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documentation/topics/relationships.md
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# Relationships
Relationships are a core component of Ash. They provide a mechanism to describe the relationships between your resources, and through those relationships you can do things like
Relationships describe the connections between resources and are a core component of Ash. Defining relationships enables you to do things like
- Loading related data
- Filtering on related data
- Managing related records through changes on a single resource
- Authorizing based on the state of related data
## Customizing default belongs_to attribute type
## Relationships Basics
By default, we assume foreign keys that we add by default (for `belongs_to` relationships) should be `:uuid`. To change this default, set the following configuration:
A relationship exists between a source resource and a destination resource. These are defined in the `relationships` block of the source resource. For example, if `MyApp.Tweet` is the source resource, and `MyApp.User` is the destination resource, we could define a relationship called `:owner` like this:
```elixir
config :ash, :default_belongs_to_type, :integer
```
defmodule MyApp.Tweet do
use Ash.Resource,
data_layer: my_data_layer
## Loading related data
attributes do
uuid_primary_key :id
attribute :body, :string
end
Loading relationships is a very common use case. There are two ways to load relationships, in the query, and on records.
### On records
Given a set of records, like `[user1, user2]`, you can load their relationships by calling your Ash Api's `load` function.
```elixir
YourApi.load(users, :friends)
```
This will fetch the friends of each user, and set them in the corresponding `friends` key.
### In the query
Loading in the query is currently pretty much the same as loading on records, but eventually data layers will be able to optimize these loads, potentially including them as joins in the main query, for example. The following will return the list of users with their friends loaded, as the above example.
```elixir
User
|> Ash.Query.load(:friends)
|> YourApi.read()
```
### More complex data loading
Multiple relationships can be loaded at once, i.e
```elixir
YourApi.load(users, [:friends, :enemies])
```
Nested relationships can be loaded:
```elixir
YourApi.load(users, friends: [:friends, :enemies])
```
The queries used for loading can be customized by providing a query as the value.
```elixir
friends = Ash.Query.sort(User, social_score: :asc)
YourApi.load(users, friends: friends)
```
Nested loads will be included in the parent load.
```elixir
friends =
User
|> Ash.Query.sort(social_score: :asc)
|> Ash.Query.load(:friends)
# Will load friends and friends of those friends
YourApi.load(users, friends: friends)
relationships do
belongs_to :owner, MyApp.User
end
end
```
## Managing related data
See [Managing Relationships](/documentation/topics/managing-relationships.md) for more information.
## Relationships Basics
All relationships have a `source` and a `destination`, as well as a corresponding `source_attribute` and `destination_attribute`. Many to many relationships have additional fields which are discussed below. Relationships will validate at compile time that their configured attributes exist. You don't need to have a corresponding "reverse" relationship for every relationship, i.e if you have a `MyApp.Tweets` resource with `belongs_to :user, User` you aren't required to have a `has_many :tweets, MyApp.Tweet`. All that is required is that the attributes used by the relationship exist.
## Kinds of relationships
There are four kinds of relationships:
- [`belongs_to`](#belongs-to)
- [`has_one`](#has-one)
- [`has_many`](#has-many)
- [`many_to_many`](#many-to-many)
Each of these relationships has a `source` resource and a `destination` resource with a corresponding attribute on the source resource (`source_attribute`), and destination resource (`destination_attribute`). Relationships will validate that their configured attributes exist at compile time.
You don't need to have a corresponding "reverse" relationship for every relationship, i.e if you have a `MyApp.Tweet` resource with `belongs_to :user, MyApp.User` you aren't required to have a `has_many :tweets, MyApp.Tweet` on `MyApp.User`. All that is required is that the attributes used by the relationship exist.
### Belongs To
```elixir
# on MyApp.Tweet
belongs_to :owner, MyApp.User
```
A `belongs_to` relationship means that there is an attribute (`source_attribute`) on the source resource that uniquely identifies a record with a matching `destination_attribute` in the destination. In the example above, the source attribute would be `owner_id`, and if you wanted to change the owner, you'd modify the `owner_id` to point to a different `MyApp.User`
A `belongs_to` relationship means that there is an attribute (`source_attribute`) on the source resource that uniquely identifies a record with a matching attribute (`destination_attribute`) in the destination. In the example above, the source attribute on `MyApp.Tweet` is `:owner_id` and the destination attribute on `MyApp.User` is `:id`.
