How do you update one object of a has_many :through relationship

Hi guys.

How do you update nested attributes for one object of a
has_many :through relationship? So I have an entity model and this
model can have many fact_values. The difference between fact and
fact_value is that the latter has an extra value attribute. This is
because facts can be common among many entities, but each entity can
have its own unique value for a specific fact. So my models are the

class Entity
  has_many :fact_values
  has_many :facts, :through => :fact_values

class FactValue
  belongs_to :entity
  belongs_to :fact

class Fact
  has_many :fact_values
  has_many :entities, :through => :fact_values

Now I've specified that Entity
accepts_nested_attributes_for :fact_values. But I want it to accept
nested attributes for a single (pre-defined) fact_value and update
only that fact_value. The way I'm going about it right now is to have
a virtual attribute in my Entity model representing that one
fact_value and update that fact_value in the entity controller. I'm
wondering if there is any automatic way to accomplish what I'm trying
to do?


I'm not sure exactly what the problem is. If you only want to update
one object, you only put that object in the fields_for(:fact_values)
part of the form.
If you're worried about the wrong object being created/updated through
accepts_nested_attributes, then the :reject_if can enable you to add

Perhaps something along the lines of

class Entity

  has_many :fact_values
  has_many :facts, :through => :fact_values
  belongs_to :anointed_fact_value

   accepts_nested_attributes_for :fact_values, :reject_if =>

   def not_updatable_fact_value(attrs)
      attrs[:id] != anointed_fact_value_id

I’m using formtastic, I see that it seems simple enough with the standard rails forms. I suppose I can ask on a formtastic gorup to see what they say.