Ruby on Rails: How to validate nested attributes on certain condition?

First of all, I want to be sure that you mean blank? rather than present?. Typically, I see this:

validate :address, :presence_of => true, :if => 'organisation.present?'

Meaning, you only want to validate address if organisation is also present.

Regarding, :accepts_nested_attributes_for, are you using this feature by passing in nested form attributes, or some such thing? I just want to make sure you absolutely need to use this functionality. If you are not actually dealing with nested form attributes, you can implement cascading validation using:

validates_associated :address

If you do need to use :accepts_nested_attributes, be sure to check out the :reject_if parameter. Basically, you can reject adding an attribute (and it's descendants) altogether if certain conditions apply:

accepts_nested_attributes_for :address, :allow_destroy => true, :reject_if => :no_organisation

def no_organisation(attributes)
  attributes[:organisation_id].blank?
end

Now, if none of the above apply, let's take a look at your syntax:

It should work, :if/:unless take symbols, strings and procs. You don't need to point to the foreign_key, but can simplify by pointing to:

:if => "organisation.blank?"

You have other validations in the Address model, correct? Is Address being validated when you don't want it to? Or is Address not being validated? I can help you test it out in the console if you can give me some additional details.

---

1. To make things easier for myself re: mass-assignment, I changed the rails config: config.active_record.whitelist_attributes = false
2. I created a gist for you to follow along

3. I have a sample project as well. Let me know if you are interested.

   Basic points:

4. Added the following to Person to ensure that either Org or Address are valid:

   validates_presence_of :organisation, :unless => "address.present?" validates_associated :address, :unless => "organisation.present?"

5. Added validation to Address to trigger errors when Org is not present: validates_presence_of :line1, :line2, :city, :zip

   I was able to produce the requirements you are seeking. Please look at the gist I created where I have a full console test plan.

---

I added a controller file to the previous gist.

Overview:

1. All you should need to create the person is: @person = current_user.people.build(params[:person])
2. :organisation_id will always be found off of the :person param node, like so: params[:person][:organisation_id] So you're if will never be true.

---

I updated the gist with the necessary changes to the controller, the model and the form.

Overview:

1. You need to cleanup your controller. You are using accepts_nested_attribute, so in the :create, you only care about params[:person]. Additionally, in the render :new, you need to setup any instance variables that the partial will use. This does NOT go back through the :new action. The :new and :edit actions also need to be simplified.
2. Your Person model needs to use the :reject_if argument because the Address fields are coming back to the :create action as :address_attributes => {:line1 => '', :line2 => '', etc}. you only want to create the association if any have values. Then your validates_presence_of for :organisation will work just fine.

3. Your form needs to pass the organisation id to the controller, rather than the organisation names

   It's all in the gist

---

Should be the final gist.

Overview:

1. Add the following to your edit action right after building the @person:

   @person.build_address if @person.address.nil? This ensure that you have the address inputs, even if the @person.address does not exist. It doesn't exist, because of the :reject_if condition on accepts_nested_attributes

2. I DRYed up the :reject_if as follows. It's a little hacky, but has some utility:

   accepts_nested_attributes_for :address, :allow_destroy => true, :reject_if => :attributes_blank?
   
   def attributes_blank?(attrs)  
     attrs.except('id').values.all?(&:blank?)  
   end  
   

   a. attrs -> the result of params[:person][:address]
   b. .except('id') -> return all key-values except for 'id'
   c. .values -> return all values from a hash as an array
   d. .all? -> do all elements in the array satisfy the following check
   e. &:blank -> ruby shorthand for a block, like this: all?{ |v| v.blank? }