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