Update README to include examples from Apollo Federation docs (#100)

* Update README to include examples from Apollo Federation docs

Co-authored-by: Doruk Gurleyen <dorukgurleyen@gmail.com>

---------

Co-authored-by: Doruk Gurleyen <dorukgurleyen@gmail.com>

README.md

@@ -1,15 +1,15 @@
 # Absinthe.Federation
 
 [![Build Status](https://github.com/DivvyPayHQ/absinthe_federation/workflows/CI/badge.svg)](https://github.com/DivvyPayHQ/absinthe_federation/actions?query=workflow%3ACI)
-[![Hex pm](http://img.shields.io/hexpm/v/absinthe_federation.svg)](https://hex.pm/packages/absinthe_federation)
+[![Hex pm](https://img.shields.io/hexpm/v/absinthe_federation.svg)](https://hex.pm/packages/absinthe_federation)
 [![Hex Docs](https://img.shields.io/badge/hex-docs-blue.svg)](https://hexdocs.pm/absinthe_federation/)
 [![License](https://img.shields.io/badge/License-MIT-blue.svg)](https://opensource.org/licenses/MIT)
 
-[Apollo Federation](https://www.apollographql.com/docs/federation/federation-spec/) support for [Absinthe](https://github.com/absinthe-graphql/absinthe)
+[Apollo Federation](https://www.apollographql.com/docs/federation) support for [Absinthe](https://hexdocs.pm/absinthe/overview.html).
 
 ## Installation
 
-Install from [Hex.pm](https://hex.pm/packages/absinthe_federation):
+Install from [Hex](https://hex.pm/packages/absinthe_federation):
 
 ```elixir
 def deps do
@@ -19,7 +19,7 @@ def deps do
 end
 ```
 
-Install from github:
+Install a specific branch from [GitHub](https://github.com/DivvyPayHQ/absinthe_federation):
 
 ```elixir
 def deps do
@@ -29,10 +29,10 @@ def deps do
 end
 ```
 
-Add the following line to your absinthe schema
+Use `Absinthe.Federation.Schema` module in your root schema:
 
 ```elixir
-defmodule MyApp.MySchema do
+defmodule Example.Schema do
   use Absinthe.Schema
 + use Absinthe.Federation.Schema
 
@@ -42,56 +42,160 @@ defmodule MyApp.MySchema do
 end
 ```
 
-## Usage
+Validate everything is wired up correctly:
 
-### Macro based schemas (recommended)
+```bash
+mix absinthe.federation.schema.sdl --schema Example.Schema
+```
+
+You should see the [Apollo Federation Subgraph Specification](https://www.apollographql.com/docs/federation/subgraph-spec) fields along with any fields you've defined. It can be helpful to add `*.graphql` to your `.gitignore`, at least at your projects root level, while testing your SDL output during development.
+
+## Usage (macro based schemas)
 
