1 files changed, 64 insertions, 0 deletions
diff --git a/app/services/merge_requests/update_assignees_service.rb b/app/services/merge_requests/update_assignees_service.rb
new file mode 100644
index 00000000000..b339a644e8c
--- /dev/null
+++ b/app/services/merge_requests/update_assignees_service.rb
@@ -0,0 +1,64 @@
+# frozen_string_literal: true
+
+module MergeRequests
+  class UpdateAssigneesService < UpdateService
+    # a stripped down service that only does what it must to update the
+    # assignees, and knows that it does not have to check for other updates.
+    # This saves a lot of queries for irrelevant things that cannot possibly
+    # change in the execution of this service.
+    def execute(merge_request)
+      return merge_request unless current_user&.can?(:update_merge_request, merge_request)
+
+      old_assignees = merge_request.assignees
+      old_ids = old_assignees.map(&:id)
+      new_ids = new_assignee_ids(merge_request)
+      return merge_request if new_ids.size != update_attrs[:assignee_ids].size
+      return merge_request if old_ids.to_set == new_ids.to_set # no-change
+
+      attrs = update_attrs.merge(assignee_ids: new_ids)
+      merge_request.update!(**attrs)
+
+      # Defer the more expensive operations (handle_assignee_changes) to the background
+      MergeRequests::HandleAssigneesChangeService
+        .new(project, current_user)
+        .async_execute(merge_request, old_assignees, execute_hooks: true)
+
+      merge_request
+    end
+
+    private
+
+    def new_assignee_ids(merge_request)
+      # prime the cache - prevent N+1 lookup during authorization loop.
+      merge_request.project.team.max_member_access_for_user_ids(update_attrs[:assignee_ids])
+      User.id_in(update_attrs[:assignee_ids]).map do |user|
+        if user.can?(:read_merge_request, merge_request)
+          user.id
+        else
+          merge_request.errors.add(
+            :assignees,
+            "Cannot assign #{user.to_reference} to #{merge_request.to_reference}"
+          )
+          nil
+        end
+      end.compact
+    end
+
+    def assignee_ids
+      params.fetch(:assignee_ids).first(1)
+    end
+
+    def params
+      ps = super
+
+      # allow either assignee_id or assignee_ids, preferring assignee_id if passed.
+      { assignee_ids: ps.key?(:assignee_id) ? Array.wrap(ps[:assignee_id]) : ps[:assignee_ids] }
+    end
+
+    def update_attrs
+      @attrs ||= { updated_at: Time.current, updated_by: current_user, assignee_ids: assignee_ids }
+    end
+  end
+end
+
+MergeRequests::UpdateAssigneesService.prepend_if_ee('EE::MergeRequests::UpdateAssigneesService')