ActionBuilding a new association in a form

I would like to use a single form to display multiple existing
associations and to permit additions to those that already exist. In
its simplest form a row in U has an association to a row R through a
row in C.

Now, in my case both R and U must exist before they may be associated
and neither may be created in the same process that creates the
association row in C. Further, the association in C contains data
unique to that association.

In the us_controller I can either use or I can use to create an empty new association for ActionView.
However, if I go with instead of then I will end
up with a nil object from the c.r_id field which will lead to errors
in the view. If I choose to use then I will avoid the nil
object error but of necessity I will also create new instances of R,
which I most definitely do not want to do.

I have looked at the railscasts on complex forms and I have purchased
and viewed the advanced forms screencasts from the Pragmatic
Programmers. I can see what is done there but I cannot seem to map
what I have seen to what I want to accomplish. In the form I want to
do a dynamic lookup on R by name and use the Iid of the selected R for
the C association. But I cannot get the form to display the selectbox
for R because for new records, unless an r instance is built along
with c then the c.r_id value is null and that is an error.

SO, how is this done? I imagine that someone who wants to add
products to an order would not wish to allow order entry clerks to add
products on the fly, so what is the technique that is used to add
multiple associations between a single U and multiple R's on one form
and at the same time does not allow instances of R to be created?


Hi, I was about to post a very similar problem although I think it is
a bit simpler so an answer would be great if someone can give us a

What I found in those tutorials about complex forms is that they
create relations on the fly to new objects (also created in the same
process) and this case is slightly different you need to establish a
relation between the object you are instantiating and an existing one.

In my case I have a model say House and I need to assign it a Category
instance, but I don't want to instantiate a new Category for that
object, I just need to associate that object with an existing category
object via its ID, how can I do that? I've been looking at fields_for
and nested models but I don't think this is the case to apply that
approach, let me know if I'm wrong.


I answered the cause my own question by tracking down exactly what the
nil object was. It turned out to be a misuse of dom_id() on my part.

I believe that the way that this problem is handled in Rails is
through the Javascript libraries. While it is possible to create both
the relation and the target at the same time (as seems canonical given
the online examples and references) it is not necessary and
appropriate safeguards in the controller will prohibit it.

In the view the question is how to reference existing row values in a
lookup table. What we need consider is what gets passed back in the
params hash. When dealing with the case where the association is the
critical element then naturally what we need pass back is a valid
foreign key reference. In the case given above that would likely be a
value assigned to params[:house][:category_id]. So, in the
controller you test that this is != nil on return (i.e in the create
or update controller methods). If it is nil then a new Category
object is being requested and you need handle that circumstance as
appropriate (Raise an error or create the object as desired).

In the view you can do the lookup on the reference table several ways
to obtain the reference row id. The standard approach is a
forms_helper select() that is dynamically populated from the reference
table, displays the some meaningful text in the drop down menu, and
returns the selection's associated row id as the value for the
foreign_key reference. An alternative is text_field-with_auto_complete

The main thing to recall is that since we are not initialising a null
record for the reference table in the controller, then we must not use
any elements of a conjectural row from that table in the view code.
In my case I was using such a reference and that was what caused my

Hope this helps. It still muddy in my own mind which is why I cannot
make my explanation clearer.