Simplify parameter key checks

This uses set union to verify the parameter suitability for the
more specialized service, and makes the service responsible for
selecting between `assignee_id` and `assignee_ids`.

app/services/merge_requests/update_assignees_service.rb

@@ -56,6 +56,13 @@ module MergeRequests
       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

lib/api/merge_requests.rb

@@ -424,8 +424,7 @@ module API
         mr_params[:force_remove_source_branch] = mr_params.delete(:remove_source_branch) if mr_params.has_key?(:remove_source_branch)
         mr_params = convert_parameters_from_legacy_format(mr_params)
 
-        service = if mr_params.keys == [:assignee_ids] || mr_params.keys == [:assignee_id]
-                    mr_params[:assignee_ids] = Array.wrap(mr_params[:assignee_ids]) if mr_params.key?(:assignee_id)
+        service = if mr_params.one? && (mr_params.keys & %i[assignee_id assignee_ids]).one?
                     ::MergeRequests::UpdateAssigneesService
                   else
                     ::MergeRequests::UpdateService