更改mongoDB聚合内的字段类型，并且$ lookup是否利用字段上的索引? [英] Change type of field inside mongoDB aggregation and does $lookup utilises index on fields or not?
问题描述
我正在使用$ lookup在mongodb中执行联接，现在我在这里遇到了问题. 我有两个集合，第一个集合包含用户所有书签品牌，第二个集合包含有关品牌的所有详细信息.现在，我试图返回用户添加书签的所有品牌详细信息.
I am performing joins in mongodb using $lookup, now i am facing a problem here. I have two collections first one to contains users all bookmarks brands and the second one contains all details about the brands.Now i am trying to return all brands details bookmarked by the user.
用户已收藏收藏夹
{"mobile_no": "8971740148", "brands": ["5829c1df334d40e20e1d1c19", "5829c1df334d40e20e1d1c20", "5829c1df334d40e20e1d1c21"]}
品牌收藏
{"_id": ObjectId("5829c1df334d40e20e1d1c19"), "brand_name": "Versace"}
{"_id": ObjectId("5829c1df334d40e20e1d1c20"), "brand_name": "Lee Cooper"}
{"_id": ObjectId("5829c1df334d40e20e1d1c21"), "brand_name": "Levis"}
下面给出了我的聚合管道代码
My aggregation pipeline code is given below
{ $match: { mobile_no: mobile_no }},
{ $unwind: { path: "$brands", includeArrayIndex: "brandsposition"}},
{ $lookup: {from: "brands",localField: "brands",foreignField: "_id",as: "user_bookmarks"}},
现在我面临的问题是上面的代码没有返回任何内容，因为我将品牌ID以字符串的形式存储在我的user_bookmarked集合中，而不是以ObjectId的形式存储，因此没有返回任何内容.现在任何人都可以告诉我如何更改聚合查询中的字段类型.
Now the problem i am facing is that above code doesn't return anything as i am storing brands ids as string in my user_bookmarked collection but not as ObjectId so nothing is being returned. Now can anyone please tell me how can i change field type inside aggregation query.
我想问的第二件事是使用$ lookup时，请告诉我，然后mongodb是否利用foreign_field上的索引.因为我使用 explain:true 在聚合管道上方运行，但是我没有发现上述查询使用的任何索引，所以我将其返回给输出.
Second thing i want to ask please tell me when using $lookup then does mongodb utilises index on foreign_field or not. Because i ran above aggregation pipeline with explain: true but i don't found any index that was utilised by above query i got this returned by the output.
db.user_bookmarked.runCommand('aggregate', {pipeline: [{ $match: { mobile_no: mobile_no }},
{ $unwind: { path: "$brands", includeArrayIndex: "brandsposition"}},
{ $lookup: {from: "brands",localField: "brands",foreignField: "_id",as: "user_bookmarks"}}], explain: true})
{
"waitedMS" : NumberLong(0),
"stages" : [
{
"$cursor" : {
"query" : {
"mobile_no" : "8971740148"
},
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test.restaurants",
"indexFilterSet" : false,
"parsedQuery" : {
"mobile_no" : {
"$eq" : "8971740148"
}
},
"winningPlan" : {
"stage" : "COLLSCAN",
"filter" : {
"mobile_no" : {
"$eq" : "8971740148"
}
},
"direction" : "forward"
},
"rejectedPlans" : [ ]
}
}
},
{
"$unwind" : {
"path" : "$brands",
"includeArrayIndex" : "brandsposition"
}
},
{
"$lookup" : {
"from" : "brands",
"as" : "user_bookmarks",
"localField" : "brands",
"foreignField" : "_id"
}
}
],
"ok" : 1
}
现在任何人都可以从这里帮助我，我已经搜索了这两个东西，这是如何更改聚合内的字段类型，并且$ lookup利用索引，但是还没有发现有用的东西，请帮助我真的很有意义.
Now can anyone please help me out of here i have searched about both of this thing which is how to change type of field inside aggregation and does $lookup utilises indexes but Haven't found anything useful please help me out of here guys it would be really appreciable.
推荐答案
您无法将字符串转换为管道中的对象ID，您将不得不遍历每个文档并使用类似方法手动将其转换(您不应该无论如何都要存储类型的混合匹配，因此从长远来看可能值得更新):
You cannot convert the string to a object Id within the pipeline, you'll have to go though each document and convert it manually, using something like (you shouldnt be storing a mix match of types anyway, so it's probably worth updating in the long run):
$ lookup是否使用索引，如果您查看此博客的统计信息，就会发现使用了索引-
as for does $lookup use index, If you look at the stats from this blog you'll see that indexes are used -
http ://guyharrison.squarespace.com/blog/2016/7/4/join-performance-in-mongodb-32-using-lookup.html
这篇关于更改mongoDB聚合内的字段类型，并且$ lookup是否利用字段上的索引?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
