Add support for using custom connections

This builds on #36, and adds support
for specifying a custom connection class to SQLAlchemyObjectType. The
implementation is mostly a copy & paste from [the identical feature in
DjangoObjectType](https://github.com/graphql-python/graphene-django/blob/master/graphene_django/types.py#L72).

Author: wichert

graphene_sqlalchemy/fields.py

@@ -11,6 +11,16 @@
 
 class UnsortedSQLAlchemyConnectionField(ConnectionField):
 
+    @property
+    def type(self):
+        from .types import SQLAlchemyObjectType
+        _type = super(ConnectionField, self).type
+        if issubclass(_type, Connection):
+            return _type
+        assert issubclass(_type, SQLAlchemyObjectType), "SQLALchemyConnectionField only accepts SQLAlchemyObjectType types, not {}".format(_type.__name__)
+        assert _type._meta.connection, "The type {} doesn't have a connection".format(_type.__name__)
+        return _type._meta.connection
+
     @property
     def model(self):
         return self.type._meta.node._meta.model

graphene_sqlalchemy/types.py

@@ -90,7 +90,8 @@ class SQLAlchemyObjectType(ObjectType):
     @classmethod
     def __init_subclass_with_meta__(cls, model=None, registry=None, skip_registry=False,
                                     only_fields=(), exclude_fields=(), connection=None,
-                                    use_connection=None, interfaces=(), id=None, _meta=None, **options):
+                                    connection_class=None, use_connection=None, interfaces=(),
+                                    id=None, _meta=None, **options):
         assert is_mapped_class(model), (
             'You need to pass a valid SQLAlchemy Model in '
             '{}.Meta, received "{}".'
@@ -114,7 +115,11 @@ def __init_subclass_with_meta__(cls, model=None, registry=None, skip_registry=Fa
 
         if use_connection and not connection:
             # We create the connection automatically
-            connection = Connection.create_type('{}Connection'.format(cls.__name__), node=cls)
+            if not connection_class:
+                connection_class = Connection
+
+            connection = connection_class.create_type(
+                '{}Connection'.format(cls.__name__), node=cls)
 
         if connection is not None:
             assert issubclass(connection, Connection), (