How to find MongoDB field name at arbitrary depth

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.

Here is one example, which I use for recursive search for Key-Value anywhere in document structure:

db.getCollection('myCollection').find({

    "$where" : function(){

        var searchKey = 'find-this';
        var searchValue = 'please find me';

        return searchInObj(obj);

        function searchInObj(obj){                            
          for(var k in obj){       
            if(typeof obj[k] == 'object' && obj[k] !== null){
              if(searchInObj(obj[k])){
                return true;
              }
            } else {
              if(k == searchKey && obj[k] == searchValue){
                return true;
              }
            }          
          }                         
          return false;
        }       
    }    
})