lmfaomongo.js

db.getCollection('posts')
    .find({})
    .forEach(function(myDoc) {
        myDoc.categories.forEach(function(category) {
           var cursor = db.getCollection('postcategories')
                .find({"_id":category});
                if (!cursor.hasNext()) {
                    printjsononeline(category);
                    var cursor2 = db.getCollection('posts').find({categories: { $in: [category] }})
                    while ( cursor2.hasNext() ) {
                       print( cursor2.next() );
                    }
                }
            });
        });