Rails - Mar 2007 - How to get more informations about an association at runtime

Hi all

I''m working on a Custom Formbuilder. I want it to create me with a
single method call checkboxes for a given HABTM association.

My model looks like this:

[code=ruby]class Member < ActiveRecord::Base
  has_and_belongs_to_many :preferred_music_styles,
    :class_name => ''MusicStyle'',
    :join_table => ''members_have_preferred_music_styles''
end
[/code]

In my view I''d like to do the following:

[code=rhtml]<%= form.checkboxes :preferred_music_styles %>[/code]

The method checkboxes() should now create a set of checkboxes; one for
each music style.

But for doing this I have to get more informations about the
"preferred_music_styles" association:

1) Does the association exist anyway?
2) Is it a HABTM association anyway?
3) If so, which class (MusicStyle) does it connect to?

I sadly have no idea how to accomplish this task. I''d be very happy if
some could give me a useful hint where to achieve this information. :-)

Thanks
Josh

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On Mar 18, 2007, at 10:49 AM, Joshua Muheim wrote:
> 1) Does the association exist anyway?
> 2) Is it a HABTM association anyway?
> 3) If so, which class (MusicStyle) does it connect to?
>
> I sadly have no idea how to accomplish this task. I''d be very
happy if
> some could give me a useful hint where to achieve this  
> information. :-)
The key is

   model.class.reflect_on_association(association)

Have a look at the classes

   ActiveRecord::Reflection::MacroReflection
   ActiveRecord::Reflection::AggregateReflection
   ActiveRecord::Reflection::AssociationReflection

for API to query the association, they live in

   activerecord/lib/active_record/reflection.rb

-- fxn


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> The key is
> 
>    model.class.reflect_on_association(association)
Thanks a lot, that''s very helpful. :-)
What''s the difference between @class_name="MusicStyle" and 
@klass=MusicStyle though?

I''m stuck right now with another problem:
I need to access the @member object from within my form builder method. 
Using self.object_name I get "member", but self.object returns nil. Am
I
looking at the wrong place?

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On Mar 18, 2007, at 2:17 PM, Joshua Muheim wrote:
>> The key is
>>
>>    model.class.reflect_on_association(association)
>
> Thanks a lot, that''s very helpful. :-)
> What''s the difference between @class_name="MusicStyle"
and
> @klass=MusicStyle though?
In the first assignement there''s a string that holds the name of a  
class. In the second one there''s a constant whose value is the very  
class, that is, an instance of the class Class.
> I''m stuck right now with another problem:
> I need to access the @member object from within my form builder  
> method.
> Using self.object_name I get "member", but self.object returns
nil.
> Am I
> looking at the wrong place?
The incantation is:

   real_object = @template.instance_variable_get("@#{@object_name}")

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> The incantation is:
> 
>    real_object =
@template.instance_variable_get("@#{@object_name}")
> 
> -- fxn
I am really thankful for your help. I''ve got my method so far:

  def checkboxes(assoc_name, options = {})
    model = @template.instance_variable_get("@#{@object_name}")
    association = model.class.reflect_on_association(assoc_name)
    raise "A has_and_belongs_to_many association is needed
(#{association.macro} given)!" unless association.macro
=:has_and_belongs_to_many
    src = [] # Will hold the HTML code for a checkbox per entry
    MusicStyle.find(:all).each do |r|
      name_param = @object_name.to_s + ''['' + assoc_name.to_s +
''_ids][]''
      value_param = r.id
      checked_param = model.send(assoc_name).include?(r)
      src << # What to do here?
    end
    src.join
  end

I don''t know if all the xxx_param vars have a correct value, but anyway
I don''t know how to create the checkbox in a clean way...

A method "check_box_tag" is not recognized by the form builder, so
what
to do? I have modified the "check_box" method so it outputs some HTML,
but I don''t want to use it because I need just plain checkboxes.

What to do?

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Oh, yet another small question:

I just realized that the checkbox needs a name in the following style:

member[preferred_music_style_ids][]

But whether do I really get why the habtm-name (preferred_music_styles) 
is singularized here nor do I see where to get this required form of the 
name from.

At the moment I hacked it together using

name_param = @object_name.to_s + ''['' +
assoc_name.to_s.singularize +
''_ids][]''

(using the singularize method), but I guess this is not very clean, is 
it? Or is it always just the singularized form, also when e.g. the 
singular and plural forms are the same?

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On Mar 18, 2007, at 4:00 PM, Xavier Noria wrote:
>> I''m stuck right now with another problem:
>> I need to access the @member object from within my form builder  
>> method.
>> Using self.object_name I get "member", but self.object
returns
>> nil. Am I
>> looking at the wrong place?
>
> The incantation is:
>
>   real_object =
@template.instance_variable_get("@#{@object_name}")
Let me add a comment to that.

The object the form builder is acting upon is not known by the form  
builder itself, I see no API for that in the current implementation.  
The default form builder is just a thin wrapper over regular form  
helper methods, and the logic to figure that out is encapsulated there.

Form builders can optionally receive a second argument with the  
object itself (passed to form helpers via :object), so the logic to  
get the actual object would be

   actual_object = object.nil? ?
     @template.instance_variable_get("@#{object_name}") :
     object

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Well that''s interesting. Why is the object itself not mandatory and the
form could evaluate the type class of the object itself? Supplying the 
type AND the object both seems a bit redundant to me...?

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