalink, through: [:post, :permalink]

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

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

• :foreign_key - Sets the foreign key, this should map to a field on the other schema, defaults to the underscored name of the current schema suffixed by _id

• :references - Sets the key on the current schema to be used for the association, defaults to the primary key on the schema

• :through - If this association must be defined in terms of existing associations. Read the section in has_many/3 for more information

• :on_delete - The action taken on associations when parent record is deleted. May be :nothing (default), :nilify_all and :delete_all. Notice :on_delete may also be set in migrations when creating a reference. If supported, relying on the database via migrations is preferred. :nilify_all and :delete_all will not cascade to child records unless set via database migrations.

• :on_replace - The action taken on associations when the record is replaced when casting or manipulating parent changeset. May be :raise (default), :mark_as_invalid, :nilify, or :delete. See Ecto.Changeset’s section on related data for more info.

• :defaults - Default values to use when building the association

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{...}

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

• :join_through - specifies the source of the associated data. It may be a string, like “posts_tags”, representing the underlying storage table or an atom, like MyApp.PostTag, representing a schema. This option is required.

• :join_keys - specifies how the schemas are associated. It expects a keyword list with two entries, the first being how the join table should reach the current schema and the second how the join table should reach the associated schema. In the example above, it defaults to: [post_id: :id, tag_id: :id]. The keys are inflected from the schema names.

• :on_delete - The action taken on associations when the parent record is deleted. May be :nothing (default) or :delete_all. :delete_all will only remove data from the join source, never the associated records. Notice :on_delete may also be set in migrations when creating a reference. If supported, relying on the database via migrations is preferred. :nilify_all and :delete_all will not cascade to child records unless set via database migrations.

• :on_replace - The action taken on associations when the record is replaced when casting or manipulating parent changeset. May be :raise (default), :mark_as_invalid, or :delete. :delete will only remove data from the join source, never the associated records. See Ecto.Changeset’s section on related data for more info.

• :defaults - Default values to use when building the association

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_delete to :delete_all or by using changeset functions such as Ecto.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)
  add :tag_id, references(:tags)
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)
  add :tag_id, references(:tags)
  timestamps
end

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

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 which belongs_to both sides of the many to many association.

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

defmodule UserOrganization do
  use Ecto.Schema

  @primary_key false
  schema "user_organisation" 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
    |> cast(params, [:user_id, :organization_id])
    |> 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, join_through: UserOrganization
  end
end

defmodule Organization do
  use Ecto.Schema

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

# Then to create the association, pass in the ID's of an existing
# User and Organization to UserOrganization.changeset
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 with a source name and field definitions.

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

• :type - the timestamps type, defaults to Ecto.DateTime.
• :usec - boolean, sets whether microseconds are used in timestamps. Microseconds will be 0 if false. Defaults to false.
• :inserted_at - the name of the column for insertion times or false
• :updated_at - the name of the column for update times or false
• :autogenerate - a module-function-args tuple used for generating both inserted_at and updated_at timestamps

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