#### Belongs to Source Attribute
#### Attribute Defaults
The `destination_attribute` defaults to `:id`.
By default, a belongs_to relationship will define an attribute called `<relationship_name>_id` of type `:uuid` on the resource. To configure this, use options like:
By default, the `source_attribute` is defined as `:<relationship_name>_id` of the type `:uuid` on the source resource and the `destination_attribute` is assumed to be `:id`. You can override the attribute names by specifying the `source_attribute` and `destination_attribute` options like so:
```elixir
belongs_to :owner, MyApp.User do
# defaults to :<relationship_name>_id (i.e. :owner_id)
source_attribute :custom_attribute_name
# defaults to :id
destination_attribute :custom_attribute_name
end
```
You can further customize the `source_attribute` using options such as:
- `d:Ash.Resource.Dsl.relationships.belongs_to|define_attribute?` to define it yourself
- `d:Ash.Resource.Dsl.relationships.belongs_to|attribute_type` to modify the default type
@ -126,6 +98,14 @@ relationships do
end
```
#### Customizing default belongs_to attribute type
Destination attributes that are added by default are assumed to be `:uuid`. To change this, set the following configuration in `config.exs`:
```elixir
config :ash, :default_belongs_to_type, :integer
```
See the docs for more: `d:Ash.Resource.Dsl.relationships.belongs_to`
### Has One
@ -135,68 +115,159 @@ See the docs for more: `d:Ash.Resource.Dsl.relationships.belongs_to`
has_one :profile, MyApp.Profile
```
A `has_one` is similar to a `belongs_to` except the "reference" attribute is on
the destination resource, instead of the source. In the example above, we'd expect a `profile_id` to be on `MyApp.Profile`, and that it is unique.
A `has_one` relationship means that there is a unique attribute (`destination_attribute`) on the destination resource that identifies a record with a matching unique attribute (`source_resource`) in the source. In the example above, the source attribute on `MyApp.User` is `:id` and the destination attribute on `MyApp.Profile` is `:user_id`.
#### Has One Attribute Defaults
A `has_one` is similar to a `belongs_to` except the reference attribute is on
the destination resource, instead of the source.
By default, the `source_attribute` is assumed to be `:id`, and `destination_attribute` defaults to `<snake_cased_last_part_of_module_name>_id`. In the above example, it would default `destination_attribute` to `user_id`.
#### Attribute Defaults
By default, the `source_attribute` is assumed to be `:id`, and `destination_attribute` defaults to `<snake_cased_last_part_of_module_name>_id`.
See the docs for more: `d:Ash.Resource.Dsl.relationships.has_one`
### Has Many
```elixir
# on MyApp.Post
has_many :comments, Comment
# on MyApp.User
has_many :tweets, MyApp.Tweet
```
A `has_many` relationship is similar to a `has_one` in that the reference attribute is on the destination resource. The only difference between this and `has_one` is that it does not expect the destination attribute is unique on the destination, and therefore will produce a list of related items.
A `has_many` relationship means that there is a non-unique attribute (`destination_attribute`) on the destination resource that identifies a record with a matching unique attribute (`source_resource`) in the source. In the example above, the source attribute on `MyApp.User` is `:id` and the destination attribute on `MyApp.Tweet` is `:user_id`.
#### Has Many Attribute Defaults
A `has_many` relationship is similar to a `has_one` because the reference attribute exists on the destination resource. The only difference between this and `has_one` is that the destination attribute is not unique, and therefore will produce a list of related items. In the example above, `:tweets` corresponds to a list of `MyApp.Tweet` records.
By default, the `source_attribute` is assumed to be `:id`, and `destination_attribute` defaults to `<snake_cased_last_part_of_module_name>_id`. In the above example, it would default `destination_attribute` to `post_id`.
#### Attribute Defaults
By default, the `source_attribute` is assumed to be `:id`, and `destination_attribute` defaults to `<snake_cased_last_part_of_module_name>_id`.
See the docs for more: `d:Ash.Resource.Dsl.relationships.has_many`
## Many To Many Relationships
Lets say that individual todo items in our app can be added to multiple lists, and every list has multiple todo items. This is a great case for `many_to_many` relationships.
