# Double has\_many help

**URL:** <https://www.ruby-forum.com/t/double-has-many-help/133043>\
**Category:** Rails\
**Created:** [March 27, 2008, 1:01am UTC](https://www.ruby-forum.com/t/double-has-many-help/133043 "2008-03-27T01:01:18Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![azazel](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/838e76/32.png) [@azazel](https://www.ruby-forum.com/u/azazel)\
**Post date:** [March 27, 2008, 1:01am UTC](https://www.ruby-forum.com/t/double-has-many-help/133043/1 "2008-03-27T01:01:18Z")

</div>

I’m a total newbie on this, and I’m learning at baby steps, I am  
really excited from what I’ve learned with Ruby on Rails, but I have  
hit a wall here, I hope someone can help me to figure this out.

I’m trying to implement what I think is a simple relation between 3  
kinds of data:  
Machine, Kind, Product

- Machine can have many Kinds (I used HABTM and a join table, and used  
check boxes in the view, ready, no problems in here)
- Product(s) must have one Kind (Used has\_many and belongs\_to, using a  
select\_tag for the view, no trouble here)

Now what is giving me some headaches:

-Machine must list the Product(s) that share the same Kind \<== How can  
I implement this? any suggestions or ideas?

I tried has\_many =\> through, but it seems that I am misunderstanding  
the method.

class Machine \< AR  
has\_and\_belongs\_to\_many :kinds  
has\_many :products, :through =\> :kinds ???  
end

All errors are very explicit telling me that I’m missing a column, but  
the thing here is, how can I call the same Kind on two different  
things, how can I call Products that have the same Kind as Machine?

Thank you.  
Roberto.

---

<div class="post-metadata">

**Author:** ![azazel](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/838e76/32.png) [@azazel](https://www.ruby-forum.com/u/azazel)\
**Post date:** [March 27, 2008, 2:17am UTC](https://www.ruby-forum.com/t/double-has-many-help/133043/2 "2008-03-27T02:17:32Z")

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If you want to use has\_many :through, the trouble with that is, it  
expects a has\_many, not a habtm association to go through.

so, you need to concretise your join tables to use it…

class Kind \< AR  
end

class MachineKind \< AR

# attributes: id, machine\_id, kind\_id

end

class Kind \< AR  
has\_many :machine\_kinds  
has\_many :kinds, :through =\> :machine\_kinds  
end

class Machine \< AR  
has\_many :machine\_kinds  
has\_many :kinds, :through =\> :machine\_kinds  
def products  
kinds.map{|kind| kind.product}.uniq  
end  
end

the “products” method inside the Machine class should solve your other  
issue, however don’t try to use it like an association (ie trying to  
run the method machine.products.build where machine is a Machine  
instance, for example - this won’t work)

It all depends what you want to do with products once you have them.

Julian.

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<div class="post-metadata">

**Author:** ![azazel](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/838e76/32.png) [@azazel](https://www.ruby-forum.com/u/azazel)\
**Post date:** [March 27, 2008, 5:50am UTC](https://www.ruby-forum.com/t/double-has-many-help/133043/3 "2008-03-27T05:50:25Z")

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Thank you for your answer Julian.

All your method is very clear, I’ll be trying it as soon as possible  
(really tired right now) but in general, yes I was going for a  
Machine.products.find(:all) to list all possible Products compatible  
with Machine, and since both Machine and Product get the same  
Kind(from the same “kinds” table) I thought they were related in some  
way I just couldn’t find out how to do that.

Anyway, I’ll try your suggestion, and dropping some lines here for  
more help. 😉

Thank you very much.  
Regards.

Roberto.