按ID分组在Spring Data MongoDB中不起作用 [英] Group by id doesn't work in spring data mongodb

问题描述

我试图在Mongodb中使用Spring数据中的聚合。经过几个阶段展开，查找，匹配并在投影后得到以下样本数据，然后尝试按 _id 分组。

Im trying to use Aggregation in Spring data with mongodb. After few stages unwind, lookup, match I come up with the sample following data which is after the projection, then I try to group it by _id.

{ "_id": 1, "name":"Maths" },
{ "_id": 1, "name":"Maths" },
{ "_id": 2, "name":"Science" },
{ "_id": 2, "name":"Science" }

以下mongo脚本运行良好。

The following mongo script is working perfectly.

{
    $project: 
    {
        name: 1
    }
}, 
{
    $group: 
    {
        _id: '$_id',
        name: {
            $first: '$name'
        }
    }
}

在春季时，

group("_id").first("name").as("name")

但显示错误引用'_id'无效！
但是，当我执行以下操作时，它的工作正常。

But it shows an error Invalid reference '_id'! But when I do something like following, its working fine.

aggregationOperationContext -> {
  return new Document("$group",
             new Document("_id", "$_id").append("name", new Document("$first", "$name")));
}

为什么不 group（）工作吗？

注意：上面的代码是

new AggregationOperation() {
    @Override
    public Document toDocument(AggregationOperationContext aggregationOperationContext) {
        // statements
    }
}

推荐答案

实际上，您认为正确的方式。当您进行投影时，仅可以投影 name ，就像 project（ name）一样，给出 _id 和 name 作为结果。

Actually the way you think is correct. When you project you just can project name only like project("name") which gives _id and name as result.

但是在这种情况下，您还应该投影 _id 字段。我认为您只投影了名称

But in this case you should project the _id field also. I think you projected only name

project("_id","name"),
group("_id").first("name").as("name")

这应该有效。我认为这可能是spring-data中的错误。

This should work. I think it might be a bug in spring-data.

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