Add restriction to number of permitted priorities per project label

app/models/project_label.rb

@@ -1,8 +1,11 @@
 class ProjectLabel < Label
+  NUMBER_OF_PRIORITIES = 1
+
   belongs_to :project
 
   validates :project, presence: true
 
+  validate :permitted_numbers_of_priorities
   validate :title_must_not_exist_at_group_level
 
   delegate :group, to: :project, allow_nil: true
@@ -20,4 +23,10 @@ class ProjectLabel < Label
       errors.add(:title, :label_already_exists_at_group_level, group: group.name)
     end
   end
+
+  def permitted_numbers_of_priorities
+    if priorities && priorities.size >= NUMBER_OF_PRIORITIES
+      errors.add(:priorities, 'Number of permitted priorities exceeded')
+    end
+  end
 end

spec/models/project_label_spec.rb

@@ -48,7 +48,18 @@ describe ProjectLabel, models: true do
 
         project_label.valid?
 
-        expect(project_label  .errors[:title]).to be_empty
+        expect(project_label.errors[:title]).to be_empty
+      end
+    end
+
+    context 'when attempting to add more than one priority to the project label' do
+      it 'returns error' do
+        subject.priorities.build
+        subject.priorities.build
+
+        subject.valid?
+
+        expect(subject.errors[:priorities]).to include 'Number of permitted priorities exceeded'
       end
     end
   end