In 4.2+, you can use the $set
aggregation pipeline operator which is nothing other than an alias to $addFields
added 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" } }
])