Rails - Jan 2009 - Removing an association on an AR model via monkey patching

How can I remove an association from an AR model with a monkey patch?

Assume:

class Foo < AR::Base
  has_many :bars
end

It''s a long story, but in one particular data loading (not running the
app. context), I''d like to remove the has_many association to :bars.

Can I do that?

Thanks,
Wes
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On Jan 16, 12:59 pm, Wes Gamble
<rails-mailing-l...-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
wrote:
> How can I remove an association from an AR model with a monkey patch?
>
> Assume:
>
> class Foo < AR::Base
>   has_many :bars
> end
>
> It''s a long story, but in one particular data loading (not running
the
> app. context), I''d like to remove the has_many association to
:bars.
>
> Can I do that?
It sounds like you want to make sure that code that does this:

@foo.bars

would raise a NoMethodFound exception?  If so, you can just reopen the
class:

class Foo
  def bars
    raise NoMethodError.new("undefined method
''bars''")
  end
end


Then,

@foo.bars  # => raises exception


However, this isn''t really removing all of remnants of the
association, so again it depends on what constitutes "removed enough"
for  your situation.


Jeff
purpleworkshops.com

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Here''s my real problem:

I am trying to load a fixture into a database which acts as a secondary 
DB to my application.

The fixture file uses Erb to generate appropriate fixtures based on 
something like this:

<% Invoice.find_all_by_im_invoice_state("Paid").each do |inv| %>
  ...generate a bunch of fixtures...
<% end %>

The problem is that the Invoice class has an association which is not 
present in the DB into which I am loading the data.  The Invoice
class''s
table is present, which is all that I need to properly generate the 
fixtures.  However, the class for this other has_many association that 
I''m not interested in is not present.

So I''d like to temporarily remove the association for the duration of 
the fixture load.  Hoping I can just monkey patch within the Erb.

Thanks,
Wes
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On Jan 16, 2009, at 3:19 PM, Wes Gamble wrote:
> Here''s my real problem:
>
> I am trying to load a fixture into a database which acts as a  
> secondary
> DB to my application.
>
> The fixture file uses Erb to generate appropriate fixtures based on
> something like this:
>
> <% Invoice.find_all_by_im_invoice_state("Paid").each do |inv|
%>
>  ...generate a bunch of fixtures...
> <% end %>
>
> The problem is that the Invoice class has an association which is not
> present in the DB into which I am loading the data.  The Invoice  
> class''s
> table is present, which is all that I need to properly generate the
> fixtures.  However, the class for this other has_many association that
> I''m not interested in is not present.
>
> So I''d like to temporarily remove the association for the duration
of
> the fixture load.  Hoping I can just monkey patch within the Erb.
>
> Thanks,
> Wes

Unless you reference the association, it won''t be accessed. Am I (Are  
we) missing something here?  Just don''t use the association.

-Rob

Rob Biedenharn		http://agileconsultingllc.com
Rob-xa9cJyRlE0mWcWVYNo9pwxS2lgjeYSpx@public.gmane.org



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When I call Invoice.find_all_by_im_invoice_state("Paid").each the 
Invoice class is loaded.

When the Invoice class is loaded, all of its class level association 
directives are executed.  When it hits the one that doesn''t have a 
backing table, it fails.

So I need to be able to modify the Invoice class to remove the offending 
association before the fixture load occurs.

Wes
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On 16 Jan 2009, at 20:44, Wes Gamble wrote:
>
> When I call Invoice.find_all_by_im_invoice_state("Paid").each the
> Invoice class is loaded.
>
> When the Invoice class is loaded, all of its class level association
> directives are executed.  When it hits the one that doesn''t have a
> backing table, it fails.
So it''s at the point where the class (not the data) is loaded that the
problem occurs ? You might be in a tight spot then because it''s going  
to be hard to remove the association before the class exists. I can  
think of two ways, neither of which is very nice.

define a new class Invoice < ActiveRecord::Base inside your fixture  
generation code. if you don''t need any of Invoice''s methods
then
you''re fine with just this

Override ActiveRecord::Base.has_many to check if self is invoice and  
the association name is the bad one. if it is, do nothing. if not let  
the normal code run.

