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CHAPTER 5 ADVANCED ACTIVE RECORD: ENHANCING YOUR MODELS
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Listing 5-20. validates_presence_of Method, in app/models/event.rb
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class Event < ActiveRecord::Base validates_presence_of :title, :location end
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The default message is can t be blank.
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Validating That a Value Is Unique
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Often, you will want to make sure that a certain field is unique. The validates_ uniqueness_of method validates whether the value of the specified attributes are unique across the system. We will use this method in our User model to make sure that each login name is unique, as shown in Listing 5-21.
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Listing 5-21. validates_uniqueness_of Method, in app/models/user.rb
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class User < ActiveRecord::Base validates_uniqueness_of :login end
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When the record is created, a check is performed to make sure that no record exists in the database with the given value for the specified attribute (that maps to a column). When the record is updated, the same check is made but disregarding the record itself. The default error message is #{name} has already been taken. The validates_uniqueness_of method can also validate whether the value of the specified attributes are unique based on multiple scope parameters. For example, you could use it to make sure that a teacher can be on the schedule only once per semester for a particular class.
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class Schedule < ActiveRecord::Base validates_uniqueness_of :teacher_id, :scope => [:semester_id, :class_id] end
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Validating the Length or Size
Sometimes you want to validate the length, or size, of a field entry. You can do this by using the validates_length_of method. We ll use this method in our User model to specify a valid range of characters for a login name, as shown in Listing 5-22. The option for specifying a range of values is :within.
CHAPTER 5 ADVANCED ACTIVE RECORD: ENHANCING YOUR MODELS
Listing 5-22. validates_length_of Method, in app/models/user.rb
class User < ActiveRecord::Base validates_length_of :login, :within => 4..40 end
If you want to ensure only the minimum or maximum, you can use the :minimum or
:maximum attributes. Table 5-7 lists the validates_length_of method options.
Table 5-7. Options for validates_length_of Option
:minimum :maximum :is :within :allow_nil :too_long :too_short :wrong_length :message
Description
Specifies the minimum size of the attribute Specifies the maximum size of the attribute Specifies the exact size of the attribute Specifies the valid range (as a Ruby Range object) of values acceptable for the attribute Specifies that the attribute may be nil; if so, the validation will be skipped Specifies the error message to add if the attribute exceeds the maximum Specifies the error message to add if the attribute is below the minimum Specifies the error message to add if the attribute is of the wrong size Specifies the error message to add if :minimum, :maximum, or :is is violated
Validating the Format of an Attribute
The validates_format_of method checks whether a value is in the correct format. Using this method requires familiarity with regular expressions (regex) or being able to steal other people s regular expressions. The classic example (and the one we need) is email. Add the method shown in Listing 5-23 to our User model.
Listing 5-23. validates_format_of Method, in app/models/user.rb
class User < ActiveRecord::Base validates_format_of :email, :with => /^[^@][\w.-]+@[\w.-]+[.][a-z]{2,4}$/i end
CHAPTER 5 ADVANCED ACTIVE RECORD: ENHANCING YOUR MODELS
Don t be put off by how complicated this looks. We simply pass in the :with option and a regex object to say what patterns we want to match.
If you want to learn more about using regular expressions, you will find many tutorials and books on Tip
the subject. One good reference is Regular Expression Recipes (Apress, 2004).
Validating Confirmation
Whenever a user changes an important piece of data (especially the password), you may want the user to confirm that entry by typing it again. This is the purpose of the validates_confirmation_of method. When you use this helper, you create a new virtual attribute called #{field_name}_confirmation. Let s add this to our User model for password confirmation, as shown in Listing 5-24.
Listing 5-24. validates_confirmation_of Method, in app/models/user.rb
class User < ActiveRecord::Base validates_confirmation_of :password end
The password attribute is a column in the users table, but the password_confirmation attribute is virtual. It exists only as an in-memory variable for validating the password. This check is performed only if password_confirmation is not nil and runs whenever the user saves the object.
Making Callbacks
You will often want to have things happen during the life cycle of the model. Certain actions need to happen during certain events pertaining to a particular model. For instance, what if you wanted to send out an email to your administrator whenever someone canceled her account Or perhaps you want to make sure to create a new model because some other model was also created. These are just a couple of examples of when you want certain actions in the life of a model to generate some associated actions. To implement this, Active Record has callbacks. Six callbacks are commonly used in Active Record models: before_create after_create
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