CODEWITHSUNDEEP
ruby-on-railsactiverecord
Blankman
Blankman

Reputation: 267010

Counting all association objects and updating a counter

I have 2 models:

class Products < ActiveRecord::Base
  has_many :purchases

class Purchase < ActiveRecord::Base
  belongs_to :Product

The products model has a column purchase_count

I want a very fast efficient way to update this counter that basically runs this sql:

UPDATE products SET purchase_count = (SELECT COUNT(*) from purchases where active=1 and product_id = 123) where id = 123

Can I do this with ActiveRecord ? If not, how can I do this with just raw sql?

Upvotes: 0

Views: 438

Answers (1)

Arun Kumar Mohan
Arun Kumar Mohan

Reputation: 11915

You should probably use a counter_cache column to make it efficient. Rails allows you to set a counter_cache option to true in the belongs_to model(purchase). But you should generate a migration to add a column to products table.

For more, look for counter_cache in http://guides.rubyonrails.org/association_basics.html#belongs-to-association-reference

If you just don't want to count all the associated objects and instead need a condition to be passed in, you need a custom counter cache. Have a look at http://douglasfshearer.com/2006/10/06/custom-counter-cache-with-conditions.html for an example.

Upvotes: 2

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