# Managing belongs\_to fields in a form

**URL:** <https://www.ruby-forum.com/t/managing-belongs-to-fields-in-a-form/50438>\
**Category:** Rails\
**Created:** [December 16, 2005, 10:37am UTC](https://www.ruby-forum.com/t/managing-belongs-to-fields-in-a-form/50438 "2005-12-16T10:37:58Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![aravet](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/49beb7/32.png) [@aravet](https://www.ruby-forum.com/u/aravet)\
**Post date:** [December 16, 2005, 10:37am UTC](https://www.ruby-forum.com/t/managing-belongs-to-fields-in-a-form/50438/1 "2005-12-16T10:37:58Z")

</div>

Hi all,

I cannot find a clean way to create/edit an object that ‘belongs\_to’  
another one, just by using form fields.  
I always need to explicitely unassemble it, store the master id in a  
hidden field, and then refetch the master from its id, and put it back  
in the object.

To summarize:  
I want to create a new member, for a given project

```
@project= ...
@member = Member.new (:project => @project)

```

=\> in the view, if I have a form with

project\<%= text\_field ‘member’, ‘project’ %\>  
  
login \<%= text\_field ‘member’, ‘name’ %\>

it doesn’t work:  
- the project fields contains/shows  
#Project:0x248aa50  
- when I submit the form, I get the error :  
“Project expected, got String”

Is there a nice Rails idioms to solve this problem?

Alain

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<div class="post-metadata">

**Author:** ![aravet](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/49beb7/32.png) [@aravet](https://www.ruby-forum.com/u/aravet)\
**Post date:** [December 16, 2005, 11:46am UTC](https://www.ruby-forum.com/t/managing-belongs-to-fields-in-a-form/50438/2 "2005-12-16T11:46:37Z")

</div>

On 12/16/05, Alain R. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> I want to create a new member, for a given project  
> - the project fields contains/shows  
> Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/).
> 
> * * *
> 
> Rails mailing list  
> removed\_email\_address@domain.invalid  
> [http://lists.rubyonrails.org/mailman/listinfo/rails](http://lists.rubyonrails.org/mailman/listinfo/rails)

You could try using the session for it. I keep both ids (  
session[:project\_id] or session[:project][:id] ) and whole objects (  
session[:project] ) inside it.

Disclaimer: I am running a low-medium load public site and I have NOT  
profiled memory use yet.

–

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<div class="post-metadata">

**Author:** ![aravet](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/49beb7/32.png) [@aravet](https://www.ruby-forum.com/u/aravet)\
**Post date:** [December 16, 2005, 1:07pm UTC](https://www.ruby-forum.com/t/managing-belongs-to-fields-in-a-form/50438/3 "2005-12-16T13:07:55Z")

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Alain R. wrote:

> ```
> #<Project:0x248aa50>
> - when I submit the form, I get the error :
> "Project expected, got String"
> 
> ```
> 
> Is there a nice Rails idioms to solve this problem?

The technique demonstrated in the 15 minute Rails movie, at the point  
where comments are added to blog posts, is to put the parent object’s ID  
in the URL that the child details will be submitted to:

In the view of a specific post, app/views/blog/show.rhtml, which also  
shows the existing comments, this form is used to add a new comment:

\<%= form\_tag :action =\> ‘comment’, :id =\> @post %\>  
\<%= text\_area “comment”, “body” %\>  
  
\<%= submit\_tag “Comment!”%\>

and the new comment is handled by this action in the controller:

def comment  
Post.find(params[:id]).comments.create(params[:comment])  
flash[:notice] = “Added your comment.”  
redirect\_to :action =\> “show”, :id =\> params[:id]  
end

regards

Justin

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<div class="post-metadata">

**Author:** ![aravet](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/49beb7/32.png) [@aravet](https://www.ruby-forum.com/u/aravet)\
**Post date:** [December 16, 2005, 2:07pm UTC](https://www.ruby-forum.com/t/managing-belongs-to-fields-in-a-form/50438/4 "2005-12-16T14:07:00Z")

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Justin

> The technique demonstrated in the 15 minute Rails movie,  
> is to put the parent object’s ID in the URL parameter

I know, and that’s what I do, but it’s not nice.

What if an object belongs\_to 5 other entities:  
def new  
…  
m = Member.new (:project =\> project,  
:section =\> section, :branch =\> branch,  
etc…)  
And what if this object already exist, and you just want to edit it?

It’s not very comfortable, having to  
1/dismantle all the parts linked to foreign keys,  
2/ship their IDs(in the url),

 3/collect the parts + the foreign keys id, and then 4/ fetch the foreign object and rebuild the main object before 5/saving.

I wonder if there isn’t a Rails idiom to solve that problem?

