Mongodb $where query always true with nodejs

Question

When I query my database with a function passed in the \"$where\" clause in nodejs, it always return me all documents in the db.

For example, if I do

Answer 1

This is how it is supposed to be. The drivers don't translate the client code into the mongo function javascript code.

Answer 2

You can use a wrapper to pass basic JSON objects, ie. (pardon coffee-script):

# That's the main wrapper.
wrap = (f, args...) ->
  "function() { return (#{f}).apply(this, #{JSON.stringify(args)}) }"

# Example 1
where1 = (flag) ->
  @myattr == 'foo' or flag

# Example 2 with different arguments
where2 = (foo, options = {}) ->
  if foo == options.bar or @_id % 2 == 0
    true
  else
    false

db.collection('coll1').count $where: wrap(where1, true), (err, count) ->
  console.log err, count

db.collection('coll1').count $where: wrap(where2, true, bar: true), (err, count) ->
  console.log err, count

Your functions are going to be passed as something like:

function () {
    return (function (flag) {
        return this.myattr === 'foo' || flag;
    }).apply(this, [true])
}

...and example 2:

function () {
    return (
        function (foo, options) {
            if (options == null) {
                options = {};
            }
            if (foo === options.bar || this._id % 2 === 0) {
                return true;
            } else {
                return false;
            }
        }
    ).apply(this, [ true, { "bar": true } ])
}

Answer 3

I'm assuming you are using Mongoose to query your database.

If you take a look at the actual Query object implementation, you'll find that only strings are valid arguments for the where prototype.

When using the where clause, you should use it along with the standard operators such as gt, lt that operates on in the path created by the where function.

Remember that Mongoose querying, as in Mongo, is by example, you may want to reconsider your query specification in a more descriptive fashion.

Answer 4

First off, keep in mind that the $where operator should almost never be used for the reasons explained here (credit goes to @WiredPrairie).

Back to your issue, the approach you'd like to take won't work even in the mongodb shell (which explicitly allows naked js functions with the $where operator). The javascript code provided to the $where operator is executed on the mongo server and won't have access to the enclosing environment (the "context bindings").

> db.test.insert({a: 42})
> db.test.find({a: 42})
{ "_id" : ObjectId("5150433c73f604984a7dff91"), "a" : 42 }
> db.test.find({$where: function() { return this.a == 42 }}) // works
{ "_id" : ObjectId("5150433c73f604984a7dff91"), "a" : 42 }
> var local_var = 42
> db.test.find({$where: function() { return this.a == local_var }})
error: {
    "$err" : "error on invocation of $where function:\nJS Error: ReferenceError: local_var is not defined nofile_b:1",
    "code" : 10071
}

Moreover it looks like that the node.js native mongo driver behaves differently from the shell in that it doesn't automatically serialize a js function you provide in the query object and instead it likely drops the clause altogether. This will leave you with the equivalent of timetables.find({}) which will return all the documents in the collection.

Answer 5

This one is works for me , Just try to store a query as a string in one variable then concat your variable in query string,

var local_var = 42

var query = "{$where: function() { return this.a == "+local_var+"}}"

db.test.find(query)

Answer 6

Store your query into a varibale and use that variable at your find query. It works..... :D