mongodb - Include all existing fields and add new fields to document

Question

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.

My document looks like this, with many fields:

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

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...
]

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?

Answer 1

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

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" } }
])