Mongoskin query check if document $exists using a variable as Query Key

Question

Using Mongoskin, I want to check if a document exists in a MongoDB collection where every document has a one unique key and an array as its value. This code works perfectly:

db.collection('buyerRec').find({ "abcd" : { $exists : true }}, { _id:0 }).toArray(function(err, doc) {
  ...
});

The "abcd" document is found. However, in the actual system's design the query key of the document is not known in advance and so I need to use a variable in place of "abcd". I can't find the combination that works. It always returns an empty array, in both failing cases-

Failing example-1:

console.log("idCode: " + typeof idCode + " " + idCode); // idCode: string abcd

db.collection('buyerRec').find({ idCode : { $exists : true }}, { _id:0 }).toArray(function(err, doc) {
  ...
});

Failing example-2:

console.log("idCode: " + typeof idCode + " " + idCode); // idCode: string abcd

var query = "\"" + idCode + "\"";
console.log("query: " + typeof query + " " + query);  // query: string "abcd"

db.collection('buyerRec').find({ query : { $exists : true }}, { _id:0 }).toArray(function(err, doc) {
  ...
});

To me, one of those 2 failing examples should have duplicated the intended operation of the first example. Can somebody please steer me as to how I need to re-code this ? Thx.

Community · Accepted Answer · 2017-05-23 10:24:20Z

1

I think your problem may be creating js object using variables for property name. you may try like the following:

 fieldName = "abcd"
 var query = {};
 query[fieldName] = {"$exists": true};
 db.collection('buyerRec').find(query)

But the above code is actually not that good, cause fieldName is not passed as parameter. In my .mongorc.js(mongo shell will load .mongorc.js every time it starts), I write function for this case, FYI.

 DBCollection.prototype.has = function(fieldName) {
   var query = {}; // construct a empty object
   query[fieldName] = {"$exists": true};
   return db.getCollection(this._shortName).find(query).pretty();
 }

Then you can just write query like:

 db.buyerRec.has("abc") // in mongo shell

But, I think you actually have to write your own function cause mine is just focusing on checking whether some key exists or not. Anyway, hope it helps.

edited May 23, 2017 at 10:24

CommunityBot

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answered Jul 26, 2015 at 3:42

Allen Chou

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1 Comment

Ric Over a year ago

That was a real bear and time soaker for me but your first code set nailed it. I can definitely understand why your code works. On my 2 failing examples the inner variable is a string so I assumed it would be evaluated. Maybe if the MongoDB/Mongoskin docs were more expansive it would say that an inner var will not be evaluated but must be included as part of the complete query object. That's how I understand it now. Thx.

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