Alain

---

<div class="post-metadata">

**Author:** ![aravet](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/49beb7/32.png) [@aravet](https://www.ruby-forum.com/u/aravet)\
**Post date:** [December 16, 2005, 3:23pm UTC](https://www.ruby-forum.com/t/managing-belongs-to-fields-in-a-form/50438/5 "2005-12-16T15:23:51Z")

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Alain R. wrote:

> > The technique demonstrated in the 15 minute Rails movie,  
> > is to put the parent object’s ID in the URL parameter
> 
> I know, and that’s what I do, but it’s not nice.

OK - your previous post said that you were using a hidden field.

> 2/ship their IDs(in the url),
> 
> 3/collect the parts + the foreign keys id, and then 4/ fetch the foreign object and rebuild the main object before 5/saving.

Your earlier question was just about members of projects (which in  
general would be many-to-many, so you might be editing a Membership  
rather than a Member). I’m not clear on whether, in your expanded  
example, it’s the Member or the Project that has section and branch - or  
the Membership?

Which relationships are genuine “belongs to” (strict containment)  
relationships, and which are just references?

> I wonder if there isn’t a Rails idiom to solve that problem?

I don’t claim any particular expertise, but this is my take:

Passing the id in the URL is appropriate when you have navigated from a  
parent to a new child. Other references from the child would normally be  
set up by controls in the child form (e.g. select lists, checkboxes…).

If you are editing an existing object, and there are relationships that  
aren’t being changed (e.g. if it isn’t changing its parent) then you  
don’t need to pass those IDs around. You have a copy of the object,  
either in the session or in the database, and you can leave those  
attributes unchanged. For relationships that _are_ changeable, there  
should be controls on the child form.

I agree that would be nice if there were standard examples covering the  
various relationship types and user interaction styles, and especially  
if scaffolding could be extended to cover some of this.

regards

Justin

---

<div class="post-metadata">

**Author:** ![aravet](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/49beb7/32.png) [@aravet](https://www.ruby-forum.com/u/aravet)\
**Post date:** [December 17, 2005, 2:39pm UTC](https://www.ruby-forum.com/t/managing-belongs-to-fields-in-a-form/50438/6 "2005-12-17T14:39:12Z")

</div>

Alain R. wrote:

> > > 3/collect the parts + the foreign keys id, and then  
> > > Which relationships are genuine “belongs to” (strict containment)  
> > > In the db, the ‘employees’ table has 2 foreign keys : company\_id, and
> 
> I’m just wondering/asking is there is a Rails idiom for this case, that  
> would make my life simpler and the code shorter/clearer.

So long as there is no risk that the company or company address has been  
deleted by another user, you don’t need to do those finds - you can just  
set the IDs using the accessors for company\_id and address\_id.

An alternative approach would be to hold the Employee/Company/Address  
combination in the session, so that when the form is submitted you could

```
- get the Employee out of the session
- apply the changes
- save it
- clear the session attribute

```

regards

Justin

---

<div class="post-metadata">

**Author:** ![aravet](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/49beb7/32.png) [@aravet](https://www.ruby-forum.com/u/aravet)\
**Post date:** [December 16, 2005, 4:06pm UTC](https://www.ruby-forum.com/t/managing-belongs-to-fields-in-a-form/50438/7 "2005-12-16T16:06:32Z")

</div>

Justin

> > > is to put the parent object’s ID in the URL parameter  
> > > I know, and that’s what I do, but it’s not nice.
> 
> OK - your previous post said that you were using a hidden field.

I’ve tried many techniques, where I each time explicitely manipulated  
the belonged\_to/foreign\_key related object/id.

> > 3/collect the parts + the foreign keys id, and then  
> > 4/ fetch the foreign object and rebuild the main object  
> > before 5/saving.
> 
> Your earlier question was just about members of projects

Actually, I’m letting one member edit his info, but the project is fixed  
in advance.  
member = Member.new (:project =\> the\_project)

> Which relationships are genuine “belongs to” (strict containment)  
> relationships, and which are just references?

For example, add a new ‘Employee’ to a a given ‘Company’, and give him a  
default work “Address”.

=\> The data sent to the form is  
e = Employe.new (:company =\> the\_company, :address =\>  
the\_company.address)

In the db, the ‘employees’ table has 2 foreign keys : company\_id, and  
address\_id.  
My problem is that when it comes to creating an ‘employee’ in a form, I  
have to EXPLICITELY manipulate its ‘address’ and ‘company’ ids/foreign  
keys after the submit, even though I already set those 2 values in an  
earlier step (see above).

( manipulate the fk == extract them, send them to the form, store them  
in hidden fields as numerical values, and after the submit perform  
multiple XXX.find( fk) to retrieve the employee previously known address  
and company, and stuff them back in the employee object.

I’m just wondering/asking is there is a Rails idiom for this case, that  
would make my life simpler and the code shorter/clearer.

Alain