Rails conditional validation in model

I have a Rails 3.2.18 app where I'm trying to do some conditional validation on a model.

In the call model there are two fields :location_id (which is an association to a list of pre-defined locations) and :location_other (which is a text field where someone could type in a string or in this case an address).

What I want to be able to do is use validations when creating a call to where either the :location_id or :location_other is validated to be present.

I've read through the Rails validations guide and am a little confused. Was hoping someone could shed some light on how to do this easily with a conditional.

I believe this is what you're looking for:

class Call < ActiveRecord::Base
  validate :location_id_or_other

  def location_id_or_other
    if location_id.blank? && location_other.blank?
      errors.add(:location_other, 'needs to be present if location_id is not present')
    end
  end
end

location_id_or_other is a custom validation method that checks if location_id and location_other are blank. If they both are, then it adds a validation error. If the presence of location_id and location_other is an exclusive or, i.e. only one of the two can be present, not either, and not both, then you can make the following change to the if block in the method.

if location_id.blank? == location_other.blank?
  errors.add(:location_other, "must be present if location_id isn't, but can't be present if location_id is")
end

Alternate Solution

class Call < ActiveRecord::Base
  validates :location_id, presence: true, unless: :location_other
  validates :location_other, presence: true, unless: :location_id
end

This solution (only) works if the presence of location_id and location_other is an exclusive or.

Check out the Rails Validation Guide for more information.