Rails validate uniqueness only if conditional

Question

I have a Question class:

class Question < ActiveRecord::Base   attr_accessible :user_id, :created_on    validates_uniqueness_of :created_on, :scope => :user_id end 

A given user can only create a single question per day, so I want to force uniqueness in the database via a unique index and the Question class via validates_uniqueness_of.

The trouble I'm running into is that I only want that constraint for non-admin users. So admins can create as many questions per day as they want. Any ideas for how to achieve that elegantly?

Answer 1

You can make a validation conditional by passing either a simple string of Ruby to be executed, a Proc, or a method name as a symbol as a value to either :if or :unless in the options for your validation. Here are some examples:

Prior to Rails version 5.2 you could pass a string:

# using a string: validates :name, uniqueness: true, if: 'name.present?' 

From 5.2 onwards, strings are no longer supported, leaving you the following options:

# using a Proc: validates :email, presence: true, if: Proc.new { |user| user.approved? }  # using a Lambda (a type of proc ... and a good replacement for deprecated strings): validates :email, presence: true, if: -> { name.present? }  # using a symbol to call a method: validates :address, presence: true, if: :some_complex_condition  def some_complex_condition   true # do your checking and return true or false end 

In your case, you could do something like this:

class Question < ActiveRecord::Base   attr_accessible :user_id, :created_on    validates_uniqueness_of :created_on, :scope => :user_id, unless: Proc.new { |question| question.user.is_admin? } end 

Have a look at the conditional validation section on the rails guides for more details: http://edgeguides.rubyonrails.org/active_record_validations.html#conditional-validation

Answer 2

The only way I know of to guarantee uniqueness is through the database (e.g. a unique index). All Rails-only based approaches involve race conditions. Given your constraints, I would think the easiest thing would be to establish a separate, uniquely indexed column containing a combination of the day and user id which you'd leave null for admins.

As for validates_uniqueness_of, you can restrict validation to non-admins through use of an if or unless option, as discussed in http://apidock.com/rails/ActiveRecord/Validations/ClassMethods/validates_uniqueness_of