mongodb 如何从数组中删除重复的条目？

How to remove duplicate entries from an array?

提问by P K

In the following example, "Algorithms in C++"is present twice.

在下面的例子中，"Algorithms in C++"出现了两次。

The $unsetmodifier can remove a particular field but how to remove an entry from a field?

该$unset修改可以删除特定的领域，但如何从一个字段中输入？

{
  "_id" : ObjectId("4f6cd3c47156522f4f45b26f"), 
  "favorites" : {
    "books" : [
      "Algorithms in C++",    
      "The Art of Computer Programming", 
      "Graph Theory",      
      "Algorithms in C++"
    ]
  }, 
  "name" : "robert"
}

采纳答案by Baba

What you have to do is use map reduce to detect and count duplicate tags .. then use $setto replace the entire books based on { "_id" : ObjectId("4f6cd3c47156522f4f45b26f"),

您需要做的是使用 map reduce 来检测和计算重复标签 .. 然后使用$set基于替换整本书{ "_id" : ObjectId("4f6cd3c47156522f4f45b26f"),

This has been discussed sevel times here .. please seee

这已经在这里讨论了七次..请参阅

Removing duplicate records using MapReduce

使用 MapReduce 删除重复记录

Fast way to find duplicates on indexed column in mongodb

在 mongodb 中的索引列上查找重复项的快速方法

http://csanz.posterous.com/look-for-duplicates-using-mongodb-mapreduce

http://csanz.posterous.com/look-for-duplicates-using-mongodb-mapreduce

http://www.mongodb.org/display/DOCS/MapReduce

http://www.mongodb.org/display/DOCS/MapReduce

How to remove duplicate record in MongoDB by MapReduce?

如何通过 MapReduce 删除 MongoDB 中的重复记录？

回答by kynan

As of MongoDB 2.2 you can use the aggregation frameworkwith an $unwind, $groupand $projectstage to achieve this:

从 MongoDB 2.2 开始，您可以使用带有,和stage的聚合框架来实现这一点：$unwind$group$project

db.users.aggregate([{$unwind: '$favorites.books'},
                    {$group: {_id: '$_id',
                              books: {$addToSet: '$favorites.books'},
                              name: {$first: '$name'}}},
                    {$project: {'favorites.books': '$books', name: '$name'}}
                   ])

Note the need for the $projectto rename the favoritesfield, since $groupaggregate fields cannot be nested.

请注意需要$project重命名favorites字段，因为$group聚合字段不能嵌套。

回答by Dennis Golomazov

The easiest solution is to use setUnion(Mongo 2.6+):

最简单的解决方案是使用setUnion(Mongo 2.6+)：

db.users.aggregate([
    {'$addFields': {'favorites.books': {'$setUnion': ['$favorites.books', []]}}}
])

Another (more lengthy) version that is based on the idea from @kynan's answer, but preserves all the other fields without explicitly specifying them (Mongo 3.4+):

另一个（更冗长）版本基于@kynan's answer的想法，但保留了所有其他字段而没有明确指定它们（Mongo 3.4+）：

> db.users.aggregate([
    {'$unwind': {
        'path': '$favorites.books',
        // output the document even if its list of books is empty
        'preserveNullAndEmptyArrays': true
    }},
    {'$group': {
        '_id': '$_id',
        'books': {'$addToSet': '$favorites.books'},
        // arbitrary name that doesn't exist on any document
        '_other_fields': {'$first': '$$ROOT'},
    }},
    {
      // the field, in the resulting document, has the value from the last document merged for the field. (c) docs
      // so the new deduped array value will be used
      '$replaceRoot': {'newRoot': {'$mergeObjects': ['$_other_fields', "$$ROOT"]}}
    },
    // this stage wouldn't be necessary if the field wasn't nested
    {'$addFields': {'favorites.books': '$books'}},
    {'$project': {'_other_fields': 0, 'books': 0}}
])

{ "_id" : ObjectId("4f6cd3c47156522f4f45b26f"), "name" : "robert", "favorites" : 
{ "books" : [ "The Art of Computer Programmning", "Graph Theory", "Algorithms in C++" ] } }    

回答by Xavier Guihot

Starting in Mongo 4.4, the $functionaggregation operator allows applying a custom javascript function to implement behaviour not supported by the MongoDB Query Language.

从 开始Mongo 4.4，$function聚合运算符允许应用自定义 javascript 函数来实现 MongoDB 查询语言不支持的行为。

For instance, in order to remove duplicates from an array:

例如，为了从数组中删除重复项：

// {
//   "favorites" : { "books" : [
//     "Algorithms in C++",
//     "The Art of Computer Programming",
//     "Graph Theory",
//     "Algorithms in C++"
//   ]},
//   "name" : "robert"
// }
db.collection.aggregate(
  { $set:
    { "favorites.books":
      { $function: {
          body: function(books) { return books.filter((v, i, a) => a.indexOf(v) === i) },
          args: ["$favorites.books"],
          lang: "js"
      }}
    }
  }
)
// {
//   "favorites" : { "books" : [
//     "Algorithms in C++",
//     "The Art of Computer Programming",
//     "Graph Theory"
//   ]},
//   "name" : "robert"
// }

This has the advantages of:

这具有以下优点：

• keeping the original order of the array (if that's not a requirement, then prefer @Dennis Golomazov's $setUnion answer)
• being more efficient than a combination of expensive $unwindand $groupstages.

• 保持数组的原始顺序（如果这不是必需的，那么更喜欢@Dennis Golomazov 的 $setUnion 答案）
• 比昂贵$unwind和$group阶段的组合更有效。

$functiontakes 3 parameters:

$function需要3个参数：

• body, which is the function to apply, whose parameter is the array to modify.
• args, which contains the fields from the record that the bodyfunction takes as parameter. In our case "$favorites.books".
• lang, which is the language in which the bodyfunction is written. Only jsis currently available.

• body，这是要应用的函数，其参数是要修改的数组。
• args，其中包含该body函数作为参数的记录字段。在我们的情况下"$favorites.books"。
• lang，这body是编写函数的语言。仅js当前可用。