MongoDB - 简单的子查询示例
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MongoDB - simple sub query example
提问by Tony Sepia
Given the data:
鉴于数据：
> db.parameters.find({})
{ "_id" : ObjectId("56cac0cd0b5a1ffab1bd6c12"), "name" : "Speed", "groups" : [ "
123", "234" ] }
> db.groups.find({})
{ "_id" : "123", "name" : "Group01" }
{ "_id" : "234", "name" : "Group02" }
{ "_id" : "567", "name" : "Group03" }
I would like to supply a parameter_id an make a query return all groups that are within the groupsarray of the given document in parameterstable.
我想提供一个参数_id 使查询返回参数表中给定文档的组数组内的所有组。
The straightforward solution seems to make several DB calls in PyMongo:
简单的解决方案似乎在 PyMongo 中进行了几次数据库调用：
- Get parameter from parameterstable based on the supplied _id
- For each element of groupsarray select a document from groupscollection
- 根据提供的 _id从参数表中获取参数
- 对于组数组的每个元素从组集合中选择一个文档
But this will have so much unnecessary overhead. I feel there must be a better, faster way to do this within MongoDB (without running custom JS in the DB). Or should I re-structure my data by normalising it a little bit (like a table of relationships), neglecting the document-based approach?
但这会有很多不必要的开销。我觉得在 MongoDB 中必须有更好、更快的方法来做到这一点（无需在 DB 中运行自定义 JS）。或者我应该通过稍微规范化数据（如关系表）来重新构建我的数据，而忽略基于文档的方法？
Again, please help me find a solution that would work from PyMongo DB interface
再次，请帮我找到一个可以从 PyMongo DB 接口工作的解决方案
回答by chridam
You can do this within a single query using the aggregation framework. In particular you'd need to run an aggregation pipeline that uses the $lookupoperator to do a left join from the parameterscollection to the groupscollection.
您可以使用聚合框架在单个查询中执行此操作。特别是，您需要运行一个聚合管道，该管道使用$lookup运算符执行从parameters集合到groups集合的左连接。
Consider running the following pipeline:
考虑运行以下管道：
db.parameters.aggregate([
{ "$unwind": "$groups" },
{
"$lookup": {
"from": "groups",
"localField": "groups",
"foreignField": "_id",
"as": "grp"
}
},
{ "$unwind": "$grp" }
])
Sample Output
样本输出
/* 1 */
{
"_id" : ObjectId("56cac0cd0b5a1ffab1bd6c12"),
"name" : "Speed",
"groups" : "123",
"grp" : {
"_id" : "123",
"name" : "Group01"
}
}
/* 2 */
{
"_id" : ObjectId("56cac0cd0b5a1ffab1bd6c12"),
"name" : "Speed",
"groups" : "234",
"grp" : {
"_id" : "234",
"name" : "Group02"
}
}
If your MongoDB server version does not support the $lookuppipeline operator, then you'd need execute two queries as follows:
如果您的 MongoDB 服务器版本不支持$lookup管道运算符，那么您需要执行以下两个查询：
# get the group ids
ids = db.parameters.find_one({ "_id": ObjectId("56cac0cd0b5a1ffab1bd6c12") })["groups"]
# query the groups collection with the ids from previous query
db.groups.find({ "_id": { "$in": ids } })
EDIT: matched the field name in the aggregation query to the field name in example dataset (within the question)
编辑：将聚合查询中的字段名称与示例数据集中的字段名称匹配（在问题内）
