Comparing Subdocuments In MongoDB Expressions

A coworker recently asked how to efficiently determine that two subdocuments are "equal".  The issue of course is that in normal MongoDB query language semantics, if you just say: 

{$eq: [ {"a":1, "b":2},  {"b":2, "a":1} ] }

the result is false because the two subdocuments are not "equal".  So how do you determine if two subdocuments are logically equal (without regard to field order) or whether all subdocuments in the collection or a group are logically equal?   

The challenge the coworker was working on was comparing index definitions across multiple shards.  There are several parts to the index definition.  The key part very much depends on the order of the fields - an index on {"a":1, "b":1} is not the same thing as the index on {"b":1, "a":1}.   

However the options on the index are not order dependent, if the specification part of the index is {"unique":true, "sparse":true} it has exactly the same effect as if it's {"sparse":true, "unique":true}.

Here are a couple of functions to the rescue.  The first one does a comparison of two objects and considers them equal if they have the same top level fields with the same values.   The second one will "normalize" an object so that no matter what order the fields are in, they will be in alphabetical order in the result document.

var unorderedEq = function(o1, o2) {
    return {$eq: [
       {$arrayToObject:{$setUnion:[{$objectToArray:o1}]}},
       {$arrayToObject:{$setUnion:[{$objectToArray:o2}]}}
    ] };
};

var normalize = function(o) {
    return {"$arrayToObject" : {"$setUnion" : [ {"$objectToArray" : o}]}}
}

Check out lots of other useful functions in my github repo here: https://github.com/asya999/bits-n-pieces/tree/master/scripts and try them out!