FEATURE: tag groups

db/migrate/20160602164008_create_tag_groups.rb

@@ -0,0 +1,17 @@
+class CreateTagGroups < ActiveRecord::Migration
+  def change
+    create_table :tag_groups do |t|
+      t.string :name,       null: false
+      t.integer :tag_count, null: false, default: 0
+      t.timestamps
+    end
+
+    create_table :tag_group_memberships do |t|
+      t.references :tag,       null: false
+      t.references :tag_group, null: false
+      t.timestamps
+    end
+
+    add_index :tag_group_memberships, [:tag_group_id, :tag_id], unique: true
+  end
+end