Provide a default implementation of resolve_type

This makes use of loads a bit safer.

app/graphql/types/base_object.rb

@@ -8,6 +8,12 @@ module Types
 
     field_class Types::BaseField
 
+    def self.accepts(*types)
+      @accepts ||= []
+      @accepts += types
+      @accepts
+    end
+
     # All graphql fields exposing an id, should expose a global id.
     def id
       GitlabSchema.id_from_object(object)
@@ -16,5 +22,13 @@ module Types
     def current_user
       context[:current_user]
     end
+
+    def self.resolve_type(object, context)
+      assignable = accepts
+
+      return self if assignable.blank?
+
+      self if assignable.any? { |cls| object.is_a?(cls) }
+    end
   end
 end

app/graphql/types/board_type.rb

@@ -4,7 +4,7 @@ module Types
   class BoardType < BaseObject
     graphql_name 'Board'
     description 'Represents a project or group board'
-
+    accepts ::Board
     authorize :read_board
 
     field :id, type: GraphQL::ID_TYPE, null: false,

ee/app/graphql/types/epic_type.rb

@@ -6,7 +6,7 @@ module Types
 
     graphql_name 'Epic'
     description 'Represents an epic'
-
+    accepts ::Epic
     authorize :read_epic
 
     expose_permissions Types::PermissionTypes::Epic

ee/spec/requests/api/graphql/mutations/issues/update_spec.rb

@@ -64,6 +64,17 @@ RSpec.describe 'Update of an existing issue' do
         expect(graphql_errors).not_to be_blank
       end
     end
+
+    context 'the epic is not an epic' do
+      let(:epic) { create(:user) }
+
+      it 'does not set the epic' do
+        post_graphql_mutation(mutation, current_user: current_user)
+
+        expect(response).to have_gitlab_http_status(:success)
+        expect(graphql_errors).not_to be_blank
+      end
+    end
   end
 
   context 'removing epic' do