Add unique contrainst to labels association

db/migrate/20170921203824_create_board_filter_labels.rb

@@ -11,6 +11,8 @@ class CreateBoardFilterLabels < ActiveRecord::Migration
       t.integer :label_id, null: false, index: true
     end
 
+    add_index :board_filter_labels, [:board_filter_id, :label_id], unique: true
+
     add_concurrent_foreign_key :board_filter_labels, :board_filters, column: :board_filter_id, on_delete: :cascade
     add_concurrent_foreign_key :board_filter_labels, :labels, column: :label_id, on_delete: :cascade
   end

db/schema.rb

@@ -219,6 +219,7 @@ ActiveRecord::Schema.define(version: 20170921203824) do
     t.integer "label_id", null: false
   end
 
+  add_index "board_filter_labels", ["board_filter_id", "label_id"], name: "index_board_filter_labels_on_board_filter_id_and_label_id", unique: true, using: :btree
   add_index "board_filter_labels", ["board_filter_id"], name: "index_board_filter_labels_on_board_filter_id", using: :btree
   add_index "board_filter_labels", ["label_id"], name: "index_board_filter_labels_on_label_id", using: :btree