### Many To Many
For example, we could define the following `many_to_many` relationship:
A `many_to_many` relationship can be used to relate many source resources to many destination resources. To achieve this, the `source_attribute` and `destination_attribute` are defined on a join resource. A `many_to_many` relationship can be thought of as a combination of a `has_many` relationship on the source/destination resources and a `belongs_to` relationship on the join resource.
For example, consider two resources `MyApp.Tweet` and `MyApp.Hashtag` representing tweets and hashtags. We want to be able to associate a tweet with many hashtags, and a hashtag with many tweets. To do this, we could define the following `many_to_many` relationship:
```elixir
# on MyApp.TodoList
many_to_many :todo_items, MyApp.TodoItem do
through MyApp.TodoListItem
source_attribute_on_join_resource :list_id
destination_attribute_on_join_resource :item_id
# on MyApp.Tweet
many_to_many :hashtags, MyApp.Tweet do
through MyApp.TweetHashtag
source_attribute_on_join_resource :tweet_id
destination_attribute_on_join_resource :hashtag_id
end
```
And then we could define the "join resource" to connect everything: `MyApp.TodoListItem`
The `through` option specifies the "join" resource that will be used to store the relationship. We need to define this resource as well:
```elixir
defmodule MyApp.TodoListItem do
defmodule MyApp.TweetHashtag do
use Ash.Resource,
data_layer: your_data_layer
attributes do
uuid_primary_key :id
end
relationships do
belongs_to :todo_list, MyApp.TodoList do
allow_nil? false
end
belongs_to :item, MyApp.TodoItem do
allow_nil? false
end
belongs_to :tweet, MyApp.Tweet, primary_key?: true, allow_nil?: false
belongs_to :hashtag, MyApp.Hashtag, primary_key?: true, allow_nil?: false
end
end
```
It is convention to name this resource `<source_resource_name><destination_resource_name>` however this is not required. The attributes on the join resource must match the `source_attribute_on_join_resource` and `destination_attribute_on_join_resource` options on the `many_to_many` relationship. The relationships on the join resource are standard `belongs_to` relationships, and can be configured as such. In this case, we have specified that the `:tweet_id` and `:hashtag_id` attributes form the primary key for the join resource, and that they cannot be `nil`.
Now that we have a resource with the proper attributes, Ash will use this automatically under the hood when
performing the relationship operations detailed above, like filtering and loading.
performing relationship operations like filtering and loading.
See the docs for more: `d:Ash.Resource.Dsl.relationships.many_to_many`
### Relationships across APIs
You will need to specify the `api` option in the relationship if the destination resource is part of a different API:
```elixir
many_to_many :authors, MyApp.OtherApi.Resource do
api MyApp.OtherApi
...
end
```
## Loading related data
There are two ways to load relationships:
- in the query using `c:Ash.Query.load/2`
- directly on records using `c:Ash.Api.load/3`
### On records
Given a single record or a set of records, it is possible to load their relationships by calling the `load` function on the record's parent API. For example:
```elixir
# user = %User{...}
YourApi.load(user, :tweets)
# users = [%User{...}, %User{...}, ....]
YourApi.load(users, :tweets)
```
This will fetch the tweets for each user, and set them in the corresponding `tweets` key.
```elixir
%User{
...
tweets: [
%Tweet{...},
%Tweet{...},
...
]
}
```
See `c:Ash.Api.load/3` for more information.
### In the query
The following will return a list of users with their tweets loaded identically to the previous example:
```elixir
User
|> Ash.Query.load(:tweets)
|> YourApi.read()
```
At present, loading relationships in the query is fundamentally the same as loading on records. Eventually, data layers will be able to optimize these loads (potentially including them as joins in the main query).
See `c:Ash.Query.load/2` for more information.
### More complex data loading
Multiple relationships can be loaded at once, i.e
```elixir
YourApi.load(users, [:tweets, :followers])
```
Nested relationships can be loaded:
```elixir
YourApi.load(users, followers: [:tweets, :followers])
```
The queries used for loading can be customized by providing a query as the value.
```elixir
followers = Ash.Query.sort(User, follower_count: :asc)
YourApi.load(users, followers: followers)
```
Nested loads will be included in the parent load.
```elixir
followers =
User
|> Ash.Query.sort(follower_count: :asc)
|> Ash.Query.load(:followers)
# Will load followers and followers of those followers
YourApi.load(users, followers: followers)
```