使用 MongoMapper 范围查询关联集合？

发布于 2024-09-27 14:03:38 字数 367 浏览 1 评论 0原文

class Comment
  include MongoMapper::Document

  scope :by_rating, lambda { |minimum| where(:rating.gte => minimum) }

  key :rating
  belongs_to :user
end

class User
  include MongoMapper::Document

  many :comments
end


User.first.comments.by_rating(3)

最后一行的查询实际上做了什么？ MongoMapper 是否足够智能，可以只执行一个具有两个 WHERE 条件（user_id 和最低评分）的查询？

原文

class Comment
  include MongoMapper::Document

  scope :by_rating, lambda { |minimum| where(:rating.gte => minimum) }

  key :rating
  belongs_to :user
end

class User
  include MongoMapper::Document

  many :comments
end


User.first.comments.by_rating(3)

What does the query on last line actually do? Is MongoMapper intelligent enough to execute only one query with two WHERE conditions (user_id and minimum rating)?

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债姬 2024-10-04 14:03:38

MongoDB 做不到这一点。这需要一个它无法做到的连接。它通过具有非常可扩展的读取性能和更轻量级的查询克服了这一限制。这不是问题。您可以通过在初始化程序中设置记录器（搜索 MongoMapper.connection）来查看此行为：

 # Change as appropriate
 MongoMapper.connection = Mongo::Connection.new(
   '127.0.0.1', 27017, :logger => Logger.new(STDOUT))

然后启动 Rails 控制台，您将看到两个查询：

 User.first.comments
 MONGODB test['users'].find({}).limit(-1)
 MONGODB test['comments'].find(
   {:user_id=>BSON::ObjectId('4e8ddd6bf2c31e7001000001')})

MongoDB can't do that. That requires a join which it cannot do. It overcomes this limitation by having very scalable read performance and lighter-weight queries. It's not an issue. You can see this behavior by setting the logger in your initializer (search for MongoMapper.connection):

 # Change as appropriate
 MongoMapper.connection = Mongo::Connection.new(
   '127.0.0.1', 27017, :logger => Logger.new(STDOUT))

Then fire up your rails console and you'll see two queries:

 User.first.comments
 MONGODB test['users'].find({}).limit(-1)
 MONGODB test['comments'].find(
   {:user_id=>BSON::ObjectId('4e8ddd6bf2c31e7001000001')})

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