Question

I have a problem with the scoped uniqueness validation in Rails. It works fine if I try to create a new object with the same set of attributes that I don’t want to repeat, directly in the child model, but when I try to create a parent with two childs that are not unique, the validation is not triggered.

Background

I have an application in Rails 3.2, with its views in HAML with simple_form.

I have two models: Page and Property. A page can have many properties, and it accepts nested attributes for property.

I want to validate that a Page must not have two properties with the same name:

#app/models/page.rb
class Page < ActiveRecord::Base
  has_many :properties
  accepts_nested_attributes_for :properties, :allow_destroy => :true
end


#app/models/property.rb
class Property < ActiveRecord::Base
  belongs_to :page
  VALID_PROPERTIES = %w(id text name xpath class css)
  validates :name, :inclusion => VALID_PROPERTIES, :uniqueness => {:scope => :page_id}
end

Of course, the property has a page_id attribute.

Like I said, when creating a new property via its form, the validation works. If I try to create a new property with the same name and the same page_id, Rails tells me that the name has already been taken.

The issue

If I create a new Page, and via nested forms, assign various properties, I am able to bypass this validation. It appears to be only a problem when a combination of page_id and property_id aren’t already present in the database, so for example if I edit a Page model, which already has a property saved, and I try to add a new one with the same name, the validation now triggers.

Answer 1

I would try with validates_associated :

class Page < ActiveRecord::Base
  has_many :properties
  accepts_nested_attributes_for :properties, :allow_destroy => :true
  validates_associated :properties 
end

Update

The Rails guide on validation states:

The validation happens by performing an SQL query into the model’s
table, searching for an existing record with the same value in that
attribute.

The 2 Properties object that you’re creating do not exists yet in the database, so the uniqueness validation can’t work. You should try with a custom validation

class Property < ActiveRecord::Base
  #...
  validate :name, :name_uniqueness

  def name_uniqueness 
    self.page.properties.select {|p| p.key == self.key}.size == 1
  end
end