嵌套数组的MongoDB投影

本文介绍了嵌套数组的MongoDB投影的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

我有一个帐户"集合，其中包含类似于以下结构的文档:

I've got a collection "accounts" which contains documents similar to this structure:

{ "email" : "john.doe@acme", "groups" : [ { "name" : "group1", "contacts" : [ { "localId" : "c1", "address" : "some address 1" }, { "localId" : "c2", "address" : "some address 2" }, { "localId" : "c3", "address" : "some address 3" } ] }, { "name" : "group2", "contacts" : [ { "localId" : "c1", "address" : "some address 1" }, { "localId" : "c3", "address" : "some address 3" } ] } ] }

通过

q = { "email" : "john.doe@acme", "groups" : { $elemMatch: { "name" : "group1" } } } p = { "groups.name" : 0, "groups" : { $elemMatch: { "name" : "group1" } } } db.accounts.find( q, p ).pretty()

我将仅成功获得我感兴趣的指定帐户的组.

I'll successfully get just the group of a specified account I'm interested in.

问题:如何在指定帐户"的某个组"中获取有限的联系人"列表?假设我有以下参数:

Question: How can I get a limited list of "contacts" within a certain "group" of a specified "account"? Let's suppose I've got the following arguments:

• 帐户:电子邮件-"john.doe@acme"
• group:名称-"group1"
• 联系人:localId数组-["c1"，"c3"，不存在的ID"]

鉴于这些论点，我希望得到以下结果:

Given these arguments I'd like to have the following result:

{ "groups" : [ { "name" : "group1", (might be omitted) "contacts" : [ { "localId" : "c1", "address" : "some address 1" }, { "localId" : "c3", "address" : "some address 3" } ] } ] }

除了生成的联系人外，我不需要其他任何东西.

I don't need anything else apart from the resulting contacts.

方法

为简单起见，所有查询都尝试仅获取一个匹配的联系人，而不是匹配的列表. 我尝试了以下查询，但均未成功:

All queries try to fetch just one matching contact instead of a list of matching contacts, for the sake of simplicity. I've tried the following queries without any success:

p = { "groups.name" : 0, "groups" : { $elemMatch: { "name" : "group1", "contacts" : { $elemMatch: { "localId" : "c1" } } } } } p = { "groups.name" : 0, "groups" : { $elemMatch: { "name" : "group1", "contacts.localId" : "c1" } } } not working: returns whole array or nothing depending on localId p = { "groups.$" : { $elemMatch: { "localId" : "c1" } } } error: { "$err" : "Can't canonicalize query: BadValue Cannot use $elemMatch projection on a nested field.", "code" : 17287 } p = { "groups.contacts" : { $elemMatch: { "localId" : "c1" } } } error: { "$err" : "Can't canonicalize query: BadValue Cannot use $elemMatch projection on a nested field.", "code" : 17287 }

感谢您的帮助！

推荐答案

2017更新

这样一个好的问题应该得到现代的回应.实际上，可以在现代MongoDB 3.2版中通过简单地 $match 和 $project 管道阶段，很像原始的普通查询操作打算的.

2017 Update

Such a well put question deserves a modern response. The sort of array filtering requested can actually be done in modern MongoDB releases post 3.2 via simply $match and $project pipeline stages, much like the original plain query operation intends.

db.accounts.aggregate([ { "$match": { "email" : "john.doe@acme", "groups": { "$elemMatch": { "name": "group1", "contacts.localId": { "$in": [ "c1","c3", null ] } } } }}, { "$addFields": { "groups": { "$filter": { "input": { "$map": { "input": "$groups", "as": "g", "in": { "name": "$$g.name", "contacts": { "$filter": { "input": "$$g.contacts", "as": "c", "cond": { "$or": [ { "$eq": [ "$$c.localId", "c1" ] }, { "$eq": [ "$$c.localId", "c3" ] } ] } } } } } }, "as": "g", "cond": { "$and": [ { "$eq": [ "$$g.name", "group1" ] }, { "$gt": [ { "$size": "$$g.contacts" }, 0 ] } ] } } } }} ])

这利用了 $filter 和 $map 运算符，仅在满足条件时从数组中返回元素条件，并且比使用 $unwind .由于流水线阶段有效地反映了.find()操作中的查询"和项目"的结构，因此此处的性能基本上与此类操作相同.

This makes use of of the $filter and $map operators to only return the elements from the arrays as would meet the conditions, and is far better for performance than using $unwind. Since the pipeline stages effectively mirror the structure of "query" and "project" from a .find() operation, the performance here is basically on par with such and operation.

请注意，如果要实际使用跨文档" 将详细信息从多个"文档而不是一个"文档中整合到一起，则通常需要某种类型的$unwind操作，以使数组项可以进行分组".

Note that where the intention is to actually work "across documents" to bring details together out of "multiple" documents rather than "one", then this would usually require some type of $unwind operation in order to do so, as such enabling the array items to be accessible for "grouping".

这基本上是方法:

db.accounts.aggregate([ // Match the documents by query { "$match": { "email" : "john.doe@acme", "groups.name": "group1", "groups.contacts.localId": { "$in": [ "c1","c3", null ] }, }}, // De-normalize nested array { "$unwind": "$groups" }, { "$unwind": "$groups.contacts" }, // Filter the actual array elements as desired { "$match": { "groups.name": "group1", "groups.contacts.localId": { "$in": [ "c1","c3", null ] }, }}, // Group the intermediate result. { "$group": { "_id": { "email": "$email", "name": "$groups.name" }, "contacts": { "$push": "$groups.contacts" } }}, // Group the final result { "$group": { "_id": "$_id.email", "groups": { "$push": { "name": "$_id.name", "contacts": "$contacts" }} }} ])

这是对多个匹配项的数组过滤"，这是.find()的基本投影功能无法完成的.

This is "array filtering" on more than a single match which the basic projection capabilities of .find() cannot do.

您有嵌套"数组，因此需要处理 $unwind 两次.以及其他操作.

You have "nested" arrays therefore you need to process $unwind twice. Along with the other operations.