Override a form field value in rails form - ruby-on-rails-3

Question

Note: I was overthinking things when I originally asked this question. The accepted answer is correct for the examples I gave - i.e. you can just pass :value to text_field, however I'd actually been having problems with date_select, which doesn't have a facility to override the value set.
As a result this has now been updated in Rails, so you can set :selected => a_date, and it will work as expected. This will be in Rails 4.
I have a model that can inherit its value from a parent model. It works something like this:
class User < ActiveRecord::Base
attr_accessible :field_name
belongs_to :company
def field_name
if self['field_name'].nil?
company['field_name']
else
self['field_name']
end
end
end
class Company < ActiveRecord::Base
attr_accessible :field_name
end
I then have a form to edit the User, but of course, if the User value is nil, then it populates the form with the value from Company, which is not what I want.
I would like to be able to override the value of the form field, so that if the User value is nil, then the value is empty.
Attempt 1
Ideally I'd be able to do:
<%= form_for #user do |f| %>
<%= f.text_field :field_name, #user['field_name'] %>
<% end %>
But that doesn't work, there doesn't seem to be a mechanism for providing an override value.
Attempt 2
So I thought about creating a second getter/setter:
def field_name_uninherited
self['field_name']
end
def field_name_uninherited=(value)
self['field_name']=value
end
Now I can use <%= f.text_field :field_name_uninherited %> and it works as expected - great! Except: when field_name is a date, or other type using multiparameter attributes, it results in this error:
1 error(s) on assignment of multiparameter attributes
I believe this is because it doesn't know that this is a date field, as it infers this from the database, and this field (with _uninherited suffix) is not in the database.
So I need some way to mark my additional method as the same type as the original database field.
A further note, the above examples (using field_name) are a simplified version. I'm actually using https://github.com/colinbm/inherits_values_from to handle the inheritance, but I don't think this is important to the question.
Obviously if there's a better way to accomplish the same goal, then I'm all ears.

Answer 1

So when it comes to displaying the value you for a user you want it to behave a bit differently?
What I'd do is use the :value option with your form field. That way you get to set the value like normal but choose what you want displayed in the form field.
<%= f.text_field :company, :value => user.field_name_uninherited %>

Answer 2

For what I understand, you want the user to put the field data and only if it's nil, populate that value with the parent (company) model. It seems to me before_save works perfectly, because it is called (as it name proposes) just before the save method is called on an ActiveRecord object.
Thus you can write this kind of callback:
class User < ActiveRecord::Base
attr_accessible :field_name
before_save :override_field
private
def override_field
if self.field_name.nil?
self.field_name = company.field_name
end
end
This way, you'll be only overriding the value if it's nil at the moment of saving, leaving that form field empty at the moment of creating a new element. Hope this works!