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I have bought this wonderful machine a computer Now I am rather an authority on gods, so I identified the machine it seems to me to be an Old Testament god with a lot of rules and no mercy Joseph Campbell
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The Validations API in Active Model, along with its supplementary functionality in Active Record allows you to declaratively define valid states for your model objects The validation methods hook into the life cycle of an Active Record model object and are able to inspect the object to determine whether certain attributes are set, have values in a given range, or pass any other logical hurdles that you specify In this chapter, we ll describe the validation methods available and how to use them effectively We ll also explore how those validation methods interact with your model s attributes and how the built-in error-messaging system messages can be used effectively in your application s user interface to provide descriptive feedback Finally, we ll cover how to use Active Model s validation functionality in your own, non-Active Record classes
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81 Finding Errors
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Validation problems are also known as (drumroll please ) errors! Every Active Record model object contains a collection of errors, accessible (unsurprisingly) as the errors attribute It s an instance of the class ActiveModel::Errors that extends ActiveSupport::OrderedHash
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8: Validations
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When a model object is valid, the errors collection is empty In fact, when you call valid on a model object, a series of steps to find errors is taken as follows (slightly simplified): 1 Clear the errors collection 2 Run validations 3 Return whether the model s errors collection is now empty or not If the errors collection ends up empty, the object is valid In cases where you have to write actual validation logic yourself, you mark an object invalid by adding items to the errors collection using its add methods Simple as that We ll cover the methods of the Errors class in some more detail later on It makes more sense to look at the validation methods themselves first
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82 The Simple Declarative Validations
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Whenever possible, you should set validations for your models declaratively by using one or more of the following class methods available to all Active Record classes Unless otherwise noted, all of the validates methods accept a variable number of attributes, plus options There are some options for these validation methods that are common to all of them, and we ll cover them at the end of the section
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Many web applications have screens in which the user is prompted to agree to terms of service or some similar concept, usually involving a check box No actual database column matching the attribute declared in the validation is required When you call this method, it will create virtual attributes automatically for each named attribute you specify I see this validation as a type of syntax sugar since it is so specific to web application programming
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class Account < ActiveRecord::Base validates_acceptance_of :privacy_policy, :terms_of_service end
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You can use this validation with or without a boolean columns on the table backing your model An attribute will be created if necessary Choose to store the value in the database only if you need to keep track of whether the user accepted the term, for auditing or other reasons Mind you, not accepting the term would prevent creation of the record, but it s good to know what is supported
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When the validates_acceptance_of validation fails, an error message is stored in the model object reading attribute must be accepted The :accept option makes it easy to change the value considered acceptance The default value is "1", which matches the value supplied by check boxes generated using Rails helper methods
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class Cancellation < ActiveRecord::Base validates_acceptance_of :account_cancellation, :accept => 'YES' end
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If you use the preceding example in conjunction with a text field connected to the account_cancellation attribute, the user would have to type the word YES in order for the cancellation object to be valid
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