add balance entity

spec/models/balance_spec.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+# == Schema Information
+#
+# Table name: balances
+#
+#  id          :bigint           not null, primary key
+#  amount      :decimal(20, 10)  default(0.0), not null
+#  created_at  :datetime         not null
+#  updated_at  :datetime         not null
+#  currency_id :bigint           not null
+#  user_id     :bigint           not null
+#
+# Indexes
+#
+#  index_balances_on_currency_id  (currency_id)
+#  index_balances_on_user_id      (user_id)
+#
+# Foreign Keys
+#
+#  fk_rails_...  (currency_id => currencies.id)
+#  fk_rails_...  (user_id => users.id)
+#
+require "rails_helper"
+
+RSpec.describe(Balance, type: :model) do
+  describe "validations" do
+    it { is_expected.to(validate_presence_of(:amount)) }
+  end
+
+  describe "associations" do
+    it { is_expected.to(belong_to(:user)) }
+    it { is_expected.to(belong_to(:currency)) }
+  end
+end