Update interface documentation

Author: jkimbo

docs/types/interfaces.rst

@@ -1,60 +1,142 @@
 Interfaces
 ==========
 
-An Interface contains the essential fields that will be implemented by
-multiple ObjectTypes.
+An *Interface* is an abstract type that defines a certain set of fields that a
+type must include to implement the interface.
 
-The basics:
-
-- Each Interface is a Python class that inherits from ``graphene.Interface``.
-- Each attribute of the Interface represents a GraphQL field.
-
-Quick example
--------------
-
-This example model defines a ``Character`` interface with a name. ``Human``
-and ``Droid`` are two implementations of that interface.
+For example, you can define an Interface ``Character`` that represents any
+character in the Star Wars trilogy:
 
 .. code:: python
 
     import graphene
 
     class Character(graphene.Interface):
-        name = graphene.String()
+        id = graphene.ID(required=True)
+        name = graphene.String(required=True)
+        friends = graphene.List(lambda: Character)
+        appears_in = graphene.List(Episode, required=True)
+
+
+Any ObjectType that implements ``Character`` will have these exact fields, with
+these arguments and return types.
+
+For example, here are some types that might implement ``Character``:
+
+.. code:: python
 
-    # Human is a Character implementation
     class Human(graphene.ObjectType):
         class Meta:
             interfaces = (Character, )
 
-        born_in = graphene.String()
+        starships = graphene.List(Starship)
+        home_planet = graphene.String()
 
-    # Droid is a Character implementation
     class Droid(graphene.ObjectType):
         class Meta:
             interfaces = (Character, )
 
-        function = graphene.String()
+        primary_function = graphene.String()
 
 
-``name`` is a field on the ``Character`` interface that will also exist on both
-the ``Human`` and ``Droid`` ObjectTypes (as those implement the ``Character``
-interface). Each ObjectType may define additional fields.
+Both of these types have all of the fields from the ``Character`` interface,
+but also bring in extra fields, ``home_planet``, ``starships`` and
+``primary_function``, that are specific to that particular type of character.
 
-The above types have the following representation in a schema:
+The full GraphQL schema defition will look like this:
 
 .. code::
 
     interface Character {
-      name: String
+        id: ID!
+        name: String!
+        friends: [Character]
+        appearsIn: [Episode]!
+    }
+
+    type Human implements Character {
+        id: ID!
+        name: String!
+        friends: [Character]
+        appearsIn: [Episode]!
+        starships: [Starship]
+        homePlanet: String
     }
 
     type Droid implements Character {
-      name: String
-      function: String
+        id: ID!
+        name: String!
+        friends: [Character]
+        appearsIn: [Episode]!
+        primaryFunction: String
     }
 
-    type Human implements Character {
-      name: String
-      bornIn: String
+Interfaces are useful when you want to return an object or set of objects,
+which might be of several different types.
+
+For example, you can define a field ``hero`` that resolves to any
+``Character``, depending on the episode, like this:
+
+.. code:: python
+
+    class Query(graphene.ObjectType):
+        hero = graphene.Field(
+            Character,
+            required=True,
+            episode=graphene.Field(Episode, required=True)
+        )
+
+        def resolve_hero(_, info, episode):
+            # Luke is the hero of Episode V
+            if episode == 5:
+                return get_human(name='Luke Skywalker')
+            return get_droid(name='R2-D2')
+
+This allows you to directly query for fields that exist on the Character interface
+as well as selecting specific fields on any type that implments the interface
+using `inline fragments <https://graphql.org/learn/queries/#inline-fragments>`_.
+
+For example, the following query:
+
+.. code::
+
+    query HeroForEpisode($episode: Episode!) {
+        hero(episode: $episode) {
+            __typename
+            name
+            ... on Droid {
+                primaryFunction
+            }
+            ... on Human {
+                homePlanet
+            }
+        }
+    }
+
+Will return the following data with variables ``{ "episode": 4 }``:
+
+.. code:: json
+
+    {
+        "data": {
+            "hero": {
+                "__typename": "Droid",
+                "name": "R2-D2",
+                "primaryFunction": "Astromech"
+            }
+        }
+    }
+
+And different data with the variables ``{ "episode": 5 }``:
+
+.. code:: json
+
+    {
+        "data": {
+            "hero": {
+                "__typename": "Human",
+                "name": "Luke Skywalker",
+                "homePlanet": "Tatooine"
+            }
+        }
     }