add withdrawl and deposit to FiatBalance and Balance

app/models/balance.rb

@@ -25,5 +25,13 @@ class Balance < ApplicationRecord
   belongs_to :user
   belongs_to :currency
 
-  validates :amount, presence: true
+  validates :amount, presence: true, numericality: { greater_than_or_equal_to: 0 }
+
+  def withdrawal!(value)
+    update!(amount: amount - value)
+  end
+
+  def deposit!(value)
+    update!(amount: amount + value)
+  end
 end

app/models/fiat_balance.rb

@@ -24,7 +24,17 @@ class FiatBalance < ApplicationRecord
 
   monetize :amount_cents
 
+  validates :amount_cents, numericality: { greater_than_or_equal_to: 0 }
+
   def amount_formatted
     amount.format
   end
+
+  def withdrawal!(value)
+    update!(amount_cents: amount_cents - value)
+  end
+
+  def deposit!(value)
+    update!(amount_cents: amount_cents + value)
+  end
 end