cant debit/credit a negative

app/models/wallet.rb

@@ -5,12 +5,18 @@ class Wallet < ActiveRecord::Base
   validates_numericality_of :amount, greater_than_or_equal_to: 0
 
   def credit(amount)
-    self.amount += amount
-    save
+    if amount.is_a?(Numeric) and amount >= 0
+      self.amount += amount
+      return save
+    end
+    false
   end
 
   def debit(amount)
-    self.amount -= amount
-    save
+    if amount.is_a?(Numeric) and amount >= 0
+      self.amount -= amount
+      return save
+    end
+    false
   end
 end

test/models/wallet_test.rb

@@ -25,4 +25,15 @@ class WalletTest < ActiveSupport::TestCase
     assert w.debit(5)
     assert_equal w.amount, expected_amount
   end
+
+  test 'cant debit/credit a negative' do
+    w = Wallet.new
+    assert_not w.credit(-5)
+    assert_not w.debit(-5)
+  end
+
+  test 'wallet amount cant < 0 after debit' do
+    w = Wallet.new
+    assert_not w.debit(5)
+  end
 end