Why does mongoose always add an s to the end of my collection name

You can try to do it. its trying to be a smart:

var dataSchema = new Schema({..}, { collection: 'data' })

This is a API structure of mongoose.model:

Mongoose#model(name, [schema], [collection], [skipInit])

The following statement is What mongoose do is that, When no collection argument is passed, The Mongoose produces a collection name by pluralizing the model name. If you don’t like this behavior, either pass a collection name or set your schemas collection name option.

For the example:

var schema = new Schema({ name: String }, { collection: 'actor' });

Or Else

schema.set('collection', 'actor')

Or Else

var collectionName = 'actor'
var M = mongoose.model('Actor', schema, collectionName);

We can add a string as a third argument to define the actual name for the collection. spanning your examples, to keep names as data and user respectively:

var Dataset = mongoose.model('data', dataSchema, 'data');
var User = mongoose.model('user', dataSchema, 'user');