how to generate barcode in c# net with example Applying Validations in Font

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Applying Validations
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It s probably a safe bet that you don t want every field in your tables to be optional. Certain fields need to be required, terms of service agreements need to be accepted, and passwords need to be confirmed. That s just the way it is when you re building web applications, and Rails understands this. Consider this example of a User model:
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class User < ActiveRecord::Base validates_presence_of :username validates_confirmation_of :password validates_acceptance_of :terms_of_service end
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CHAPTER 5 ADVANCED ACTIVE RECORD: ENHANCING YOUR MODELS
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Like associations, validations are a set of high-level macros that let you selectively apply common validation requirements to your model s attributes. In this section, we ll create a full set of validations for our events application, and you ll see first-hand how easy it is to perform basic validations with Active Record. We re going to start by building a couple custom validation methods, and then we will apply some of the built-in validations.
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Building Custom Validation Methods
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In our events application, we would like to make sure that no one creates an event that happens in the past. First, we need to create a method so that we can ask an Event if its occurs_on field is in the past. This method is useful outside validations, because we might want to indicate on the interface later when an event has happened. Let s add that method now and call it is_in_the_past . Add the code shown in Listing 5-17 to the Event model.
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Listing 5-17. is_in_the past Method, in app/models/event.rb
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def is_in_the_past occurs_on < Date.today end
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This gets us a step closer to our goal. When building validations, Active Record gives us a nice object to use called errors. Whenever we want to add a validation error to the list of errors, we just say event.add(column_name, error_message). So, lets implement a method called has_not_occurred that uses this functionality, as shown in Listing 5-18.
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Listing 5-18. has_not_occurred Method, in app/models/event.rb
def has_not_occurred errors.add("occurs_on", "is in the past") if occurs_on && is_in_the_past end
This will check if we should apply the error by evaluating the if statement. If that if statement is true, we want to add an error into the errors object. Note that before we test to see if it s in the past, we make sure that occurs_on is not nil. This is so that our test won t throw an error. If occurs_on is nil, that will be handled by another validator. So, how do we tell Active Record that this method should be run before a save We use the validate class method and pass it a symbol with the name of the method. At the top of our Event class, add the code shown in Listing 5-19.
CHAPTER 5 ADVANCED ACTIVE RECORD: ENHANCING YOUR MODELS
Listing 5-19. validate Method, in app/models/event.rb
class Event < ActiveRecord::Base #... validate :has_not_occurred #... end
This just lets Active Record know to pay attention to our new has_not_occurred method. Later, we ll write tests to make sure that this is working. But you can also go to the console and try to create an invalid object to see if it will report back errors for you. The easiest way to get to errors in an Active Record object is with events. errors.full_messages. After you do that, let s look through the built-in validations.
Using Built-in Validations
Rails has a myriad of built-in validators. We re going to cover a few of them here, as we apply them to our events application. Check the API for details on all the Rails validators (http://api.rubyonrails.com/classes/ActiveRecord/Validations/ClassMethods.html). Just as a reference as we get started, there are two common options that you can pass into any built-in validator. These are described in Table 5-6.
Table 5-6. Default Options for All Validators Option
:message :on
Description
Specifies the error message shown if validation fails. Specifies when this validation happens. The default is :save. Other options are :create and :update.
Example
:message => "too long" :on => :create
Validating That a Value Has Been Entered
You can use validates_presence_of to make sure that a user has entered something into a field. This is very useful in many cases. In fact, we are going to add this validator to our Event model, as shown in Listing 5-20.
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