add balance entity

app/models/balance.rb

@@ -0,0 +1,29 @@
+# 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)
+#
+class Balance < ApplicationRecord
+  belongs_to :user
+  belongs_to :currency
+
+  validates :amount, presence: true
+end

app/models/user.rb

@@ -25,6 +25,7 @@ class User < ApplicationRecord
     :recoverable, :rememberable, :validatable
 
   has_many :documents, class_name: "UserDocument", dependent: :destroy
+  has_many :balances, dependent: :restrict_with_error
 
   validates :first_name, :last_name, :email, presence: true
   validates :email, uniqueness: true