# Intra-data source Relationships

{% hint style="success" %}
This is the official documentation of the `agent_ruby` Ruby agent.
{% endhint %}

We've seen that when building data sources, collections must fulfill the contract they defined [when declaring their capabilities](https://docs.forestadmin.com/developer-guide-agents-ruby/data-sources/custom/translation/capabilities).

The same is also true for [Intra-data source relationships](https://docs.forestadmin.com/developer-guide-agents-ruby/data-sources/custom/structure#relationships) which are declared on the structure of a collection: declared relations must be handled when processing `filters`, `projection`, and `aggregations`.

## In practice

`one-to-many` and `many-to-many` relationships do not require additional work: Forest Admin will automatically call the destination collection with a valid filter.

On the other hand, `many-to-one` and `one-to-one` relationships require the implementer to make all fields from the target collection available on the source collection (under a prefix).

### Example

If a structure declaration contains the following statement

```ruby
module App
  module Collections
    class Movie < ForestAdminDatasourceToolkit::Collection
      include ForestAdminDatasourceToolkit::Schema
      include ForestAdminDatasourceToolkit::Components::Query:: ConditionTree

      def initialize(datasource)
        super(datasource, 'Movie')

        # [...]

        add_field('director', ManyToOneSchema.new(
          foreign_key: 'director_id',
          foreign_key_target: 'id',
          foreign_collection: 'People'
        ))
      end
    end
  end
end

```

Then the collection must accept references to fields from the `people` collection under the `director` prefix in all method parameters.

```ruby
datasource.collection('Movie').list(
  caller,
  ForestAdminDatasourceToolkit::Components::Query::Filter.new(
    condition_tree: ForestAdminDatasourceToolkit::Components::Query::ConditionTree::ConditionTreeBranch.new(
      aggregator: 'and',
      conditions: [
        ForestAdminDatasourceToolkit::Components::Query::ConditionTree::ConditionTreeLeaf.new(
          field: 'title',
          operator: 'Equal',
          value: 'E.T.'
        ),
        ForestAdminDatasourceToolkit::Components::Query::ConditionTree::ConditionTreeLeaf.new(
          field: 'director:firstName',
          operator: 'Equal',
          value: 'Steven'
        ),
      ],
    ),
    sort: ForestAdminDatasourceToolkit::Components::Query::Sort.new(
      [
        {
            field: 'director:birthDate',
            ascending: true
        }
      ]
    )
  )
)
```

should return

```json
{
  "id": 34,
  "title": "E.T",
  "director": { "firstName": "Steven", "lastName": "Spielberg" }
}
```