# Absinthe.Federation

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[Apollo Federation](https://www.apollographql.com/docs/federation/federation-spec/) support for [Absinthe](https://github.com/absinthe-graphql/absinthe)

## Installation

Install from [Hex.pm](https://hex.pm/packages/absinthe_federation):

```elixir
def deps do
  [
    {:absinthe_federation, "~> 0.2.53"}
  ]
end
```

Install from github:

```elixir
def deps do
  [
    {:absinthe_federation, github: "DivvyPayHQ/absinthe_federation", branch: "main"}
  ]
end
```

Add the following line to your absinthe schema

```elixir
defmodule MyApp.MySchema do
  use Absinthe.Schema
+ use Absinthe.Federation.Schema

  query do
    ...
  end
end
```

## Usage

### Macro based schemas (recommended)

```elixir
defmodule MyApp.MySchema do
  use Absinthe.Schema
+ use Absinthe.Federation.Schema

  query do
+   extends()

    field :review, :review do
      arg(:id, non_null(:id))
      resolve(&ReviewResolver.get_review_by_id/3)
    end
    ...
  end

  object :product do
+   key_fields("upc")
+   extends()

    field :upc, non_null(:string) do
+     external()
    end

    field(:reviews, list_of(:review)) do
      resolve(&ReviewResolver.get_reviews_for_product/3)
    end

+   field(:_resolve_reference, :product) do
+     resolve(&ProductResolver.get_product_by_upc/3)
+   end
  end
end
```

### SDL based schemas (experimental)

```elixir
defmodule MyApp.MySchema do
  use Absinthe.Schema
+ use Absinthe.Federation.Schema

  import_sdl """
    extend type Query {
      review(id: ID!): Review
    }

    extend type Product @key(fields: "upc") {
      upc: String! @external
      reviews: [Review]
    }
  """

  def hydrate(_, _) do
    ...
  end
```

### Resolving structs in \_entities queries

If you need to resolve your struct to a specific type in your schema you can implement the `Absinthe.Federation.Schema.EntityUnion.Resolver` protocol like this:

```elixir
defmodule MySchema do
  @type t :: %__MODULE__{
          id: String.t()
        }

  defstruct id: ""

  defimpl Absinthe.Federation.Schema.EntityUnion.Resolver do
    def resolve_type(_, _), do: :my_schema_object_name
  end
end
```

### Federation v2

If you are wanting to use Apollo Federation v2 a workaround is required for the `@link` directive
since absinthe doesn't currently support directives on macro-based schemas (until [this PR](https://github.com/absinthe-graphql/absinthe/pull/1176) is merged). The rest of the
federation v2 directives have macros you can use, e.g. `shareable`, `tag`, `override_from`, `inaccessible`.
For an example schema using federation v2 see the [Apollo subgraph compatibility test suite repo pull request](https://github.com/apollographql/apollo-federation-subgraph-compatibility/pull/119).

```elixir
defmodule MyApp.MySchema do
  use Absinthe.Schema
  use Absinthe.Federation.Schema

+ import_sdl """
+   schema @link(url: "https://specs.apollo.dev/federation/v2.0",
+               import: [
+                 "@key",
+                 "@shareable",
+                 "@provides",
+                 "@external",
+                 "@tag",
+                 "@extends",
+                 "@override",
+                 "@inaccessible"
+               ]) {
+     query: RootQueryType
+     mutation: RootMutationType
+   }
+ """

  query do
    ...
  end
end
```

## More Documentation

See additional documentation, including guides, in the [Absinthe.Federation hexdocs](https://hexdocs.pm/absinthe_federation).

## Contributing

Refer to the [Contributing Guide](./CONTRIBUTING.md).

## License

See [LICENSE](./LICENSE.md)