Fix: /assignee should add assignees to MR and replace in an issue

app/services/slash_commands/interpret_service.rb

@@ -92,9 +92,19 @@ module SlashCommands
 
     desc 'Assign'
     explanation do |users|
-      "Assigns #{users.map(&:to_reference).to_sentence}." if users.any?
+      if issuable.is_a?(Issue)
+        "Assigns #{users.map(&:to_reference).to_sentence}." if users.any?
+      else
+        "Assigns #{users.last.to_reference}." if users.any?
+      end
+    end
+    params do
+      if issuable.is_a?(Issue)
+        ['@user1 @user2']
+      else
+        ['@user']
+      end
     end
-    params '@user'
     condition do
       current_user.can?(:"admin_#{issuable.to_ability_name}", project)
     end
@@ -105,7 +115,8 @@ module SlashCommands
       next if users.empty?
 
       if issuable.is_a?(Issue)
-        @updates[:assignee_ids] = users.map(&:id)
+        # EE specific. In CE we should replace one assignee with another
+        @updates[:assignee_ids] = issuable.assignees.pluck(:id) + users.map(&:id)
       else
         @updates[:assignee_id] = users.last.id
       end

spec/services/slash_commands/interpret_service_spec.rb

@@ -376,15 +376,21 @@ describe SlashCommands::InterpretService, services: true do
       let(:content) { "/assign @#{developer.username}" }
 
       context 'Issue' do
-        it 'fetches assignee and populates assignee_id if content contains /assign' do
+        it 'fetches assignees and populates them if content contains /assign' do
+          user = create(:user)
+          issue.assignees << user
+
           _, updates = service.execute(content, issue)
 
-          expect(updates).to eq(assignee_ids: [developer.id])
+          expect(updates[:assignee_ids]).to match_array([developer.id, user.id])
         end
       end
 
       context 'Merge Request' do
         it 'fetches assignee and populates assignee_id if content contains /assign' do
+          user = create(:user)
+          merge_request.update(assignee: user)
+
           _, updates = service.execute(content, merge_request)
 
           expect(updates).to eq(assignee_id: developer.id)