go - Efficient paging in MongoDB using mgo

Question

^{I've searched and found no Go solution to the problem, not with or without using mgo.v2, not on StackOverflow and not on any other site. This Q&A is in the spirit of knowledge sharing / documenting.}

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Let's say we have a users collection in MongoDB modeled with this Go struct:

type User struct {
    ID      bson.ObjectId `bson:"_id"`
    Name    string        `bson:"name"`
    Country string        `bson:"country"`
}

We want to sort and list users based on some criteria, but have paging implemented due to the expected long result list.

To achieve paging of the results of some query, MongoDB and the mgo.v2 driver package has built-in support in the form of Query.Skip() and Query.Limit(), e.g.:

session, err := mgo.Dial(url) // Acquire Mongo session, handle error!

c := session.DB("").C("users")
q := c.Find(bson.M{"country" : "USA"}).Sort("name", "_id").Limit(10)

// To get the nth page:
q = q.Skip((n-1)*10)

var users []*User
err = q.All(&users)

This however becomes slow if the page number increases, as MongoDB can't just "magically" jump to the x^th document in the result, it has to iterate over all the result documents and omit (not return) the first x that need to be skipped.

MongoDB provides the right solution: If the query operates on an index (it has to work on an index), cursor.min() can be used to specify the first index entry to start listing results from.

This Stack Overflow answer shows how it can be done using a mongo client: How to do pagination using range queries in MongoDB?

Note: the required index for the above query would be:

db.users.createIndex(
    {
        country: 1,
        name: 1,
        _id: 1
    }
)

There is one problem though: the mgo.v2 package has no support specifying this min().

How can we achieve efficient paging that uses MongoDB's cursor.min() feature using the mgo.v2 driver?

Answer 1

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