Rails - May 2006 - What association do I want here, has_ or belongs_to?

I thought I had a handle on this one but it''s causing me grief and
I''m
hoping somebody can set me straight.  I have an object AlertTemplate 
which will contain a fill in the blank style hunk of web content.  I 
then have an object, Alert, which will have a relationship to one of 
those AlertTemplate objects, plus some data.
So, in human speak, I''d think that an Alert has_one AlertTemplate.  But
I can''t really say that AlertTemplate belongs_to Alert, because each 
template may be used by many alerts.  So AlertTemplate just as an id 
column, but Alert has a template_id column.

In my form for creating an alert I have:

collection_select(:alert, :template, (AlertTemplate.find :all), :id, 
:name)

to produce a dropdown for choosing which template you want to associate 
with the new Alert object.

However in my controller''s create method when I do an 
Alert.new(params[:alert])
I get an error :

ActiveRecord::AssociationTypeMismatch in AlertController#create

AlertTemplate expected, got String

All the stack trace tells me is that the error happens on the Alert.new 
line.  But that''s where I''m lost.  Which code expected an
AlertTemplate?
What String did it get?

Interestingly I tried an experiment in irb.  I created an AlertTemplate 
object t, then created an empty Alert a, and said a.template = t and 
tried to save it.  The Alert object got saved -- but template_id 
remained null.  That leads me to believe that I''ve got me id fields 
wrong?

Thanks!

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On Friday, May 26, 2006, at 3:09 PM, Duane wrote:
>I thought I had a handle on this one but it''s causing me grief and
I''m
>hoping somebody can set me straight.  I have an object AlertTemplate
>which will contain a fill in the blank style hunk of web content.  I
>then have an object, Alert, which will have a relationship to one of
>those AlertTemplate objects, plus some data.
>So, in human speak, I''d think that an Alert has_one AlertTemplate. 
But
>I can''t really say that AlertTemplate belongs_to Alert, because
each
>template may be used by many alerts.  So AlertTemplate just as an id
>column, but Alert has a template_id column.
>
>In my form for creating an alert I have:
>
>collection_select(:alert, :template, (AlertTemplate.find :all), :id,
>:name)
>
>to produce a dropdown for choosing which template you want to associate
>with the new Alert object.
>
>However in my controller''s create method when I do an
>Alert.new(params[:alert])
>I get an error :
>
>ActiveRecord::AssociationTypeMismatch in AlertController#create
>
>AlertTemplate expected, got String
>
>All the stack trace tells me is that the error happens on the Alert.new
>line.  But that''s where I''m lost.  Which code expected an
AlertTemplate?
>What String did it get?
>
>Interestingly I tried an experiment in irb.  I created an AlertTemplate
>object t, then created an empty Alert a, and said a.template = t and
>tried to save it.  The Alert object got saved -- but template_id
>remained null.  That leads me to believe that I''ve got me id fields
>wrong?
>
>Thanks!
>
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>Posted via http://www.ruby-forum.com/.
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>Rails mailing list
>Rails@lists.rubyonrails.org
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I have a couple of tricks I use for creating ''template''
objects.  One of
them is to populate a ''created_by'' attribute with the current
user id.
I then use the convention that when ''created_by'' is NULL, that
item is a
template object.

Then you can find all the ''template'' objects by searching for
those
''created_by'' NULL.  If a user calls ''new''
with a params[:id]
correstponding to a template object, I clone that object and use it to  
populate the form.

You can use other fields to keep track of which ones are templates.   
Association ids are also good. You will need a special action to edit  
the template object as well.

This avoids having to explicity create a ''template'' class,
which I think
is DRYer.

_Kevin

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Found it, I think:
> collection_select(:alert, :template, (AlertTemplate.find :all), :id, 
> :name)
if I replace :template with ''template_id'', then it does the
right thing.
Which now I understand, now that I see it.  The dropdown is providing 
back the id, not the object itself.

D

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Duane wrote:
> Found it, I think:
> 
>> collection_select(:alert, :template, (AlertTemplate.find :all), :id, 
>> :name)
> 
> if I replace :template with ''template_id'', then it does
the right thing.
> Which now I understand, now that I see it.  The dropdown is providing 
> back the id, not the object itself.
> 
> D
Nope, that''s not working.  That allows me form to work, but I
can''t
access alert.template as a nested object, which is really what I need. 
What the heck am I missing?  Does the template object have to have an id 
reference back to alert?  Even though it doens''t belong to only one of 
them?

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Duane wrote:
> So AlertTemplate just as an id column, but Alert has a template_id column.
Shouldn''t the Alert table have an "alert_template_id" column
instead?

Jeff

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