return only a single property "_id"

Question

I would like to know if there is a way to return only the _id, user_id and total without the items subdocument.

{     "_id" : 122,     "user_id" : 123456,     "total" : 100,     "items" : [             {                     "item_name" : "my_item_one",                     "price" : 20             },             {                     "item_name" : "my_item_two",                     "price" : 50             },             {                     "item_name" : "my_item_three",                     "price" : 30             }     ] } 

Answer 1

The second parameter of find lets you select fields. So you can use this (note that the _id field is always selected anyway):

db.mycollection.find({}, {"user_id": 1, "total": 1}); 

You can also exclude certain fields, so this would be equivalent:

db.mycollection.find({}, {"items": 0}); 

You can exclude _id field by doing:

db.mycollection.find({}, {"user_id": 1, "_id": 0}); 

Answer 2

What I was looking for is returning a list of ids, so return only _id but as an array for all documents. I did it as such:

db. mycollection.distinct("_id", {}) 

The first parameter is your distict fields, and second is your query. More on distinct here. Hopefully this helps someone with a similar requirement. Cheers!