Add callback for managers assignment

When we change the managers group of a staging_workflow we trigger an
`before_update` to update the managers group of related the staging
projects.

src/api/app/controllers/webui/staging/workflows_controller.rb

@@ -72,23 +72,11 @@ def destroy
   def update
     authorize @staging_workflow
 
-    begin
-      Staging::Workflow.transaction do
-        old_managers_group = @staging_workflow.managers_group
-        new_managers_group = Group.find_by(title: params[:managers_title])
-
-        @staging_workflow.staging_projects.each do |staging_project|
-          staging_project.unassign_managers_group(old_managers_group)
-          staging_project.assign_managers_group(new_managers_group)
-          staging_project.store
-        end
-
-        @staging_workflow.managers_group = new_managers_group
-        @staging_workflow.save!
-      end
-      flash[:success] = 'Managers group if staging was successfully assigned'
-    rescue
-      flash[:error] = "Sorry, something unexpected happended and the group couldn't be assigned to this staging"
+    @staging_workflow.managers_group = Group.find_by(title: params[:managers_title])
+    if @staging_workflow.save
+      flash[:success] = 'Managers group was successfully assigned'
+    else
+      flash[:error] = "Managers group couldn't be assigned: #{@staging_workflow.errors.full_messages.to_sentence}."
     end
 
     redirect_to edit_staging_workflow_path(@staging_workflow)

src/api/app/models/staging/workflow.rb

@@ -30,6 +30,7 @@ def ready_to_stage
   validates :managers_group, presence: true
 
   after_create :create_staging_projects
+  before_update :update_staging_projects_managers_group
 
   def unassigned_requests
     target_of_bs_requests.stageable.where.not(id: ignored_requests | staged_requests)
@@ -60,4 +61,21 @@ def create_staging_projects
       staging_project.store
     end
   end
+
+  def update_staging_projects_managers_group
+    return unless changes[:managers_group_id]
+
+    old_managers_group = Group.find(changes[:managers_group_id].first)
+    new_managers_group = managers_group
+
+    staging_projects.each do |staging_project|
+      staging_project.unassign_managers_group(old_managers_group)
+      staging_project.assign_managers_group(new_managers_group)
+      staging_project.store
+    end
+
+    # FIXME: This assignation is need because after store a staging_project
+    # the object is reloaded and we lost the changes.
+    self.managers_group = new_managers_group
+  end
 end

src/api/spec/controllers/staging/staged_requests_controller_spec.rb

@@ -39,6 +39,8 @@
   describe 'POST #create' do
     context 'invalid user' do
       before do
+        staging_workflow
+
         login other_user
         post :create, params: { staging_project_name: staging_project.name, format: :xml },
                       body: "<requests><number>#{bs_request.number}</number></requests>"