Rails - Jul 2006 - counter_cache, has_many and belongs_to

Alex Wayne wrote:
> I am a bit confused about counter_cache here.  The API docs 
>
http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html
> say that only the belongs_to association can take the :counter_cache 
> option.  When I try to use it on a has_many I get an "unknown key(s): 
> counter_cache" error.
> 
> Why would this be the case?  belongs_to means that this model has a 
> field with an id of a record form another table.  belongs_to 
> associations only ever return a single object.  So why would you need to 
> cache the number of associated objects?  Wouldnt the has_many 
> association need the counter cache instead since you never know how many 
> records are linked?
> 
> Maybe I''m just not understanding something, but here is the
snippet I am
> using:
> 
>   class Coupon
>     has_many :coupon_uses, :counter_cache => :uses_count
>   end
> 
> Now I could just do "coupon.coupon_uses.size" but there will be
very
> large number of associated objects and would rather not even visit the 
> table if I don''t need to.
Ah hah!

Since the counter cache gets updated when the child object saves, the 
counter_cache call has to be there.  The parent object doesnt need to 
know about it.  The counter caching behaviour is in the child even 
though the actual data field is in the parent.  This make my code more 
like.

  class Coupon
    has_many :coupon_uses
  end

  class CouponUses
    belongs_to :coupon, :counter_cache => :uses_count
  end

-- 
Posted via http://www.ruby-forum.com/.

I am a bit confused about counter_cache here.  The API docs 
http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html 
say that only the belongs_to association can take the :counter_cache 
option.  When I try to use it on a has_many I get an "unknown key(s): 
counter_cache" error.

Why would this be the case?  belongs_to means that this model has a 
field with an id of a record form another table.  belongs_to 
associations only ever return a single object.  So why would you need to 
cache the number of associated objects?  Wouldnt the has_many 
association need the counter cache instead since you never know how many 
records are linked?

Maybe I''m just not understanding something, but here is the snippet I
am
using:

  class Coupon
    has_many :coupon_uses, :counter_cache => :uses_count
  end

Now I could just do "coupon.coupon_uses.size" but there will be very 
large number of associated objects and would rather not even visit the 
table if I don''t need to.

-- 
Posted via http://www.ruby-forum.com/.