add currency entity

app/models/currency.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+# == Schema Information
+#
+# Table name: currencies
+#
+#  id         :bigint           not null, primary key
+#  name       :string           not null
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
+#
+class Currency < ApplicationRecord
+  validates :name, presence: true
+end