Question

我有一个如下所示的作者集合：

{ "_id" : ObjectId("332ddf"), 
  "authors" : "Mark Twain", 
  "publisher" : "NY", 
  "books" : [ "The Adventures of Tom Sawyer", "The Prince and the Pauper" ] }

{ "_id" : ObjectId("4ef342"), 
  "authors" : "F. Scott Fitzgerald", 
  "publisher" : "NY", 
  "books" : [ "The Adventures of Tom Sawyer", 
              "The Great Gatsby", "This Side of Paradise" ] }

我正在尝试使用聚合来创建新集合。为了聚合，我尝试了：

db.author.aggregate([
   { "$unwind": "$books"},
   { $project: 
     { 
        book: "$books",
        authors: ["$authors"]

     }
     }
])

但我得到duplicate key error collection来解开书籍。如何使用聚合获得所需的集合？

Answer 1

实际上你不需要$project阶段。您需要做的就是使用$unwind运算符“denormalize”“Books”数组，然后使用“book”$group将文档“{3}}”并使用$push运算符返回“作者”数组”

var cursor = db.getCollection('authors').aggregate([
    { "$unwind": "$books" }, 
    { "$group": { 
        "_id": "$books", 
        "authors": { "$push": "$authors" }
    }}
])

聚合查询产生如下内容：

{ "_id" : "This Side of Paradise", "authors" : [ "F. Scott Fitzgerald" ] }
{ "_id" : "The Great Gatsby", "authors" : [ "F. Scott Fitzgerald" ] }
{ "_id" : "The Prince and the Pauper", "authors" : [ "Mark Twain" ] }
{
        "_id" : "The Adventures of Tom Sawyer",
        "authors" : [
                "Mark Twain",
                "F. Scott Fitzgerald"
        ]
}

顺便说一句，预期结果中的_id复合字段没有多大意义，所以我删除了它，但如果您确实觉得需要它，那么只需将"_id": "$books"替换为"_id": { "book": "$books" }小组赛。

现在让我们看看如何插入到另一个集合中。在下面的评论中提到的一种方法是使用$out运算符，它必须是聚合管道中的最后一个阶段。

{ "$out": "newCollection" }

如果您需要在将文档插入新集合之前处理结果客户端，则应使用“批量”操作。

MongoDB 3.2或更新版

var requests = [];
var count = 0;

cursor.forEach(function(document) {
    // Do something with the document and push a new operation to the stack
    requests.push({ insertOne : document });
    count++;
    if(count % 1000 === 0) {
        db.newCollection.bulkWrite(requests);
        requests = [];
        count = 0;
    }
});

db.newCollection.bulkWrite(requests);

MongoDB 3.0向后

var bulk = db.newCollection.initializeUnorderedBulkOp();
var count = 0;

cursor.forEach(function(document) {
    // Do something with the document and push a new operation to the stack
    bulk.insert(document);
    count++;
    if (count % 1000 === 0) {
        bulk.execute(); 
        bulk = db.newCollection.initializeUnorderedBulkOp();
    }
})

if (count > 0 )  {
    bulk.execute();
}

匹配MongoDB中数组中的每个对象

1 个答案:

MongoDB 3.2或更新版

MongoDB 3.0向后