Possible to alias a belongs_to association in Rail

问题:

I have a model with a belongs_to association:

class Car < ActiveRecord::Base
  belongs_to :vendor
end

So I can call car.vendor. But I also want to call car.company! So, I have the following:

class Car < ActiveRecord::Base
  belongs_to :vendor
  def company
    vendor
  end
end

but that doesn't solve the assignment situation car.company = 'ford', so I need to create another method for that. Is there a simple alias mechanism I can use for associations? Can I just use alias_method :company, :vendor and alias_method :company=, :vendor=?

回答1:

No it doesn't look for company_id for instance change your code as follows

In Rails3

class Car < ActiveRecord::Base
   belongs_to :vendor
   belongs_to :company, :class_name => :Vendor,:foreign_key => "vendor_id"
end

In Rails4

We can use alias attribute.

alias_attribute :company, :vendor

回答2:

In Rails 4, you should simply be able to add alias_attribute :company, :vendor to your model.

回答3:

Short Version:

1. Generate model with migration

   $ rails generate model Car vendor:references name:string ...

2. Add following line in Car model i.e car.rb file

   class Car < ActiveRecord::Base
     belongs_to :company, :class_name => 'Vendor', :foreign_key => 'vendor_id'
   end
   

3. Now you have @car.company instance method.

For a Detailed explanation read ahead [Optional if you understood the above !!]

Detailed Version:

The model Car will have an association with the model Vendor (which is obvious). So there should be a vendor_id in the table cars.

1. In order to make sure that the field vendor_id is present in the cars table run the following on the command line. This will generate the right migration. The vendor:references is important. You can have any number of attributes after that.

   $ rails generate model Car vendor:references name:string

2. Or else in the existing migration for create_table :cars just add the line t.references :vendor

   class CreateCars < ActiveRecord::Migration
     def change
       create_table :cars do |t|
         t.string :name
         ...
         t.references :vendor
         t.timestamps
       end
     end
   end
   

3. The final thing that you need to do is edit the model Car. So add this code to your car.rb file

   class Car < ActiveRecord::Base
     belongs_to :company, :class_name => 'Vendor', :foreign_key => 'vendor_id'
   end
   

After you do the third step you will get the following instance methods for the model Car provided by Rails Associations

1. @car.company

   • When you do @car.company it will return a #<Vendor ...> object. To find that #<Vendor ...> object it will go look for the vendor_id column in the cars table because you have mentioned :foreign_key => 'vendor_id'

   • You can set the company for a car instance by writing

     @car.company = @vendor || Vendor.find(params[:id]) #whichever Vendor object you want
     @car.save
     

     This will save the id of that Vendor object in the vendor_id field of the cars table.

Thank You.

回答4:

class Car < ActiveRecord::Base
  belongs_to :vendor
  belongs_to :company, :class_name => :Vendor
end