Validation Based Upon Associated Record

I have two models: User and Lesson. I only want lessons to be assigned to users that are admins.

What is the best way to ensure this? I am currently attempting to create a custom validator like so:

class BelongsToAdminValidator < ActiveModel::EachValidator  
  def validate_each(object, attribute, value)  
    unless value.admin?
      object.errors[attribute] << (options[:message] || "must belong to an admin")  
    end  
  end  
end

But this leads to Rspec saying:

undefined method `admin?开发者_高级运维' for nil:NilClass

Which makes sense.

Is a custom validator the best way to do this? Or should I be checking if the to-be-assigned user is an admin in a controller?

I think that the easiest way of doing it is:

# Lesson model
validate :allow_only_admins

private
def allow_only_admins
  errors.add(:user, "must be admin user!") if (user.blank? || !user.admin?)
end

I assumed that you have association named user and that your user object responds to admin? method.