MongoDB excluding fields from aggregate, is that possible?

Question

Im trying to exclude some fields from my booking aggregate.

{ "user":0, "paymentType":0, "_id":0 }

without this it works great, but with it I get the errors:

assert: command failed: {
    "ok" : 0,
    "errmsg" : "A pipeline stage specification object must contain exactly one field.",
    "code" : 40323,
    "codeName" : "Location40323"
} : aggregate failed

Is this possible to do, or can I only do it with find() ?

db.getCollection('booking').aggregate([{
    $match: {
        checkin : {$lte: (1512145439)},
        checkout: {$gte: (1483203600)},
    }
}, 
    {
     "user":0, 
     "paymentType":0, 
     "_id":0
    }
, {
    $lookup: {
        from: "users",
        localField: "user",
        foreignField: "_id",
        as: "users"
    }   
}, {
    $unwind: "$user"
}, {
    $addFields: {
        "country": "$users.country"
    }
}, {
    $project: {
        "users": 0
    }    
}, {
    $project: {
        "booking": 0
    }
}])

If I do like this, it will exclude the fields:

db.getCollection('booking').find({},        
        {
         "user":0, 
         "paymentType":0, 
         "_id":0
        })

Answer 1

you can use $unset or $project

$unset and $project : The $unset is an alias for the $project stage that removes/excludes fields

{ $unset: [ "<field1>", "<field2>", ... ] }

{ $project: { "<field1>": 0, "<field2>": 0, ... } }//Return all but the specified fields

Answer 2

You've to use the field exclusion/inclusion inside the $project stage in aggregation pipeline.

On a side note use $addFields when you need to add new fields / overwrite fields and use $project when you want to limit fields.

From the docs,

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