Using $exists in a MongoDB expression

Question

For background, if I want to compare two fields, I can\'t use the following syntax (because it compares to the literal string \"$lastName\" rather than the contents of the $last

Answer 1

You will need to use the $or logical operator to do this.

{
   "$or": [
      {
         "$expr": {
            "$ne": [
               "$firstName",
               "$lastName"
            ]
         }
      },
      {
         "fullName": {
            "$exists": true
         }
      }
   ]
}

You last query failed because mongod thinks $exists is the expression you are passing the the $expr operator.