Include all existing fields and add new fields to document

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?

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" } }
])

You can use $$ROOT to references the root document. Keep all fields of this document in a field and try to get it after that (depending on your client system: Java, C++, ...)

 [
    {
        $project: {
            custom_field: "$obj.obj_field1",
            document: "$$ROOT"

        }
    },
    ... //group, match, and whatever...
]

>>> There's something like "include all fields" keyword that I can use in this case or some another solution?

Unfortunaly, there is no operator to "include all fields" in aggregation operation. The only reason, why, because aggregation is mostly created to group/calculate data from collection fields (sum, avg, etc.) and return all the collection's fields is not direct purpose.

To add new fields to your document you can use $addFields

from docs

and to all the fields in your document, you can use $$ROOT

db.collection.aggregate([

{ "$addFields": { "custom_field": "$obj.obj_field1" } },
{ "$group": {
        _id : "$field1",
        data: { $push : "$$ROOT" }
    }}
])