7: Active Record Associations in Java

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7: Active Record Associations
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>> u = Userfirst >> uavatar => #<Avatar id: 1, url: "/avatars/smiling", user_id: 1> >> ucreate_avatar(:url => '/avatars/frowning') => #<Avatar id: 2, url: "/avatars/4567", user_id: 1> >> Avatarall => [#<Avatar id: 1, url: "/avatars/smiling", user_id: nil>, #<Avatar id: 2, url: "/avatars/4567", user_id: 1>]
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The last line from that console session is the most interesting, because it shows that our initial avatar is now no longer associated with the user Of course, the previous avatar was not removed from the database, which is something that we want in this scenario So, we ll use the :dependent => :destroy option to force avatars to be destroyed when they are no longer associated with a user
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class User < ActiveRecord::Base has_one :avatar, :dependent => :destroy end
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With some additional fiddling around in the console, we can verify that it works as intended In doing so, you might notice that Rails only destroys the avatar that was just removed from the user, so bad data that was in your database from before will still remain Keep this in mind when you decide to add :dependent => :destroy to your code and remember to manually clear orphaned data that might otherwise remain
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Using has one together with has many As I alluded to earlier, has_one is sometimes used to single out one record of significance alongside an already established has_many relationship For instance, let s say we want to easily be able to access the last timesheet a user was working on:
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class User < ActiveRecord::Base has_many :timesheets has_one :latest_sheet, :class_name => 'Timesheet', :order => 'created_at desc' end
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I had to specify a :class_name, so that Active Record knows what kind of object we re associating (It can t figure it out based on the name of the association, :latest_sheet) When adding a has_one relationship to a model that already has a has_many defined to the same related model, it is not necessary to add another belongs_to method call to the target object, just for the new has_one That might seem a little counterintuitive
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76 One-to-One Relationships
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at first, but if you think about it, the same foreign key value is being used to read the data from the database
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has one options
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The options for has_one associations are similar to the ones for has_many For your convenience, we briefly cover the most relevant ones here
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Allows you to set up a polymorphic association; this is covered in 9
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:class name
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Allows you to specify the class this association uses When you re doing has_one :latest_timesheet, :class_name => 'Timesheet', :class_name => 'Timesheet' specifies that latest_timesheet is actually the last Timesheet object in the database that is associated with this user Normally, this option is inferred by Rails from the name of the association
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Active Record
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:conditions
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Allows you to specify conditions that the object must meet to be included in the association The conditions are specified the same as if you were using ActiveRecord::Base#find
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class User < ActiveRecord::Base has_one :manager, :class_name => 'Person', :conditions => {:type => "manager"}
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Here manager is specified as a person object that has type = "manager" I tend to almost always use :conditions in conjunction with has_one When Active Record loads the association, it s grabbing one of potentially many rows that have the right foreign key Absent some explicit conditions (or perhaps an :order clause), you re leaving it in the hands of the database to pick a row
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:dependent
The :dependent option specifies how Active Record should treat associated objects when the parent object is deleted (The default is to do nothing with associated objects, which will leave orphaned records in the database) There are a few different values that you can pass, and they work just like the :dependent option of has_many If you pass :destroy to it, you tell Rails to destroy the associated object when it is no longer associated with the primary object Passing :delete will destroy the associated