MongoDB $project: Retain previous pipeline fields [duplicate]

Answer 1

Only _id will be projected by default. You have to specify what other fields have to be projected.

{ $project: { field1:1,field2:1, field3: { $gt: ['$field1', 10] } } }

or

{ $project: { field1:'$field1',field2:'$field2', field3: { $gt: ['$field1', 10] } } }

Answer 2

MongoDB 2.6 introduced a new $$ROOT aggregation variable which almost does what you want: it will include the full document being processed in the current pipeline stage, so you don't need to know or specify what fields to include.

Taking your example aggregation and adding a fields projection with $$ROOT:

db.pipeline.aggregate(
    { $project: {
        field3: {
            $gt: ['$field1', 10]
        },
        fields: "$$ROOT"
    }}
)

The output will include both new calculated field(s) and all the original fields:

{
    "_id" : ObjectId("53b56cc542974939144cee2a"),
    "field3" : false,
    "fields" : {
        "_id" : ObjectId("53b56cc542974939144cee2a"),
        "field1" : 1,
        "field2" : 2,
        "field3" : 3
    }
}

MongoDB 3.3.11 introduces the addFields aggregation stage that does what you want. See this entry in MongoDB issue tracker: SERVER-5781 - Implement $addFields aggregation stage for using expression language to add new fields to a document.

Answer 3

where $project needs to specify which fields to pass through, $addFields will return all fields and add or replace specified fields.

{ $addFields: { field3: { $gt: ['$field1', 10] } } }

will achieve exactly what you want.

Please note, this feature was added in Mongo version 3.4.