add currency entity

spec/models/currency_spec.rb

@@ -0,0 +1,18 @@
+# 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
+#
+require "rails_helper"
+
+RSpec.describe(Currency, type: :model) do
+  describe "validations" do
+    it { is_expected.to(validate_presence_of(:name)) }
+  end
+end