查询Mongoid/rails 3中的嵌入式对象(“小于"，Min运算符和排序) [英] Querying embedded objects in Mongoid/rails 3 ("Lower than", Min operators and sorting)

问题描述

我正在使用带有蒙古型的滑轨3. 我有一个带有嵌入式价格集合的股票集合:

I am using rails 3 with mongoid. I have a collection of Stocks with an embedded collection of Prices :

class Stock
  include Mongoid::Document
  field :name, :type => String
  field :code, :type => Integer
  embeds_many :prices

class Price
  include Mongoid::Document
  field :date, :type => DateTime
  field :value, :type => Float
  embedded_in :stock, :inverse_of => :prices

我想获得自给定日期以来的最低价格低于给定价格p的股票，然后能够对每种股票的价格进行排序.

I would like to get the stocks whose the minimum price since a given date is lower than a given price p, and then be able to sort the prices for each stock.

但是看来Mongodb不允许这样做. 因为这行不通:

But it looks like Mongodb does not allow to do it. Because this will not work:

@stocks = Stock.Where(:prices.value.lt => p)

此外，似乎mongoDB无法对嵌入式对象进行排序.

Also, it seems that mongoDB can not sort embedded objects.

那么，是否有替代方法可以完成此任务?

So, is there an alternative in order to accomplish this task ?

也许我应该将所有内容都放在一个集合中，以便可以轻松运行以下查询:

Maybe i should put everything in one collection so that i could easily run the following query:

@stocks = Stock.Where(:prices.lt => p)

但是我真的很想在查询后得到按股票名称分组的结果(例如，具有一系列定单价格的不同股票).我听说过使用组函数进行映射/归约的情况，但是我不确定如何在Mongoid中正确使用它.

But i really want to get results grouped by stock names after my query (distinct stocks with an array of ordered prices for example). I have heard about map/reduce with the group function but i am not sure how to use it correctly with Mongoid.

http://www.mongodb.org/display/DOCS/Aggregation

SQL中的等效内容如下:

The equivalent in SQL would be something like this:

SELECT name, code, min(price) from Stock WHERE price<p GROUP BY name, code

感谢您的帮助.

推荐答案

MongoDB/Mongoid允许您执行此操作.您的示例可以使用，语法不正确.

MongoDB / Mongoid do allow you to do this. Your example will work, the syntax is just incorrect.

@stocks = Stock.Where(:prices.value.lt => p) #does not work

@stocks = Stock.where('prices.value' => {'$lt' => p}) #this should work

而且，它仍然可以链接，因此您也可以按名称订购:

And, it's still chainable so you can order by name as well:

@stocks = Stock.where('prices.value' => {'$lt' => p}).asc(:name)

希望这会有所帮助.

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