Database
db={"user":[{"_id":"9efb42e5-514d-44bd-a4b8-6f74e6313ec2","name":"Haralt","age":21,"bloodlineId":"c59a2d02-f304-49a8-a52a-44018fc15fe6","villageId":"foovillage"}],"bloodlines":[{"_id":"c59a2d02-f304-49a8-a52a-44018fc15fe6","name":"Tevla","legacy":0}]}
Query
db.user.aggregate([{"$match":{_id:"9efb42e5-514d-44bd-a4b8-6f74e6313ec2"}},{"$lookup":{from:"bloodlines",localField:"bloodlineId",foreignField:"_id",as:"bloodline"}},{$project:{"age":1,"bloodlineId":{$arrayElemAt:["$bloodline",0]},"name":1,"villageId":1}}])