Block a change field in mongoDB

Question

I have a collection in MongoDB which makes an increase, the field was initially defined as an Integer, but I find that after the increase was converted to double.

But then I make an update of the document and see that changes to Long.

Is there any way to block these changes in Mongo?

Thanks in advance

Answer 1

Since MongoDB doesn't have a fixed schema per collection, there's no way to prevent such changes on the database side. Make sure that you use the same data type for the field everywhere, including its update operations. The C# driver is pretty smart about this.

Be careful when working with the shell, it can be irritating. Per default, the mongo shell will treat every number as a double, e.g.:

> db.Inc.find().pretty();
{ "_id" : 1, "Number" : 1000023272226647000 }
// this number is waaayyy larger than the largest 32 bit int, but there's no
// NumberLong here. So it must be double.
> db.Inc.update({}, {$inc: {"Number" : 1 }});
> db.Inc.find().pretty();
{ "_id" : 1, "Number" : 1000023272226647000 }
// Yikes, the $inc doesn't work anymore because of precision loss

Let's use NumberLong:

> db.Inc.insert({"Number" : NumberLong("1000023272226647000")});
> db.Inc.update({}, {$inc: {"Number" : 1}});
> db.Inc.find();
{ "Number" : 1000023272226647000, "_id" : 1 }
// Yikes! type conversion changed to double again! Also note 
// that the _id field moved to the end

Let's use NumberLong also in $inc:

> db.Inc.insert({"Number" : NumberLong("1000023272226647000")});
> db.Inc.update({}, {$inc: {"Number" : NumberLong("1")}});
> db.Inc.find();
{ "_id" : 1, "Number" : NumberLong("1000023272226647001") }
// This actually worked

In C#, both of the following updates work, Number remains a long:

class Counter { public long Number {get;set;} public ObjectId Id {get;set;} }
var collection = db.GetCollection("Counter");
collection.Insert(new Counter { Number = 1234 }); 
collection.Update(Query.Null, Update<Counter>.Inc(p => p.Number, 1)); // works
collection.Update(Query.Null, Update.Inc("Number", 1)); // works too

Answer 2

MongoDB is schema-less. Schamalessness provides for easier changes in your data structure but at the cost of the database not enforcing things like type constraints. You need to be disciplined in your application code to ensure that things are persisted in the way you want them to be.

If you need to ensure that the data is always of type Integer then it's recommended to have your application access MongoDB through a data access layer within the application. The data access layer can enforce type constraints (as well as any other constraints you want to put on your objects).

Short answer: There is no way to enforce this in MongoDB.