Ensure that we have only one list per label per board

app/models/list.rb

@@ -6,6 +6,7 @@ class List < ActiveRecord::Base
 
   validates :board, :list_type, presence: true
   validates :label, :position, presence: true, if: :label?
+  validates :label_id, uniqueness: { scope: :board_id }, if: :label?
   validates :position, numericality: { only_integer: true, greater_than_or_equal_to: 0 }, if: :label?
 
   delegate :name, to: :label, allow_nil: true, prefix: true

db/migrate/20160803161903_add_unique_index_to_lists_label_id.rb

+class AddUniqueIndexToListsLabelId < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :lists, [:board_id, :label_id], unique: true
+  end
+
+  def down
+    remove_index :lists, column: [:board_id, :label_id] if index_exists?(:lists, [:board_id, :label_id], unique: true)
+  end
+end

db/schema.rb

@@ -550,6 +550,7 @@ ActiveRecord::Schema.define(version: 20160810142633) do
     t.datetime "updated_at",             null: false
   end
 
+  add_index "lists", ["board_id", "label_id"], name: "index_lists_on_board_id_and_label_id", unique: true, using: :btree
   add_index "lists", ["board_id"], name: "index_lists_on_board_id", using: :btree
   add_index "lists", ["label_id"], name: "index_lists_on_label_id", using: :btree
 

spec/models/list_spec.rb

@@ -17,6 +17,12 @@ describe List do
     it { is_expected.to validate_presence_of(:position) }
     it { is_expected.to validate_numericality_of(:position).only_integer.is_greater_than_or_equal_to(0) }
 
+    it 'validates uniqueness of label scoped to board_id' do
+      create(:list)
+
+      expect(subject).to validate_uniqueness_of(:label_id).scoped_to(:board_id)
+    end
+
     context 'when list_type is set to backlog' do
       subject { described_class.new(list_type: :backlog) }