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] } } }