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You may have noticed in your generated scaffolding that you use a helper method called errors.full_messages to print out a helpful error message. That helper isn t black magic; it s a bit of code that asks the model associated with the form for its list of errors (also referred to as the errors collection) and returns a nicely formatted block of HTML to show the user.
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NOTE You may have noticed that you call methods in Ruby with a dot (.) For instance, you say
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@article.errors to get the error collection back. However, Ruby documentation has an idiomatic convention of
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using the # symbol along with the class name to let the reader know that there is a method it can call on an object. For example, on the Article class, you can use the method @article.title as Article#title, because it s something that acts on a particular @article but not the Article class itself. You ve also seen that you can write the code Article.count, because you don t need to know about a particular @article, but only Article objects in general. Keep this convention in mind when you re reading Ruby documentation.
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The secret to this is that every Active Record object has an automatic attribute added to it called errors. To get started, create a fresh Article object: >> article = Article.new => #<Article id: nil, title: nil, body: nil, published_at: nil, created_at: nil, updated_at: nil, excerpt: nil, location: nil> >> article.errors.any => false This seems odd: you know this new article should have errors, because it s invalid you didn t give it a title or a body. This is because you haven t triggered the validations yet. You can cause them to occur a couple of ways. The most obvious way is to attempt to save the object: >> article.save => false Every time you ve used save before, the model has happily chirped true back to you. But this time, save returns false. This is because before the model allows itself to be saved, it runs through its gauntlet of validations, and one or more of those validations failed. You would be right to guess that if you tried article.errors.any again, it would return true: >> article.errors.any => true Let s interrogate the errors collection a little more closely with the full_messages method: >> article.errors.full_messages => ["Title can't be blank", "Body can't be blank"] Voil ! Look how helpful the model is being. It s passing back an array of error messages. If there is only one attribute that you care about, you can also ask the errors collection for a particular attribute s errors:
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>> article.errors.on(:title) => "can't be blank" Notice that because you tell it which attribute you re looking for, the message returns a slightly different result than before. What if you ask for an attribute that doesn t exist or doesn t have errors >> article.errors.on(:nonexistent) => nil You get back nil, which lets you know that you didn t find anything. Another helpful method is size, which, as your saw earlier, works with all arrays: >> article.errors.size => 2 Saving isn t the only way you can cause validations to run. You can ask a model object if it s valid : >> article.valid => false If you try that on a new object, the errors collection magically fills up with your pretty errors.
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In this chapter, you ve become familiar with using the console to work with models. You ve learned how to create, read, update, and destroy model objects. Also, you ve briefly looked into how to see the simple errors caused by the validations you set up on your model in the previous chapter. The next chapter discusses how to create relationships (called associations) between your models, and you begin to see how Active Record helps you work with your data in extremely powerful ways. It also expands on the concept of validations and shows how you can do a lot more with validates. You see that Rails provides a bevy of prewritten validators and an easy way to write your own customized validators.
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