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CHAPTER 5 ADVANCED ACTIVE RECORD: ENHANCING YOUR MODELS
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Table 5-5. Common has_many Options Option
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:class_name
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Specifies the class name of the association. Used when the class name can t be inferred from the association name. Specifies the conditions that the associated objects must meet in order to be included as a WHERE SQL fragment. Specifies the foreign key used for the association in the event that it doesn t adhere to convention of being the lowercase, singular name of target class with _id appended. Specifies the order in which the associated objects are returned as an ORDER BY SQL fragment. Specifies that the associated objects should be removed when this object is. If set to :destroy, the associated objects are deleted using the destroy method. If set to :delete, the associated objects are deleted without calling their destroy method. If set to :nullify, the associated objects foreign keys are set to NULL.
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has_many :registered_users, :class_name => 'User'
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has_many :events, :conditions => "active = 1"
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has_many :events, :foreign_key => 'activity_id'
:order
has_many :events, :order => "occurs_on DESC" has_many :events, :dependent => :destroy
:dependent
There s much more to has_many associations than we can possibly hope to cover here, so be sure to check out the Rails API documentation (http://api.rubyonrails.org/ classes/ActiveRecord/Associations/ClassMethods.html) for the full scoop.
Applying Association Options
It s time to apply what you ve learned to our domain model. Specifically, we ll use the :order option to apply a default order to the User.has_many :events declaration, and use the :dependent option to make sure that when we delete a user, all their events are deleted as well.
CHAPTER 5 ADVANCED ACTIVE RECORD: ENHANCING YOUR MODELS
Specifying a Default Order
When we access a user s events, we want to make sure that they come back in the order in which they will occur. Specifically, we want the oldest to be at the bottom and the furthest in the future to be at the top of the list. We can do this by configuring our has_many association with a default order using the :order option. Add the :order option to the has_many :events declaration, as shown in Listing 5-5.
Listing 5-5. :order Option Added to has_many
has_many :events, :order => "occurs_on DESC"
We give the name of the field that we want to order by, and then we say either ASC (ascending) or DESC (descending) to indicate the order in which the results should be returned. Since time moves forward (to bigger numbers), we want to make sure that we are going back in time, so we use the DESC keyword here.
Note
ASC and DESC are SQL keywords. We re actually specifying a SQL fragment here, which we ll
discuss in the Building Conditions for Advanced Finding section later in this chapter.
We can also specify a secondary order by adding a comma in between arguments. Let s say we wanted to sort by the title of the event after we sort by the date. So, if two events occur on the same day, they will be ordered first by the date, and then by the lexical order of the title. Listing 5-6 shows the event title added to the :order option.
Listing 5-6. Adding the Title to the :order Option for has_many
has_many :events, :order => "occurs_on DESC, title ASC"
You ll notice that we use ASC for ordering on the title. This is because as letters go up in the alphabet, their value goes up. So, to sort alphabetically, use the ASC keyword.
Specifying Dependencies
Frequently, you ll have dependencies between your models. For instance, in our events application, if we delete users, we want to make sure they don t have events in the system. Said another way, an Event is dependent on its User. We can let Active Record take
CHAPTER 5 ADVANCED ACTIVE RECORD: ENHANCING YOUR MODELS
care of this for us automatically by specifying the :dependent option to our association. Listing 5-7 shows all the options to has_many :events, including the :dependent option.
Listing 5-7. :dependent Option Added to has_many
has_many :events, :order => "occurs_on DESC, title ASC", :dependent => :destroy
By passing in the symbol :destroy, we are saying that not only are events dependent, but also that when the owner is deleted, we want to call the destroy method on every related event. This ensures that any *_destroy callbacks on the event instances will be called (we ll talk about callbacks later in this chapter, in the Making Callbacks section). If we wanted to skip the callbacks, we could use the :delete option in place of :destroy, which will delete the records directly via SQL. Let s say we just wanted to set the foreign key column (user_id) to NULL in the events table, instead of completely destroying the event. This would essentially orphan the events. We can do this by using the :nullify option in place of :destroy.
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