ruby-on-rails - Rails + PostgreSQL : How to get products that have been ordered at least once + get the order # and the manager id

Question

我有这些模型:

class Product < ActiveRecord::Base
  has_many :orders
end

class Order < ActiveRecord::Base
 belongs_to :product
end

我想获取至少订购过一次的所有产品的列表，以及下订单时的时间戳 (orders.created_at) 和订单代码 (orders.no)和处理订单的管理员 (orders.processed_by_admin_id)。最后，我想列出 orders.created_at 的输出。

经过一番谷歌搜索后，我得到了这样的结果:

Product.joins(:orders)
       .group("(products.id) having count(products.id) > 0")
       .select("products.*", "max(orders.created_at) as last_order_time")
       .sort_by(&:last_order_time).reverse

这让我获得了至少订购过一次的产品列表，并按时间戳排序。但是，我被困在这里，无法获取 orders.no 和 orders.processed_by_admin_id。我不确定我遵循的程序是否正确。

所需的输出类似于:

products.* | last_order_time | order_no | order_processed_by_admin_id

任何建议表示赞赏。

Answer 1

I want to get a list of all products which have been ordered at least once along with the timestamp of when the order has been made (orders.created_at) and the code of the order (orders.no) and the admin who processed the orderd (orders.processed_by_admin_id). And finally, I would want to list the output by orders.created_at.

当编写复杂的查询时，我喜欢先编写简单的 SQL。

让我们从查询开始，获取每种产品的最新订单。

select product_id, id, 
  row_number() over ( 
    partition by product_id order by created_at 
  ) as recency_ranking 
from orders
;

这称为 window function 。对于每种产品，最近订单的 recency_ranking 为 1。现在我们可以将此查询加入到我们的 products 表中。

select p.*,
  x.id as order_id,
  x.no as order_number,
  x.order_processed_by_admin_id,
  x.created_at as order_created_at
from products p
inner join (
  select product_id, 
    id, 
    created_at, 
    row_number() over ( 
      partition by product_id order by created_at 
    ) as recency_ranking,
    no,
    order_processed_by_admin_id 
  from orders
) x
  on x.product_id = p.id
    and x.recency_ranking = 1
order by x.created_at
;

当我们像这样加入另一个查询时，它称为子查询。请注意我们如何加入recency_ranking = 1。

要在 ActiveRecord 中运行整个查询，我建议 find_by_sql:

products = Product.find_by_sql('select p.*, x.id ...')
products.first.order_created_at #=> '2019-01-01 ...'