Add Custom Validation Message as a rails til

README.md

@@ -219,6 +219,7 @@ _358 TILs and counting..._
 - [Code Statistics For An Application](rails/code-statistics-for-an-application.md)
 - [Conditional Class Selectors in Haml](rails/conditional-class-selectors-in-haml.md)
 - [Creating Records of Has_One Associations](rails/creating-records-of-has-one-associations.md)
+- [Custom Validation Message](rails/custom-validation-message.md)
 - [Hash Slicing](rails/hash-slicing.md)
 - [Ignore Poltergeist JavaScript Errors](rails/ignore-poltergeist-javascript-errors.md)
 - [Migrating Up Down Up](rails/migrating-up-down-up.md)

rails/custom-validation-message.md

@@ -0,0 +1,31 @@
+# Custom Validation Message
+
+When using Rails validations, a standard error message will be provided
+whenever there is a violation. Consider the scenario when there is a
+uniqueness validation on the email attribute and it is violated:
+
+```ruby
+# User model
+validates_uniqueness_of :email
+
+# Users controller
+new_user.errors.full_messages
+#=> ["Email has already been taken"]
+```
+
+Sometimes you don't want the default validation message. The validation
+declaration can be given a `message` option to specify an alternate
+validation message.
+
+```ruby
+# User model
+validates_uniqueness_of :email, message: 'is not available'
+
+# Users controller
+new_user.errors.full_messages
+#=> ["Email is not available"]
+```
+
+Keep in mind that `full_messages` will prepend the model name to the front
+of the message. You'll want to ensure that the resulting message is
+coherent.