如何编码/解码mongodb游标？

Question

I need to build a list of "pages" so part of this there will be a cursor. The issue is that I can't find a way to encode(to string) and decode the cursor. Any idea? The Cursor interface has no "encoding" method(there is ID, though undocumented) and there is no way to create a new cursor from a string(or int).

type Cursor interface {

    // Get the ID of the cursor.
    ID() int64

    // Get the next result from the cursor.
    // Returns true if there were no errors and there is a next result.
    Next(context.Context) bool

    Decode(interface{}) error

    DecodeBytes() (bson.Reader, error)

    // Returns the error status of the cursor
    Err() error

    // Close the cursor.
    Close(context.Context) error
}

Why do I need the cursor encoded?

To provide pagination to the end client through a html or JSON API.

Answer 1

MongoDB does not provide a serializable cursor. Cursor is not serializable. The recommended workaround is to use a range query and sort on a field that generally changes in a consistent direction over time such _id.

function printStudents(startValue, nPerPage) {
  let endValue = null;
  db.students.find( { _id: { $lt: startValue } } )
             .sort( { _id: -1 } )
             .limit( nPerPage )
             .forEach( student => {
               print( student.name );
               endValue = student._id;
             } );

  return endValue;
}

There is a go package minquery that tries to make the cursor query/serialization more convenient. You may find it helpful.