# Relationships

Relationships describe connections between resources and are a core component of Ash. Define relationships in the `relationships` block of a resource.

## Best Practices for Relationships

- Be descriptive with relationship names (e.g., use `:authored_posts` instead of just `:posts`)
- Configure foreign key constraints in your data layer if they have them (see `references` in AshPostgres)
- Always choose the appropriate relationship type based on your domain model

### Relationship Types

- For Polymorphic relationships, you can model them using `Ash.Type.Union`; see the “Polymorphic Relationships” guide for more information.

```elixir
relationships do
  # belongs_to - adds foreign key to source resource
  belongs_to :owner, MyApp.User do
    allow_nil? false
    attribute_type :integer  # defaults to :uuid
  end

  # has_one - foreign key on destination resource
  has_one :profile, MyApp.Profile

  # has_many - foreign key on destination resource, returns list
  has_many :posts, MyApp.Post do
    filter expr(published == true)
    sort published_at: :desc
  end

  # many_to_many - requires join resource
  many_to_many :tags, MyApp.Tag do
    through MyApp.PostTag
    source_attribute_on_join_resource :post_id
    destination_attribute_on_join_resource :tag_id
  end
end
```

The join resource must be defined separately:

```elixir
defmodule MyApp.PostTag do
  use Ash.Resource,
    data_layer: AshPostgres.DataLayer

  attributes do
    uuid_primary_key :id
    # Add additional attributes if you need metadata on the relationship
    attribute :added_at, :utc_datetime_usec do
      default &DateTime.utc_now/0
    end
  end

  relationships do
    belongs_to :post, MyApp.Post, primary_key?: true, allow_nil?: false
    belongs_to :tag, MyApp.Tag, primary_key?: true, allow_nil?: false
  end

  actions do
    defaults [:read, :destroy, create: :*, update: :*]
  end
end
```

## Loading Relationships

```elixir
# Using code interface options (preferred)
post = MyDomain.get_post!(id, load: [:author, comments: [:author]])

# Complex loading with filters
posts = MyDomain.list_posts!(
  query: [load: [comments: [filter: [is_approved: true], limit: 5]]]
)

# Manual query building (for complex cases)
MyApp.Post
|> Ash.Query.load(comments: MyApp.Comment |> Ash.Query.filter(is_approved == true))
|> Ash.read!()

# Loading on existing records
Ash.load!(post, :author)
```

Prefer to use the `strict?` option when loading to only load necessary fields on related data.

```elixir
MyApp.Post
|> Ash.Query.load([comments: [:title]], strict?: true)
```

## Managing Relationships

There are two primary ways to manage relationships in Ash:

### 1. Using `change manage_relationship/2-3` in Actions
Use this when input comes from action arguments:

```elixir
actions do
  update :update do
    # Define argument for the related data
    argument :comments, {:array, :map} do
      allow_nil? false
    end

    argument :new_tags, {:array, :map}

    # Link argument to relationship management
    change manage_relationship(:comments, type: :append)

    # For different argument and relationship names
    change manage_relationship(:new_tags, :tags, type: :append)
  end
end
```

### 2. Using `Ash.Changeset.manage_relationship/3-4` in Custom Changes
Use this when building values programmatically:

```elixir
defmodule MyApp.Changes.AssignTeamMembers do
  use Ash.Resource.Change

  def change(changeset, _opts, context) do
    members = determine_team_members(changeset, context.actor)

    Ash.Changeset.manage_relationship(
      changeset,
      :members,
      members,
      type: :append_and_remove
    )
  end
end
```

### Quick Reference - Management Types
- `:append` - Add new related records, ignore existing
- `:append_and_remove` - Add new related records, remove missing
- `:remove` - Remove specified related records
- `:direct_control` - Full CRUD control (create/update/destroy)
- `:create` - Only create new records

### Quick Reference - Common Options
- `on_lookup: :relate` - Look up and relate existing records
- `on_no_match: :create` - Create if no match found
- `on_match: :update` - Update existing matches
- `on_missing: :destroy` - Delete records not in input
- `value_is_key: :name` - Use field as key for simple values

For comprehensive documentation, see the [Managing Relationships](https://hexdocs.pm/ash/relationships.html#managing-relationships) section.

### Examples

Creating a post with tags:
```elixir
MyDomain.create_post!(%{
  title: "New Post",
  body: "Content here...",
  tags: [%{name: "elixir"}, %{name: "ash"}]  # Creates new tags
})

# Updating a post to replace its tags
MyDomain.update_post!(post, %{
  tags: [tag1.id, tag2.id]  # Replaces tags with existing ones by ID
})
```