Merge branch '3488-fix-service-desk' into 'master'

Resolve "Service Desk filter Error 500"

Closes #3488

See merge request gitlab-org/gitlab-ee!2969

changelogs/unreleased-ee/3488-fix-service-desk.yml

+---
+title: Fix searching by assignee in the service desk
+merge_request: 2969
+author:
+type: fixed

doc/development/ee_features.md

@@ -27,6 +27,12 @@ Merging changes from GitLab CE to EE can result in numerous conflicts.
 To reduce conflicts, EE code should be separated in to the `EE` module
 as much as possible.
+When referencing constants *outside* of the `EE` namespace from within it, you
+should always use absolute constants - e.g., `::User` instead of `User`. This
+will prevent `::EE::User` from being picked instead of `::User`. Follow this
+rule even if the constant doesn't exist in the `EE` namespace at present - it
+may be added in the future.
+
 ### Classes vs. Module Mixins
 If the feature being developed is not present in any form in CE,

ee/app/controllers/ee/projects/issues_controller.rb

@@ -12,7 +12,7 @@ module IssuesController
       def service_desk
         if params[:assignee_id].present?
-          assignee = User.find_by_id(params[:assignee_id])
+          assignee = ::User.find_by_id(params[:assignee_id])
           @users.push(assignee) if assignee
         end

spec/ee/spec/controllers/projects/issues_controller_spec.rb

@@ -227,6 +227,12 @@ def get_service_desk(extra_params = {})
         expect(assigns(:issues)).to contain_exactly(service_desk_issue_2)
       end
+
+      it 'allows an assignee to be specified by id' do
+        get_service_desk(assignee_id: other_user.id)
+
+        expect(assigns(:users)).to contain_exactly(other_user, support_bot)
+      end
     end
     context 'when Service Desk is not available on the project' do