MongoDB sorting order and unique fields - database

Question

I'm using a mongoose schema like this one:
{
numero: {
type: Number,
required: true,
unique: true
},
capacidad: {
type: Number,
required: true
}
}
When I retrieve the collection's documents (p.e. using Model.find({})), I get the documents sorted by _id.
My questions are:
MongoDB creates an index for handling the unique: true requirement but it does not use it as default sorting mechanism?
If I do Model.find({}).sort("numero") does this use the index for handling uniqueness or must build another for my query?
If I define my own index (schema.index({ numero: 1 }), am I duplicating work?
Summarizing, what are the best practices for maintaining a collection sorted for querying?

Answer 1

First lets make a note that you are talking more specifically on how Mongoose does things as a mediator with MongoDB. What I mean by this is that in mongoose schema defining something like this:
numero: {
type: Number,
required: true,
unique: true
}
Actually means that (by using unique: true) you ARE creating a unique index on the field numero and the the index: true part is optional as per the documentation.
So you do not need to actually create another index.
That also should answer your question about Model.find({}).sort("numero") using the index as well.
As far as best practices go you should review the way you are querying your data and very importantly what type of data you have in order to figure out what kind of index you need. For example if you have lat/lng data you should probably be using the Geospatial Index etc.
You also do not want to go crazy on the indexing since that brings other issues as well.
Also very important tool to review what MongoDB is doing is the explain operator which gives you information on the query plan.
You should use it often to analyze and optimize your queries and figure out where your bottlenecks are.
Also you can view your collection statistics via
db.getCollection('your-collection-name').stats()
That would give you good information on the current indexes on the collection their size etc.
Hope this helps.