包括所有现有字段并向文档添加新字段 [英] Include all existing fields and add new fields to document

问题描述

我想定义一个$ project聚合阶段，可以指示它添加一个新字段并包括所有现有字段，而不必列出所有现有字段.

I would like to define a $project aggregation stage where I can instruct it to add a new field and include all existing fields, without having to list all the existing fields.

我的文档如下所示，其中包含许多字段:

My document looks like this, with many fields:

{
    obj: {
        obj_field1: "hi",
        obj_field2: "hi2"
    },
    field1: "a",
    field2: "b",
    ...
    field26: "z"
}

我要进行这样的聚合操作:

I want to make an aggregation operation like this:

[
    {
        $project: {
            custom_field: "$obj.obj_field1",
            //the next part is that I don't want to do
            field1: 1,
            field2: 1,
            ...
            field26: 1
        }
    },
    ... //group, match, and whatever...
]

在这种情况下，是否可以使用类似包括所有字段"这样的关键字，或者可以通过其他方式避免单独列出每个字段?

Is there something like an "include all fields" keyword that I can use in this case, or some other way to avoid having to list every field separately?

推荐答案

在4.2及更高版本中，您可以使用

In 4.2+, you can use the $set aggregation pipeline operator which is nothing other than an alias to $addFieldsadded in 3.4

$addFields阶段等效​​于 $project 阶段，在输入文档中明确指定所有现有字段并添加新字段.

The $addFields stage is equivalent to a $project stage that explicitly specifies all existing fields in the input documents and adds the new fields.

db.collection.aggregate([
    { "$addFields": { "custom_field": "$obj.obj_field1" } }
])

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