Question

I have an open source project that deals with mongodb database. I am trying to make a function that queries the database to check if entry exists.

The problem is when if_exists() returning true or false it returns undefined since the mongodb driver function is asynchronous. The file is Query.js and I have tried the solution here to workaround the problem What is the right way to make a synchronous MongoDB query in Node.js? but still I get an undefined result with the get method.

What is the best way to make this work?

The output from the unit tests is as the following:

running unit tests...
add query test
exists tests:
get: undefined
{}
should be true: undefined
get: undefined
{}
should be false:undefined
Captains Logs listening on port 3000
Captains_Logs v0.5.0-21
[ { name: 'rhcp', _id: 50cbdcbe9c3cf97203000002 } ]
[ { name: 'os', _id: 50cbdcbe9c3cf97203000001 } ]

You can browse the whole codes at WeaponXI/cplog

Or for a quick look the query.js code is:

var DB = require('../../lib/db.js').DB;

function methods() {
  //query object
  var Q = {};
  //will act as our private variables to workaround asynchronous functions.
  //will delete non-required ones when done -- we don't have to, but just for continuity.
  exports.privates = {};




  //add tag to collection
  Q.add = function(tag) {
    if (typeof tag === "string") {
      //maybe we are adding a tag by name
      var obj = {
        name: tag
      };
    } else if (typeof tag === "object" && tag.name) {
      //maybe the tag object was specified, and tag's name was provided
      var obj = tag;
    }

    require('mongodb').connect(DB.mongo_url, function(err, db) {
      db.collection('tags', function(err, coll) {
        coll.insert(obj, {
          safe: true
        }, function(err, result) {
          console.log(result);

        });

      });
    });
  }
  var callback = {
    _set: function(key, val) {
      exports.privates[key] = val;
      //console.log(JSON.stringify(privates));
    },
    _get: function(key) {
      console.log("get: "+exports.privates.key);
      console.log(JSON.stringify(exports.privates));
      return exports.privates[key];
    },
    _unset: function(key) {
      delete privates[key];
    }
  }
  var if_exists = function(query, where, callback) {

    require('mongodb').connect(DB.mongo_url, function(err, db) {
      db.collection(where, function(err, coll) {
        coll.findOne(query, function(e, r) {
          //console.log(r);
          if (r === null) {
            callback._set("does_exist", false);
          } else {
            callback._set("does_exist", true);
          }

        });
      });
    });

    var result = callback._get("does_exist");
    // delete privates.does_exist;

    return result;
  }

  Q.if_exists = function(query, where) {
    if_exists(query, where, callback);

  }



  return Q;
}

var query = exports.query = methods();

function unit_test_add() {
  console.log("add query test");
  query.add("os");
  query.add({
    name: "rhcp"
  });
}

function unit_test_if_exists() {
  console.log("exists tests:");
  console.log("should be true: " + query.if_exists({
    name: "os"
  }, "tags"));
  console.log("should be false:" + query.if_exists({
    name: "ossuruk"
  }, "tags"));

}

function unit_tests() {
  console.log("running unit tests...");
  unit_test_add();
  unit_test_if_exists();

}
unit_tests();

Solution:

Query.js Query.test.js Gists

Thanks JohnnyHK!

Answer 1

You cannot use an asynchronous result as the return value from a function. It’s that simple. You have to deliver the asynchronous result to the caller via a callback that is provided as a parameter to the function (or use futures/promises and effectively defer that step, but that’s more involved).

if_exists should look like this instead:

var if_exists = function(query, where, callback) {

  require('mongodb').connect(DB.mongo_url, function(err, db) {
    db.collection(where, function(err, coll) {
      coll.findOne(query, function(e, r) {
        //console.log(r);
        if (r === null) {
          callback(e, false);
        } else {
          callback(e, true);
        }
        // You should either close db here or connect during start up
        // and leave it open.
        db.close();
      });
    });
  });
}