ruby on rails - Validation Based Upon Associated Record -

i have 2 models: user , lesson. want lessons assigned users admins.

what best way ensure this? attempting create custom validator so:

class belongstoadminvalidator < activemodel::eachvalidator     def validate_each(object, attribute, value)       unless value.admin?       object.errors[attribute] << (options[:message] || "must belong admin")       end     end   end 

but leads rspec saying:

undefined method `admin?' nil:nilclass 

which makes sense.

is custom validator best way this? or should checking if to-be-assigned user admin in controller?

i think easiest way of doing is:

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

i assumed have association named user , user object responds admin? method.