GraphQL support for Notes created in discussions

A new `discussion_id` argument on the `createNote` mutation allows
people to create a note within that discussion.

The ability to lazy-load Discussions has been added, so
GraphQL.object_from_id can treat Discussions the same as AR objects and
batch load them.

https://gitlab.com/gitlab-org/gitlab-ce/issues/62826
https://gitlab.com/gitlab-org/gitlab-ee/issues/9489

3 files changed, 39 insertions, 0 deletions

diff --git a/app/graphql/gitlab_schema.rb b/app/graphql/gitlab_schema.rb
index 152ebb930e2..7edd14e48f7 100644
--- a/app/graphql/gitlab_schema.rb
+++ b/app/graphql/gitlab_schema.rb
@@ -66,6 +66,8 @@ class GitlabSchema < GraphQL::Schema
 
       if gid.model_class < ApplicationRecord
         Gitlab::Graphql::Loaders::BatchModelLoader.new(gid.model_class, gid.model_id).find
+      elsif gid.model_class.respond_to?(:lazy_find)
+        gid.model_class.lazy_find(gid.model_id)
       else
         gid.find
       end
diff --git a/app/graphql/mutations/notes/create/note.rb b/app/graphql/mutations/notes/create/note.rb
index 3c571a4538f..5236e48026e 100644
--- a/app/graphql/mutations/notes/create/note.rb
+++ b/app/graphql/mutations/notes/create/note.rb
@@ -5,6 +5,35 @@ module Mutations
     module Create
       class Note < Base
         graphql_name 'CreateNote'
+
+        argument :discussion_id,
+                  GraphQL::ID_TYPE,
+                  required: false,
+                  description: 'The global id of the discussion this note is in reply to'
+
+        private
+
+        def create_note_params(noteable, args)
+          discussion_id = nil
+
+          if args[:discussion_id]
+            discussion = GitlabSchema.object_from_id(args[:discussion_id])
+            authorize_discussion!(discussion)
+
+            discussion_id = discussion.id
+          end
+
+          super(noteable, args).merge({
+            in_reply_to_discussion_id: discussion_id
+          })
+        end
+
+        def authorize_discussion!(discussion)
+          unless Ability.allowed?(current_user, :read_note, discussion, scope: :user)
+            raise Gitlab::Graphql::Errors::ResourceNotAvailable,
+              "The discussion does not exist or you don't have permission to perform this action"
+          end
+        end
       end
     end
   end
diff --git a/app/graphql/types/notes/discussion_type.rb b/app/graphql/types/notes/discussion_type.rb
index c4691942f2d..a3fb28298f6 100644
--- a/app/graphql/types/notes/discussion_type.rb
+++ b/app/graphql/types/notes/discussion_type.rb
@@ -8,8 +8,16 @@ module Types
       authorize :read_note
 
       field :id, GraphQL::ID_TYPE, null: false
+      field :reply_id, GraphQL::ID_TYPE, null: false, description: 'The ID used to reply to this discussion'
       field :created_at, Types::TimeType, null: false
       field :notes, Types::Notes::NoteType.connection_type, null: false, description: "All notes in the discussion"
+
+      # The gem we use to generate Global IDs is hard-coded to work with
+      # `id` properties. To generate a GID for the `reply_id` property,
+      # we must use the ::Gitlab::GlobalId module.
+      def reply_id
+        ::Gitlab::GlobalId.build(object, id: object.reply_id)
+      end
     end
   end
 end