MongoDB advanced query

Question

Let's say I have a Collection with two properties: amount and amountreceived that are decimals.

I want to write a query that will return items from the collection that meet the criteria of the amount being greater than the amount received.

Currently I have a javascript function that looks like this:

f = function() { return this.amount > this.amountreceived;}

I also have a collection set that looks like this:

item1 : amount = 50, amountreceived = 0;
item2 : amount = 50, amountreceived = 25;

If I run db.MyCollection.find(f) the only result returned is item1.

Any ideas on why that query will not return both results?

Answer 1

I'm assuming you are using the C# driver? Because BSON does not have a decimal data type the C# driver has to represent the .NET decimal value in some way. The default representation is as a string, which is good for not losing any precision but bad for doing queries.

You can also ask the C# driver to store .NET decimal values as BSON doubles, which would involve some loss of precision and could overflow, but would be good for queries.

Answer 2

Are you sure that you are doing this correctly? Do you have a counter-example for my example below?

Here's my sample test script where this works. (db.foo is empty)

MongoDB shell version: 1.6.5
connecting to: test
> db.foo.insert( {_id : 1, amount : 50, amtreceived : 100 } )
> db.foo.insert( {_id : 2, amount : 50, amtreceived : 25 } )
> db.foo.insert( {_id : 3, amount : 50, amtreceived : 0 } )
> db.foo.find()
{ "_id" : 1, "amount" : 50, "amtreceived" : 100 }
{ "_id" : 2, "amount" : 50, "amtreceived" : 25 }
{ "_id" : 3, "amount" : 50, "amtreceived" : 0 }
> f = function() { return this.amount > this.amtreceived; }
function () {
    return this.amount > this.amtreceived;
}
> db.foo.find(f) // expect 2 & 3
{ "_id" : 2, "amount" : 50, "amtreceived" : 25 }
{ "_id" : 3, "amount" : 50, "amtreceived" : 0 }