MongoDB update. Trying to set one field from a property of another

Question

What I\'m trying to do is pretty straightforward, but I can\'t find out how to give one field the value of another.

I simply want to update one field with the character

Answer 1

1. Your are using exists in the wrong way.
   Syntax: { field: { $exists: } }

2. You use of $where is also incorrect
   Use the $where operator to pass either a string containing a JavaScript expression or a full JavaScript function to the query system

   db.myCollection.find( { $where: "this.credits == this.debits" } ); db.myCollection.find( { $where: "obj.credits == obj.debits" } );

   db.myCollection.find( { $where: function() { return (this.credits == this.debits) } } );

   db.myCollection.find( { $where: function() { return obj.credits == obj.debits; } } );

I think you should use Map-Reduce for what you are trying to do.