How to compare 2 mongodb collections?

Question

Im trying to 'compare' all documents between 2 collections, which will return true only and if only all documents inside 2 collections are exactly equal.

I've been searching for the methods on the collection, but couldnt find one that can do this.

I experimented something like these in the mongo shell, but not working as i expected :

db.test1 == db.test2

or

db.test1.to_json() == db.test2.to_json()

Please share your thoughts ! Thank you.

Answer 1

You can try using mongodb eval combined with your custom equals function, something like this.

Your methods don't work because in the first case you are comparing object references, which are not the same. In the second case, there is no guarantee that to_json will generate the same string even for the objects that are the same.

Instead, try something like this:

var compareCollections = function(){
    db.test1.find().forEach(function(obj1){
        db.test2.find({/*if you know some properties, you can put them here...if don't, leave this empty*/}).forEach(function(obj2){
            var equals = function(o1, o2){
                // here goes some compare code...modified from the SO link you have in the answer.
            };

            if(equals(ob1, obj2)){
                // Do what you want to do
            }
        });
    });
};

db.eval(compareCollections);

With db.eval you ensure that code will be executed on the database server side, without fetching collections to the client.