db={"bookings":[{"_id":1,"vehicle_id":"1","vehicle_energy_type":"electric","battery_level":10},{"_id":2,"vehicle_id":"1","vehicle_energy_type":"electric"},{"_id":3,"vehicle_id":"1","vehicle_energy_type":"diesel"},],"vehicles":[{"_id":"1","energy_type":"electric"}]}

db.bookings.aggregate([{"$lookup":{"from":"vehicles","localField":"vehicle_id","foreignField":"_id","as":"vehicles"}},{"$unwind":"$vehicles"},{$set:{vehicle_energy_type:"$vehicles.energy_type",battery_level:{"$cond":{"if":{$eq:["$vehicles.energy_type","electric"]},"then":10,"else":"$$REMOVE"}},vehicles:"$$REMOVE"}},{"$merge":{"into":"bookings","on":"_id","whenMatched":"merge"}}])