recursion - How to find MongoDB field name at arbitrary depth

Question

I imported some sort-of sloppy XML data into a Mongo database. Each Document has nested sub-documents to a depth of around 5-10. I would like to find() documents that have a particular value of a particular field, where the field may appear at any depth in the sub-documents (and may appear multiple times).

I am currently pulling each Document into Python and then searching that dictionary, but it would be nice if I could state a filter prototype where the database would only return documents that have a particular value of the field name somewhere in their contents.

Here is an example document:

{
    "foo": 1,
    "bar": 2,
    "find-this": "Yes!",
    "stuff": {
        "baz": 3,
        "gobble": [
            "wibble",
            "wobble",
            {
                "all-fall-down": 4,
                "find-this": "please find me"
            }                
        ],
        "plugh": {
            "plove": {
                "find-this": "Here too!"
            }
        }
   }
}

So, I'd like to find documents that have a "find-this" field, and (if possible) to be able to find documents that have a particular value of a "find-this" field.

Answer 1

You are right in the certain statement of a BSON document is not an XML document. Since XML is loaded into a tree structure that comprises of "nodes", searching on an arbitary key is quite easy.

A MonoDB document is not so simple to process, and this is a "database" in many respects, so it is generally expected to have a certain "uniformity" of data locations in order to make it easy to both "index" and search.

Nonetheless, it can be done. But of course this does mean a recursive process executing on the server and this means JavaScript processing with $where.

As a basic shell example, but the general function is just a string argument to the $where operator everywhere else:

db.collection.find(
  function () {
    var findKey = "find-this",
        findVal = "please find me";

    function inspectObj(doc) {
      return Object.keys(doc).some(function(key) {
        if ( typeof(doc[key]) == "object" ) {
          return inspectObj(doc[key]);
        } else {
          return ( key == findKey && doc[key] == findVal );
        }
      });
    }
    return inspectObj(this);
  }
)

So basically, test the keys present in the object to see if they match the desired "field name" and content. If one of those keys happens to be an "object" then recurse into the function and inspect again.

JavaScript .some() makes sure that the "first" match found will return from the search function giving a true result and returning the object where that "key/value" was present at some depth.

Note that $where essentially means traversing your whole collection unless there is some other valid query filter than can be applied to an "index" on the collection.

So use with care, or not at all and just work with re-structring the data into a more workable form.

But this will give you your match.