add balance entity

db/migrate/20210811121726_create_balances.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+class CreateBalances < ActiveRecord::Migration[6.1]
+  def change
+    create_table(:balances) do |t|
+      t.references(:user, null: false, foreign_key: true)
+      t.references(:currency, null: false, foreign_key: true)
+
+      t.decimal(:amount, precision: 20, scale: 10, null: false, default: 0)
+
+      t.timestamps
+    end
+  end
+end