How to improve the query so as to reduce the time taken?

Question

Consider i have 15 categories and each category has 6 sub-categories and now i have items table where i have to find 3 items from each sub-category based on the latest purchased date.

category 1 ---> level 1 ---> 3 items with latest date
category 1 ---> level 2 ---> 3 items with latest date
  ...
  ...
  ...
category 15 ---> level 5 ---> 3 items with latest date
category 15 ---> level 6 ---> 3 items with latest date

I have tried

@categories.each do |value|
   @sub-categories.each do |value1|
      array = Item.find(:all, :conditions => ["customer_id IN (?) AND category_id = ? AND sub-category_id = ?", @customer, value.id, value1.id], :order => 'created_at DESC', :limit => 3)
            array.each do |value2|
                   @latest_item_of_each_customer << value2
            end
          end
        end

This would repeat fro 90 times as 15categories x 6sub-categories, so it would take much time. please tell me how i can reduce the time by efficient query.

Answer 1

Depending on the size of the data set you are pulling in, one way to speed up a nested loop like this would be to eager load some of those associations.

@categories = Category.all(:include => :sub_categories)

This should reduce the amount of queries you are making and therefor improve performance.

If you want to read up some more on eager loading take a look at this http://guides.rubyonrails.org/active_record_querying.html#eager-loading-associations

Answer 2

1) First thing is... what is the use of category_id in item table, as we already have sub_category_id, and from sub_category we can get category and you want items for each subcategory.

  @latest_item_of_each_customer = []
  @sub_categories = SubCategory.all

  @sub_categories.each do |value1|
    @latest_item_of_each_customer << Item.find(:all, :conditions => ["customer_id IN (?) and sub_category_id = ?", @customer, value1.id], :order => 'created_at DESC', :limit => 3)
  end

  @latest_item_of_each_customer  = @latest_item_of_each_customer.flatten