-> Note: Implementing the reference resolver with function capture does not work at the moment. Hence, the below example uses an anonymous function.
+The following sticks close to the Apollo Federation documentation to better clarify how to achieve the same outcomes with the `Absinthe.Federation` module as you'd get from their JavaScript examples.
+
+### [Defining an entity](https://www.apollographql.com/docs/federation/entities#defining-an-entity)
 
 ```elixir
-defmodule MyApp.MySchema do
+defmodule Products.Schema do
   use Absinthe.Schema
-+ use Absinthe.Federation.Schema
+  use Absinthe.Federation.Schema
 
-  query do
-+   extends()
+  extend schema do
+    directive(:link,
+      url: "https://specs.apollo.dev/federation/v2.3",
+      import: ["@key", ...]
+    )
+  end
 
-    field :review, :review do
-      arg(:id, non_null(:id))
-      resolve(&ReviewResolver.get_review_by_id/3)
+  object :product do
+    directive :key, fields: "id"
+
+    # Any subgraph contributing fields MUST define a _resolve_reference field.
+    # Note that implementing the reference resolver with function capture does not work at the moment. Hence, the examples below use an anonymous function.
+    field :_resolve_reference, :product do
+      resolve(fn %{__typename: "Product", id: id} = entity, _info ->
+        {:ok, Map.merge(entity, %{name: "ACME Anvil", price: 10000})}
+      end)
     end
+
+    field :id, non_null(:id)
+    field :name, non_null(:string)
+    field :price, :int
+  end
+
+  query do
     ...
   end
+end
+```
+
+Your `:_resolve_reference` must return one of the following:
+
+```elixir
+{:ok, %Product{id: id, ...}}
+```
+
+```elixir
+{:ok, %{__typename: "Product", id: id, ...}}
+```
+
+```elixir
+{:ok, %{"__typename" => "Product", "id" => id, ...}}
+```
+
+```elixir
+{:ok, nil}
+```
+
+It is easier to just merge a subgraph's contributed fields back onto the incoming entity reference than rely on a struct to set the `__typename`.
+
+### [Contributing entity fields](https://www.apollographql.com/docs/federation/entities#contributing-entity-fields)
+
+Each subgraph, by default, must return different fields. See the Apollo documentation should you need to [override this behavior](https://www.apollographql.com/docs/federation/entities/resolve-another-subgraphs-fields).
+
+```elixir
+defmodule Inventory.Schema do
+  use Absinthe.Schema
+  use Absinthe.Federation.Schema
+
+  extend schema do
+    directive(:link,
+      url: "https://specs.apollo.dev/federation/v2.3",
+      import: ["@key", ...]
+    )
+  end
 
   object :product do
-+   key_fields("upc")
-+   extends()
+    directive :key, fields: "id"
 
-    field :upc, non_null(:string) do
-+     external()
+    # In this case, only the `Inventory.Schema` should resolve the `inStock` field.
+    field :_resolve_reference, :product do
+      resolve(fn %{__typename: "Product", id: id} = entity, _info ->
+        {:ok, Map.merge(entity, %{in_stock: true})}
+      end)
     end
 
-    field(:reviews, list_of(:review)) do
-      resolve(&ReviewResolver.get_reviews_for_product/3)
-    end
+    field :id, non_null(:string)
+    field :in_stock, non_null(:boolean)
+  end
 
-+   field(:_resolve_reference, :product) do
-+     resolve(fn parent, args, context ->
-        ProductResolver.get_product_by_upc(parent, args, context)
+  query do
+    ...
+  end
+end
+```
+
+### [Referencing an entity without contributing fields](https://www.apollographql.com/docs/federation/entities#referencing-an-entity-without-contributing-fields)
+
+```elixir
+defmodule Reviews.Schema do
+  use Absinthe.Schema
+  use Absinthe.Federation.Schema
+
+  extend schema do
+    directive(:link,
+      url: "https://specs.apollo.dev/federation/v2.3",
+      import: ["@key", ...]
+    )
+  end
+
+  # Stubbed entity, marked as unresolvable in this subgraph.
+  object :product do
+    directive :key, fields: "id", resolvable: false
+
+    field :id, non_null(:string)
+  end
+
+  object :review do
+    field :id, non_null(:id)
+    field :score, non_null(:int)
+    field :description, non_null(:string)
+
+    # This subgraph only needs to resolve the key fields used to reference the entity.
+    field :product, non_null(:product) do
+      resolve(fn %{product_id: id} = _parent, _args, _info ->
+        {:ok, %{id: id}}
       end)
-+   end
+    end
+  end
+
+  query do
+    field :latest_reviews, non_null(list(:review)) do
+      resolve(&ReviewsResolver.find_many/2)
+    end
   end
 end
 ```
 
 ### Macro based schema with existing prototype
 
-If you are already using a schema prototype
+If you are already using a schema prototype.
 
 ```elixir
-defmodule MyApp.MySchema do
+defmodule Example.Schema do
   use Absinthe.Schema
-+ use Absinthe.Federation.Schema, prototype_schema: MyApp.MySchemaPrototype
++ use Absinthe.Federation.Schema, prototype_schema: Example.SchemaPrototype
 
   query do
     ...
@@ -100,7 +204,7 @@ end
 ```
 
 ```elixir
-defmodule MyApp.MySchemaPrototype do
+defmodule Example.SchemaPrototype do
   use Absinthe.Schema.Prototype
 + use Absinthe.Federation.Schema.Prototype.FederatedDirectives
 
@@ -113,7 +217,7 @@ end
 ### SDL based schemas (experimental)
 
 ```elixir
-defmodule MyApp.MySchema do
+defmodule Example.Schema do
   use Absinthe.Schema
 + use Absinthe.Federation.Schema
 
@@ -131,6 +235,7 @@ defmodule MyApp.MySchema do
   def hydrate(_, _) do
     ...
   end
+end
 ```
 
 ### Resolving structs in \_entities queries
@@ -156,7 +261,7 @@ end
 You can import Apollo Federation v2 directives by extending your top-level schema with the `@link` directive.
 
 ```elixir
-defmodule MyApp.MySchema do
+defmodule Example.Schema do
   use Absinthe.Schema
   use Absinthe.Federation.Schema
 
@@ -189,13 +294,13 @@ end
 `@link` directive supports namespacing and directive renaming (only on **Absinthe >= 1.7.2**) according to the specs.
 
 ```elixir
-defmodule MyApp.MySchema do
+defmodule Example.Schema do
   use Absinthe.Schema
   use Absinthe.Federation.Schema
 
 + extend schema do
 +   directive :link,
-+     url: "https://specs.apollo.dev/federation/v2.0",
++     url: "https://specs.apollo.dev/federation/v2.3",
 +     import: [%{"name" => "@key", "as" => "@primaryKey"}], # directive renaming
 +     as: "federation" # namespacing
 + end