如何在 mongoid + rails3 中找到超过 2 个团队参与者的记分卡答案

【问题标题】：How to find Scorecards having more than 2 team participants in mongoid + rails3如何在 mongoid + rails3 中找到超过 2 个团队参与者的记分卡
【发布时间】：2014-03-20 12:24:50
【问题描述】：

您好，我有以下型号

class Scorecard
  include Mongoid::Document
  include Mongoid::Timestamps

  embeds_many :team_participants
end

我想找出所有超过 2 个 team_participants 的记分卡。我试过关注

Scorecard.where('team_participants.count' => {"$gte" => 3}).count

但这对我不起作用。

感谢任何帮助

【问题讨论】：

这可能会帮助你stackoverflow.com/questions/4705024/…

标签： ruby-on-rails ruby-on-rails-3 mongodb ruby-on-rails-3.1 mongoid

【解决方案1】：

根据这个谷歌小组讨论 https://groups.google.com/forum/#!topic/mongoid/oyNRf3hJ5Cc 似乎无法计算 where 子句中嵌入文档的长度。但如果你真的需要这个功能，你可以做类似的事情。

Scorecard.all.map{|c| c if c.team_participants.length >  3}).compact

但它会降低性能，因为它会搜索整个文档。

或者，您可以查看链接 MongoDB query based on count of embedded document

【讨论】：

【解决方案2】：

下面的测试展示了如何通过 MongoDB 聚合框架做你想做的事。但是，如果选择使用它，请仔细测试其性能。为了获得更高的缩放性能，您应该考虑添加一个显式字段，例如 team_participants_count 跟踪 team_participants 的数组大小，并且该显式字段可以在正常查询中使用。如果您尝试其他解决方案，例如其他 cmets 中引用的 '$where'，你也应该仔细衡量他们的表现。

享受吧！

test/unit/scorecard_test.rb

require 'test_helper'
require 'pp'

class ScorecardTest < ActiveSupport::TestCase
  def setup
    Scorecard.delete_all
  end

  test "select by array size comparison" do
    docs = [
        {team_participants:[{name:'a'}]},
        {team_participants:[{name:'b'},{name:'c'}]},
        {team_participants:[{name:'d'},{name:'e'},{name:'f'}]},
        {team_participants:[{name:'g'},{name:'h'},{name:'i'},{name:'j'}]}
    ]
    Scorecard.create(docs)
    assert_equal(4, Scorecard.count)
    pp Scorecard.collection.aggregate(
        [{'$unwind' => '$team_participants'},
         {'$group' => {'_id' => '$_id',
                       'team_participants' => {'$push' => '$team_participants'},
                       'team_participants_count' => {'$sum' => 1},
                       'created_at' => {'$first' => '$created_at'},
                       'updated_at' => {'$first' => '$updated_at'}}},
         {'$match' => {'team_participants_count' => {'$gte' => 3}}}
        ])
  end
end

$ rake 测试

Run options:

# Running tests:

[1/1] ScorecardTest#test_select_by_array_size_comparison[{"_id"=>"535b1a90a3f576ea5100000a",
  "team_participants"=>
   [{"_id"=>"535b1a90a3f576ea5100000b", "name"=>"g"},
    {"_id"=>"535b1a90a3f576ea5100000c", "name"=>"h"},
    {"_id"=>"535b1a90a3f576ea5100000d", "name"=>"i"},
    {"_id"=>"535b1a90a3f576ea5100000e", "name"=>"j"}],
  "team_participants_count"=>4,
  "created_at"=>2014-04-26 02:31:44 UTC,
  "updated_at"=>2014-04-26 02:31:44 UTC},
 {"_id"=>"535b1a90a3f576ea51000006",
  "team_participants"=>
   [{"_id"=>"535b1a90a3f576ea51000007", "name"=>"d"},
    {"_id"=>"535b1a90a3f576ea51000008", "name"=>"e"},
    {"_id"=>"535b1a90a3f576ea51000009", "name"=>"f"}],
  "team_participants_count"=>3,
  "created_at"=>2014-04-26 02:31:44 UTC,
  "updated_at"=>2014-04-26 02:31:44 UTC}]
Finished tests in 0.031745s, 31.5010 tests/s, 31.5010 assertions/s.
1 tests, 1 assertions, 0 failures, 0 errors, 0 skips

【讨论】：