Allow to query discussion design on GraphQL

Expose noteable on GraphQL discussion.

Changelog: added

app/graphql/types/noteable_union.rb

+# frozen_string_literal: true
+
+module Types
+  class NoteableUnion < BaseUnion
+    graphql_name 'NoteableUnion'
+    description 'Represents an object that supports notes.'
+
+    possible_types Types::IssueType, Types::DesignManagement::DesignType, Types::MergeRequestType
+
+    def self.resolve_type(object, context)
+      case object
+      when Issue
+        Types::IssueType
+      when ::DesignManagement::Design
+        Types::DesignManagement::DesignType
+      when MergeRequest
+        Types::MergeRequestType
+      else
+        raise 'Unsupported issuable type'
+      end
+    end
+  end
+end

app/graphql/types/notes/discussion_type.rb

@@ -19,6 +19,8 @@ module Types
             description: "Timestamp of the discussion's creation."
       field :notes, Types::Notes::NoteType.connection_type, null: false,
             description: 'All notes in the discussion.'
+      field :noteable, Types::NoteableUnion, null: true,
+            description: 'Object which the discussion belongs to.'
 
       # DiscussionID.coerce_result is suitable here, but will always mark this
       # as being a 'Discussion'. Using `GlobalId.build` guarantees that we get
@@ -26,6 +28,14 @@ module Types
       def reply_id
         ::Gitlab::GlobalId.build(object, id: object.reply_id)
       end
+
+      def noteable
+        noteable = object.noteable
+
+        return unless Ability.allowed?(context[:current_user], :"read_#{noteable.to_ability_name}", noteable)
+
+        noteable
+      end
     end
   end
 end

spec/graphql/types/notes/discussion_type_spec.rb

@@ -13,6 +13,7 @@ RSpec.describe GitlabSchema.types['Discussion'] do
       resolved
       resolved_at
       resolved_by
+      noteable
     ]
 
     expect(described_class).to have_graphql_fields(*expected_fields)

spec/support/shared_examples/requests/api/graphql/noteable_shared_examples.rb

@@ -31,6 +31,28 @@ RSpec.shared_examples 'a noteable graphql type we can query' do
       expect(graphql_data_at(*path_to_noteable, :discussions, :nodes))
         .to match_array(expected)
     end
+
+    it 'can fetch discussion noteable' do
+      create(discussion_factory, project: project, noteable: noteable)
+      fields =
+        <<-QL.strip_heredoc
+          discussions {
+            nodes {
+              noteable {
+                __typename
+                ... on #{noteable.class.name.demodulize} {
+                  id
+                }
+              }
+            }
+          }
+        QL
+
+      post_graphql(query(fields), current_user: current_user)
+
+      data = graphql_data_at(*path_to_noteable, :discussions, :nodes, :noteable, :id)
+      expect(data[0]).to eq(global_id_of(noteable))
+    end
   end
 
   describe '.notes' do