如何制作模型在Rails 4中不包含重复项

Question

I want an elegant way to make this work in rails 4:

# this is a neted_attribute for the model User
class Category < ActiveRecord::Base
  belongs_to :user
  attr_accessible name, code

  # to make sure that when the user try to create a Category where exists
  # a category that has the same attributes it assigns
  # the current category to the existing one
  before_save do
    self = Category.where(name: name, code: code).first_or_initialize
  end
end

You don't need to be an expert to that this won't even parse. It is merely an example to convey my idea.

Just to be clear I don't want validations (I'm very well aware of them). And I can achieve this in many ways, but they would be too hacky and complex for a general and common problem like this.

What I want is a way to enforce this feature in the model itself regardless of its relation with other models.

Thanks in advance for your time and efforts.

Answer 1

You cannot do this. As you probably noted, Ruby does not allow you to assign to self , as the concept makes no sense. In the before_save filter, you could change id to match the existing record, but the save would fail because ActiveRecord would try to insert a new record in the database with the same id as an existing record, resulting in a database uniqueness violation.

Answer 2

In a concern:

module CreateCategory
  def find_or_create_category(name, code)
    cat = Category.where(name: name, code: code).first_or_create
    self.categories << cat
    cat
  end
end

In user.rb:

include CreateCategory

When you create the category, use @category = user.find_or_create_category(name, code)