mongodb aggregate show specific fields code example

Example 1: mongodb project all fields

db.collection.aggregate([
    { "$addFields": { "custom_field": "$obj.obj_field1" } }
])

Example 2: mongodb aggregate over nested fields

> db.demo99.aggregate([ { $project: { Count: { $objectToArray: "$Details" } } }, { $unwind: "$Count" }, { $group: { _id: "$Count.k", count: { $sum: "$Count.v.Number"} } } ])