Question

将简单的.find().sort().skip().limit()迁移到Aggregation Framework时遇到了问题，导致分页中断：同一文档出现在多个页面上。最终，我意识到$limit影响$sort的结果顺序。下面我将描述如何重现步骤以查看问题（在Mongo 3.4.17和3.6.7上已选中）。

> // 1. Create a simple coleection and fill it with test data: 
> for(var i = 0; i < 10; ++i) db.SomeCollection.insert({sortField: "just_some_string"});
> db.SomeCollection.count();
10
>
> // 2. Check, that with .find().sort().limit() the order of results does not depend on the `limit()`'s value:
db.SomeCollection.find().sort({"sortField": 1}).limit(5)
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb531"), "sortField" : "just_some_string" }
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb532"), "sortField" : "just_some_string" }
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb533"), "sortField" : "just_some_string" }
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb534"), "sortField" : "just_some_string" }
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb535"), "sortField" : "just_some_string" }
> db.SomeCollection.find().sort({"sortField": 1}).limit(8)
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb531"), "sortField" : "just_some_string" }
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb532"), "sortField" : "just_some_string" }
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb533"), "sortField" : "just_some_string" }
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb534"), "sortField" : "just_some_string" }
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb535"), "sortField" : "just_some_string" }
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb536"), "sortField" : "just_some_string" }
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb537"), "sortField" : "just_some_string" }
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb538"), "sortField" : "just_some_string" }
>
> // 3. Logically the same query but using Aggregation Framework
> db.SomeCollection.aggregate([{$sort: {"sortField": 1}}, {$limit: 5}]);
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb534"), "sortField" : "just_some_string" }
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb535"), "sortField" : "just_some_string" }
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb532"), "sortField" : "just_some_string" }
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb531"), "sortField" : "just_some_string" }
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb533"), "sortField" : "just_some_string" }
> db.SomeCollection.aggregate([{$sort: {"sortField": 1}}, {$limit: 8}]);
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb538"), "sortField" : "just_some_string" }
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb535"), "sortField" : "just_some_string" }
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb532"), "sortField" : "just_some_string" }
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb534"), "sortField" : "just_some_string" }
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb536"), "sortField" : "just_some_string" }
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb531"), "sortField" : "just_some_string" }
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb533"), "sortField" : "just_some_string" }
{ "_id" : ObjectId("5ba3ddf3a07260b77dddb537"), "sortField" : "just_some_string" }

我的问题是：

为什么.aggregate()中带有$ sort和$ limit的结果顺序与.find().sort().limit()不同？
为什么.aggregate()中结果的顺序取决于$limit的值？

Answer 1

所有这些文档都具有相同的sortField值，因此顺序不确定。在_id上添加辅助排序，以确保结果一致。

db.SomeCollection.aggregate([{$sort: {"sortField": 1, _id: 1}}, {$limit: 5}]);

$ sort更改结果顺序后，MongoDB的$ limit值合计

1 个答案: