Rails validate uniqueness only if conditional

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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

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