A simple module that allows validation of only certain attributes of any given model. Created for CoverHound.com.

ConditionalValidations.rb

#
# At CoverHound, we use conditional validations all over the form. However, there is no proper way to do
# this in Rails. Instead, we can provide an array of attributes (validated_fields attribute)
# and ensure they are the only ones to get validated.
#

module ConditionalValidations
  attr_accessor :validated_fields
  
  def field_is_required?(field)
    unless validated_fields.blank?
      validated_fields.include?(field)
    end
  end

  def conditionally_validate(attribute, options=nil)
    unless options.nil? 
      # Passing options for built-in validators
      # http://guides.rubyonrails.org/active_record_validations_callbacks.html#validation-helpers
      unless options[:if].nil?
        options[:if] = "#{options[:if]} && field_is_required?(:#{attribute})"
        validates attribute, options
      else
        validates attribute, options.merge(if: "field_is_required?(:#{attribute})")
      end
    else
      # Block validation
      # will initiate a validator on attribute using a method called validate_#{attribute}
      validate :"validate_#{attribute}", if: "field_is_required?(:#{attribute})"
    end
  end
  
  def fields_valid?(fields)
    # Note that when validations are run, class variable @errors changes. As such, we have
    # to keep track of whether there were errors originally and repopulate them as necessary. 
    original_errors = self.errors.messages.count
    
    mock = self.dup                       # duplicate the model in a non-persisted way.
    mock.validated_fields = fields        # ...tell model what fields to validate.
    validity = mock.valid?                # run validations and change @errors.

    # If there were errors on the original model, run valid? so that it populates @errors.
    # Otherwise, remove any errors left over from the mock run.
    (original_errors > 0) ? self.valid? : self.errors.clear
    return validity
  end
end

ActiveRecord::Base.extend ConditionalValidations

I love this module. It's very useful. However I wonder if you could help me adapt it to my specific use case?

I have it working fine in a standard form scenario, but what I'm trying to do is set the attributes to validate on a child object through the accepts_nested_attributes_for method.

The problem is that I can put the following in the update action for my parents class in the controller:

@child.validated_fields = [ :title, :email ]

But because the attributes are being pass through the childs_attributes option in the parameters hash it isn't getting picked up by the Child model.

Can you help? I though about hacking the childs_attributes in the parameters hash but that just feels ugly and not very rails-ish.....

Woah, I just saw this! Sorry I never answered your question -- did you ever find a solution?