Selectively retrieving depending on existence of key in map

Question

Is it possible to selectively retrieve depending on the existence of keys in a map in mongodb? And if so, how do you go about doing it?

Suppose I have a document that looks like this for example..

{ "_id": 1234,
    "parentfield1" : {
        "childfield1" : { ...},
        "childfield2" : { ...},
        "childfield5" : { ...}, // There might be many childfields.. > 50
    },
}

How would I be able to selectively retrieve from the document a/some particular childfields given multiple options to choose from? Some of which may not exist in the document.

i.e.

input "childfield1", "childfield2", "childfield3"

-> output 

{ "_id": 1234,
  "parentfield1": {
      "childfield1" : { ... },
      "childfield2" : { ... },
  },
}

Is it even doable? Is it possible to do efficiently also? Any help would be great (python, go).

score 0 · Accepted Answer · answered Feb 08 '13 at 16:26

Yes, that's the purpose of the projection parameter of find:

db.collection.find({_id: 1234}, {
    'parentfield1.childfield1': 1,
    'parentfield1.childfield2': 1,
    'parentfield1.childfield3': 1
});

If a specified field isn't present in a given doc, the other matching fields will still be included.

Build up the projection parameter object programmatically if you want it to be dynamic.

Selectively retrieving depending on existence of key in map

1 Answers1