Ecto.Schema

Defines an association.

This macro is used by belongs_to/3, has_one/3 and has_many/3 to define associations. However, custom association mechanisms can be provided by developers and hooked in via this macro.

Read more about custom associations in Ecto.Association.

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

The current model belongs to zero or one records of the other model. The other model 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 model 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 be set per schema via @foreign_key_type

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.Model
  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 model and you wish to use it for commenting on tasks and posts.

Because Ecto does not tie a model to a given table, we can achieve this by specifying the table on the association definition. Let’s start over and define a new Comment model:

defmodule Comment do
  use Ecto.Model
  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: comment”. You can choose whatever name you want, the point here is that this particular table will never exist.

Now in your Post and Task models:

defmodule Post do
  use Ecto.Model
  schema "posts" do
    has_many :comments, {"posts_comments", Comment}, foreign_key: :assoc_id
  end
end

defmodule Task do
  use Ecto.Model
  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, ensuring we keep databases 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

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

where build/2 is defined in Ecto.Model. You can also use assoc/2 in both Ecto.Model 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))

Finally, if for some reason you wish to query one of comments table directly, you can also specify the tuple source in the query syntax:

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

Defines a field on the model 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.local or Ecto.UUID.generate as they would then be the same for all records

• :autogenerate - Annotates the field to be autogenerated before insertion if not value is set.

• :read_after_writes - When true, the field only sent on insert if not nil and always read back from the repository during inserts and updates.

  For relational databases, this means the RETURNING option of those statements are used. For this reason, MySQL does not support this option and will raise an error if a model 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 model.

The current model has zero or more records of the other model. The other model 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 model, defaults to the underscored name of the current model suffixed by _id

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

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

Examples

defmodule Post do
  use Ecto.Model
  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.Model
  schema "posts" do
    has_many :comments, Comment
    has_one :permalink, Permalink
    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.Model
  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 expect 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 model Comment.

This :through associations 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 to avoid repetition allowing the developer to easily retrieve data that is often seem 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).authors #=> [%Author{...}, ...]

Finally, :through can be used with multiple associations (not only 2) and with associations of any kind, including belongs_to and others :through associations. When the :through association is expected to return one or no item, 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 model.

The current model has zero or one records of the other model. The other model 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 model, defaults to the underscored name of the current model suffixed by _id

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

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

Examples

defmodule Post do
  use Ecto.Model
  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{...}

Defines a schema with a source name and field definitions.

Generates :inserted_at and :updated_at timestamp fields.

When using Ecto.Model, 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

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