Scopes in Java

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91 Scopes
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That s pretty nice, but what if we don t want our default scope to be included in our queries In this case Active Record takes care of us through the unscoped method
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>> Timesheetunscopedorder("submitted_at DESC") => [#<Timesheet id: 2, status: "submitted">]
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Similarly to overriding our default scope with a relation when creating new objects, we can supply unscoped as well to remove the default attributes
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>> Timesheetunscopednew => #<Timesheet id: nil, status: nil>
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917 Using Scopes for CRUD
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You have a wide range of Active Record s CRUD methods available on scopes, which gives you some powerful abilities For instance, let s give all our underutilized timesheets some extra hours
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Active Record
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>> utimesheetsunderutilizedcollect(&:total_hours) => [37, 38] >> utimesheetsunderutilizedupdate_all("total_hours = total_hours + 2") => 2 >> utimesheetsunderutilizedcollect(&:total_hours) => [37, 38] # whoops, cached result >> utimesheets(true)underutilizedcollect(&:total_hours) => [39] # results after telling association to reload
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Scopes including a where clause using hashed conditions will populate attributes of objects built off of them with those attributes as default values Admittedly it s a bit difficult to think of a plausible use case for this feature, but we ll show it in an example First, we add the following scope to Timesheet:
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scope :perfect, submittedwhere(:total_hours => 40)
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Now, building an object on the perfect scope should give us a submitted timesheet with 40 hours
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> Timesheetperfectbuild => #<Timesheet id: nil, submitted: true, user_id: nil, total_hours: 40 >
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As you ve probably realized by now, the new Arel underpinnings of Active Record are tremendously powerful and truly elevate the Rails 3 platform
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9: Advanced Active Record
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92 Callbacks
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This advanced feature of Active Record allows the savvy developer to attach behavior at a variety of different points along a model s life cycle, such as after initialization, before database records are inserted, updated or removed, and so on Callbacks can do a variety of tasks, ranging from simple things such as logging and massaging of attribute values prior to validation, to complex calculations Callbacks can halt the execution of the life-cycle process taking place Some callbacks can even modify the behavior of the model class on the fly We ll cover all of those scenarios in this section, but first let s get a taste of what a callback looks like Check out the following silly example:
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class Beethoven < ActiveRecord::Base before_destroy :last_words protected def last_words loggerinfo "Friends applaud, the comedy is over" end end
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So prior to dying (ehrm, being destroy d), the last words of the Beethoven class will always be logged for posterity As we ll see soon, there are 14 different opportunities to add behavior to your model in this fashion Before we get to that list, let s cover the mechanics of registering a callback
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921 Callback Registration
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Overall, the most common way to register a callback method is to declare it at the top of the class using a typical Rails macro-style class method However, there s a less verbose way to do it also Simply implement the callback as a method in your class In other words, I could have coded the prior example as follows:
class Beethoven < ActiveRecord::Base protected def before_destroy loggerinfo "Friends applaud, the comedy is over" end end
This is a rare case of the less-verbose solution being bad In fact, it is almost always preferable, dare I say it is the Rails way, to use the callback macros over implementing
92 Callbacks
callback methods, for the following reasons:
Macro-style callback declarations are added near the top of the class definition,
making the existence of that callback more evident versus a method body potentially buried later in the file
Macro-style callbacks add callback methods to a queue That means that more than
one method can be hooked into the same slot in the life cycle Callbacks will be invoked in the order in which they were added to the queue
Callback methods for the same hook can be added to their queue at different levels
of an inheritance hierarchy and still work they won t override each other the way that methods would
Callbacks defined as methods on the model are always called last
922 One-Liners
Active Record
Now, if (and only if) your callback routine is really short,1 you can add it by passing a block to the callback macro We re talking one-liners!