Unions
======

Union types are very similar to interfaces, but they don't get
to specify any common fields between the types.

The basics:

- Each Union is a Python class that inherits from ``graphene.Union``.
- Unions don't have any fields on it, just links to the possible ObjectTypes.

Quick example
-------------

This example model defines several ObjectTypes with their own fields.
``SearchResult`` is the implementation of ``Union`` of this object types.

.. code:: python

    import graphene

    class Human(graphene.ObjectType):
        name = graphene.String()
        born_in = graphene.String()

    class Droid(graphene.ObjectType):
        name = graphene.String()
        primary_function = graphene.String()

    class Starship(graphene.ObjectType):
        name = graphene.String()
        length = graphene.Int()

    class SearchResult(graphene.Union):
        class Meta:
            types = (Human, Droid, Starship)


Wherever we return a SearchResult type in our schema, we might get a Human, a Droid, or a Starship.
Note that members of a union type need to be concrete object types;
you can't create a union type out of interfaces or other unions.

The above types have the following representation in a schema:

.. code::

    type Droid {
      name: String
      primaryFunction: String
    }

    type Human {
      name: String
      bornIn: String
    }

    type Ship {
      name: String
      length: Int
    }

    union SearchResult = Human | Droid | Starship

Resolving Types
~~~~~~~~~~~~~~~

When defining Unions, as with Interfaces, we need to tell the schema how to resolve the type of a returned object.

This can be achieved by:

- defining an `is_type_of`-method on each `ObjectType`
- defining the attribute `possible_types` on the Meta class
- defining a `resolve_type` on the Union

Examples:
^^^^^^^^^

An example with `is_type_of` and `Meta.possible_types`:

.. code:: python

    class one_object:
        one = 'one'

    class two_object:
        two = 'two'

    class One(ObjectType):
        one = String()

        @classmethod
        def is_type_of(cls, root, info):
            return isinstance(root, one_object)

    class Two(ObjectType):
        class Meta:
            possible_types = (two_object,)
        two = String()

    class MyUnion(Union):
        class Meta:
            types = (One, Two)


An example with `resolve_type`:

.. code:: python

    class One(ObjectType):
        one = String()

    class Two(ObjectType):
        two = String()

    class MyUnion(Union):
        class Meta:
            types = (One, Two)

        @classmethod
        def resolve_type(cls, instance, info)
            if hasattr(instance, 'one'):
                return One
            else:
                return Two