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The has_one declaration on the Employee model tells Active Record that it can expect to find at least one record in the addresses table that has an employee_id matching the primary key of a row in the employees table. The Address model, in turn, declares that each of its records belongs_to a particular Employee. Telling the Address model that it belongs_to :employee is saying, in effect, that each Address object references a particular Employee. We could even go so far as to say that Employee is the parent and Address is the child. The child model is dependent on the parent and therefore references it. Figure 5-2 demonstrates the has_one relationship.
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Figure 5-2. The one-to-one relationship between employees and addresses
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CHAPTER 5 ADVANCED ACTIVE RECORD: ENHANCING YOUR MODELS
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Let s get inside a console session (./script/console) and see how this comes together. You ll be able to follow along as we create objects and relate them to one another. First, we ll create an employee and an address, as follows:
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>> => >> => employee = Employee.create(:name => 'Dwight Schrute') #<Employee:0x210dcb5> address = Profile.create(:street => '1725 Slough Avenue') #<Address:0x230dbb4>
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Although we ve successfully created a new employee and a new address, we haven t yet associated them with each other. If we ask the Employee object for its address, it will respond with nil.
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>> employee.address => nil
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To make the association happen, we specify it like any regular assignment on the
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Employee object, and then call save, like so: >> => >> => employee.address = address #<Address:0x230dbb4> employee.save true
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Assignment is assignment whether it s a name attribute to which you re assigning the value Joe or an association method to which you re assigning an object. Now when we ask the Employee object for its address, it will happily respond with one.
>> employee.address => #<Address:0x230dbb4>
That s all there is to it. While this is pretty good, we can actually do a bit better. We can create and save the address in one shot, and have it perform the association automatically, like this:
>> employee.create_address(:street => '1725 Slough Avenue ') => #<Address:0x210cbc2>
Using the create_address method for creating a new address will initialize the Address object, set its foreign key, and save it to the database. This works for any has_one association, no matter what it s named. Active Record automatically generates the create_#{association_name} method for you. So, if you had a User model set up with an association like has_one :account, you would get the create_account method automatically.
CHAPTER 5 ADVANCED ACTIVE RECORD: ENHANCING YOUR MODELS
These alternatives for doing the same thing might seem confusing, but they re really just variations on the same theme. In all cases, you are creating two objects (the parent and the child) and telling them about one another. Whether you choose to do this in a multistep operation or all on one line is entirely up to you. Earlier we said that declaring a has_one association causes Active Record to automatically add a suite of methods to make working with the association easier. Table 5-2 shows a summary of the methods that are added when we declare a has_one and belongs_to relationship between Employee and Address, where @employee is an Employee instance.
Table 5-2. Methods Added by the has_one Association in the Employee/Address Example Method
@employee.address @employee.address=(address) @employee.address.nil @employee.build_address(attributes={})
Description
Returns the associated (Address) object; nil is returned if none is found Assigns the associated (Address) object, extracts the primary key, and sets it as the foreign key Returns true if there is no associated Address object Returns a new Address object that has been instantiated with attributes and linked to @employee through a foreign key but has not yet been saved Returns a new Address object that has been instantiated with attributes and linked to @employee through a foreign key and that has already been saved
@employee.create_address(attributes={})
While we re using the Employee.has_one :address example here, the rules work for any object associated to another using has_one. Here are some examples, along with sample return values:
@user.profile #=> #<Profile:0x130dcb1> @user.profile.nil #=> false @user.build_profile(:bio => 'eats trees and leaves') #=> #<Profile:0x130dcb1> @user.create_profile(:bio => 'eats trees and leaves') #=> #<Profile:0x130dcb1>
The has_one declaration can also include an options hash to specialize its behavior if necessary. Table 5-3 lists the most common options. For a complete list of all options, consult the Rails API documentation (http://api.rubyonrails.org/classes/ActiveRecord/ Associations/ClassMethods.html).
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