Question

考虑我有15个类别和6个子类别，我有表项目，其中我有一组记录，我必须以下列方式获取

category 1 ---> level 1 ---> 3 items with maximum price
category 1 ---> level 2 ---> 3 items with maximum price
  ...
  ...
  ...
category 15 ---> level 6 ---> 3 items with maximum price

和

@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 => 'price DESC', :limit => 3)
            array.each do |value2|
                   @max_price_item_of_each_customer << value2
            end
          end
        end

但这会花费很多时间，因为这会迭代。那么我怎么能以这种方式改变这个时间呢？任何帮助表示赞赏。

Answer 1

尝试：

@max_price_item_of_each_customer = []
@categories.each do |value|   
      @max_price_item_of_each_customer +=  Item.find(:all, :conditions => ["customer_id IN (?) AND category_id = ? AND sub-category_id in (?)", @customer, value.id, @sub-categories.map(&:id)], :order => 'price DESC', :limit => 3)            
end

Answer 2

这一切都取决于您正在使用的记录的规模，但如果您使用合理的设置，这应该更快，并将您的查询减少到1.

@customer_id = 1
@categories  = [1, 2, 3]
@subs        = [4, 5, 6]

@max_price_item_of_each_customer = []
items = Item.where(customer_id: @customer, category_id: @categories, subcategory_id: @subcategories)
items.group_by{|item| item.category_id}.each_pair do |category_id, category_items|
  category_items.group_by{|item| item.subcategory_id}.each_pair do |subcategory_id, subcategory_items|
    @max_price_item_of_each_customer += subcategory_items.sort{|x, y| y.price <=> x.price }.first(3)
  end
end

Answer 3

如果您使用Postgresql，下面的解决方案可能会有效。

从items表中选择一组3个项ID，按price降序排序，按category_id和subcategory_id分组。您可以使用Postgres array_agg在分组后收集项ID。
选择项目行，其中项目ID位于这些分组的项目ID中。之后，按category_id升序，subcategory_id升序和price降序

结果为ActiveRecord::Relation，因此您可以像往常一样迭代这些项目。由于结果是扁平化的（但已经按类别，子类别和价格排序），您需要自己分离不同的类别和子类别。

grouped_item_ids = Item.where(customer_id: customer_id).
  select("items.category_id, items.subcategory_id, (array_agg(items.id order by items.price desc))[1:3] AS item_ids").
  group("items.category_id, items.subcategory_id").map {|item| item["item_ids"]}
@items = Item.where(id: grouped_item_ids.flatten).
  order("items.category_id ASC, items.subcategory_id ASC, items.price desc")

Answer 4

以下查询适用于我

   @max_price_item_of_each_customer =Item.find_by_sql(["SELECT i1.* FROM item i1
      LEFT OUTER JOIN item i2 ON (i1.category_id = i2.category_id AND i1.sub-category_id = i2.sub-category_id AND i1.id < i2.id)
      WHERE i1.customer_id IN (?) AND i1.category_id IN (?)
      GROUP BY i1.id HAVING COUNT(*) < 3
      ORDER BY price DESC", @customer, @categories.map(&:id)])

如何在rails中有效地执行查询？

4 个答案: