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

• :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: :integer 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, :update, or :delete. See Ecto.Changeset’s section on related data for more info.

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

• :primary_key - if the underlying belongs_to field is a primary key

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

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 with 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.

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).

The embedded may or may not have a primary key. Ecto use the primary keys to detect if an embed is being updated or not. If a primary is not present and you still want the list of embeds to be updated, :on_replace must 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 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 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"}]

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

When defining an inline embed, the :primary_key option may be given to customize the embed primary key type.

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 :with option 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 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 use the primary keys to detect if an embed is being updated or not. If a primary 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

• :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, :update, 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)
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

When defining an inline embed, the :primary_key option may be given to customize the embed primary key type.

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 :with option 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(: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.

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 - Allow this association to be defined in terms of existing associations. Read the section on :through associations for more info

• :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, :update, 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)

Although we used the :through association in the example above, Ecto also allows developers to dynamically build the through associations using the Ecto.assoc/2 function:

assoc(post, [:comments, :author])

In fact, given :through associations are read-only, using the Ecto.assoc/2 format is the preferred mechanism for working with through associations. Use the schema-based one only if you need to store the through data alongside of the parent struct, in specific cases such as preloading.

: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
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

• :unique - When true, checks if the associated entries are unique. This is done by checking the primary key of the associated entries during repository operations. Keep in mind this does not guarantee uniqueness at the database level. For such it is preferred to set a unique index in the database. For example: create unique_index(:posts_tags, [:post_id, :tag_id])

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 "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

# 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

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 :naive_datetime.
• :usec - sets whether microseconds are used in timestamps. Microseconds will be 0 if false. Defaults to true.
• :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.