Fred

>
>
> So I need to be able to modify the Invoice class to remove the  
> offending
> association before the fixture load occurs.
>
> Wes
> -- 
> Posted via http://www.ruby-forum.com/.
>
> >

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Frederick Cheung wrote:
> On 16 Jan 2009, at 20:44, Wes Gamble wrote:
> 
>> When the Invoice class is loaded, all of its class level association
>> directives are executed.  When it hits the one that doesn''t
have a
>> backing table, it fails.
> 
> So it''s at the point where the class (not the data) is loaded that
the
> problem occurs ? You might be in a tight spot then because it''s
going
> to be hard to remove the association before the class exists. 
Actually, the class needs to exist and I just need to temporarily remove 
the association.
> define a new class Invoice < ActiveRecord::Base inside your fixture
> generation code. if you don''t need any of Invoice''s
methods then
> you''re fine with just this
I need to be able to do a find on Invoice, which is why I have the 
problem in the first place, so this probably won''t work.
> Override ActiveRecord::Base.has_many to check if self is invoice and
> the association name is the bad one. if it is, do nothing. if not let
> the normal code run.
This may work and I will give it a shot.  Thanks for the idea.

Wes
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On Jan 16, 2009, at 4:54 PM, Wes Gamble wrote:
>
> Frederick Cheung wrote:
>> On 16 Jan 2009, at 20:44, Wes Gamble wrote:
>>
>>> When the Invoice class is loaded, all of its class level
association
>>> directives are executed.  When it hits the one that
doesn''t have a
>>> backing table, it fails.
>>
>> So it''s at the point where the class (not the data) is loaded
that
>> the
>> problem occurs ? You might be in a tight spot then because
it''s going
>> to be hard to remove the association before the class exists.
>
> Actually, the class needs to exist and I just need to temporarily  
> remove
> the association.
Can you show us the has_many part of your Invoice class?

Product is a model in one of my client projects.  I can do:
$ script/console
Loading development environment.
irb> Product
=> Product
irb> class Product; has_many :foobars; end
=> [:build_to_foobars]
irb> p=Product.find(:first)
=> #<Product:0x3edb0e0 @attributes={...}>
irb> p.methods.include?(''foobars'')
=> true
irb> p.foobars
NameError: uninitialized constant Product::Foobar

Notice that there is no error when the:
   has_many :foobars
gets processed.  When I find a product, it has a foobars method, but  
it doesn''t work.

Perhaps you are defining your association is a way that causes pre- 
mature loading of the associated model?
>
>> define a new class Invoice < ActiveRecord::Base inside your fixture
>> generation code. if you don''t need any of Invoice''s
methods then
>> you''re fine with just this
>
> I need to be able to do a find on Invoice, which is why I have the
> problem in the first place, so this probably won''t work.
>
>> Override ActiveRecord::Base.has_many to check if self is invoice and
>> the association name is the bad one. if it is, do nothing. if not let
>> the normal code run.
>
> This may work and I will give it a shot.  Thanks for the idea.
>
> Wes
''Cause this seems like a lot of work that might not even be necessary.

-Rob

Rob Biedenharn		http://agileconsultingllc.com
Rob-xa9cJyRlE0mWcWVYNo9pwxS2lgjeYSpx@public.gmane.org



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The has_many part of the Invoice class is actually provided by a plugin. 
And inside of that plugin, a set_table_name was being done to point to 
the offensive table.

I got this to work by making the inclusion of that plugin dependent on 
the setting of RAILS_ENV.  Which isn''t pretty, but works.

Not sure that I will leave the change in, but I did (finally) get my 
data loaded.  Thankfully, I have Erb to make the fixtures as flexible as 
I need them to be.

Thanks for all of the help, everyone.

Wes

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