Mongodb – not evil, just misunderstood

Mongodb – not evil, just misunderstood
siddharth-ravichandran.com

-Yes you can make aggregate results using map reduce (In javascript with a single core, idempotent) and you can store the aggregates as separate documents. I do not see what is so magical about this.

-It is true that you do not have a set schema thus no migrations, but you do have to have a schema in mind for nearly everything as something has to make sense of that field.

If you add fields to a document and you already have documents in there, you are going to have to add that data to them unless you want them to be nil. If you rename a field you have to iterate all documents and add rename it. If you remove a field you have to remove it from all existing documents if you do not want to have that data around anymore. Certainly you do not HAVE to do any of this but if you are fetching meaningful data from a field you need to.

And you need to index on whatever you want to search on or the performance is terrible as Mongo is not very good on the disk so you have to keep that in mind too. I think most people were attracted to Mongo due to "no migrations" and schema but when you work with it almost any use case you find yourself using an unenforced schema and doing script based "migrations" The recent Mongo criticism is that it markets itself as a all purpose DB but in reality document stores are not that.

I ended up figuring out a workaround but it left a bad taste in my mouth. My workaround was to create a temporary collection and insert all the previous records, with their Category attribute updated, in a single .insert call.

I am more than sure that an SQL UPDATE query using a subselect for the COUNT(*) would have probably executed in less than a couple seconds. That's what's so sad about MongoDB. And I even had everything indexed, it made almost no difference. The write/read locks are _murder_.

    houses.db.eval(function(){
		const tmp = db[Math.random().toString(36).slice(2)];
		tmp.insert(
			db.houses.find().map(function(e){
				e.Block = db.houses.count({
					Street: e.Street,
					Sector: {$lte: e.Sector},
					Quadrant: 1
				});
				return e;
			})
		);
		tmp.renameCollection("houses", true);
	}, {nolock: true}, function(err){
		dbCallback(err);
		console.log("Done!");
		process.exit();
	});