Gives the previous cursor in the scroll block

[GCorbel]

In addition to the next cursor, blocks in Mongo::Scrollable#scroll and Mongoid::Criteria::Scrollable gives an additional cursor with allow to fetch records before the first one returned.

The previous cursor is created based on the first record with a previous flag set to true. For example, if we have Feed::Item with a name from 0 to 3, we have this behavior :

cursor = nil
first_previous_cursor = nil

Feed::Item.asc(:name).limit(2).scroll do |record, next_cursor|
  puts record.name # => 0, 1
  cursor = next_cursor
end

Feed::Item.asc(:name).limit(2).scroll(cursor) do |record, _, previous_record|
  puts record.name # => 2, 3
  first_previous_cursor = previous_cursor
end

Feed::Item.asc(:name).limit(2).scroll(first_previous_cursor) do |record|
  puts record.name # => 0, 1
  second_previous_cursor = previous_cursor
end

[GCorbel]

This gem was hard to understand. I don't know why there is a code for mongo and another for mongoid instead of calling the mongo code with mongoid. Conventions used, with no space, are hard to read.

I still have to add a cursor to get the first page but I think it will be in another PR.

[GCorbel]

This :

1. Select records before the limit in the criteria
2. Reverse the sort
3. Take only the number of records in limit
4. Resort the order

Given that, if we have this list :

1
2
3
4
5 # <= Cursor here
6 

We want to have 2 records before the cursor and keep the order. Returned values have to be 3, 4.

To do that, we first select only records be the cursor :

1
2
3
4

We reverse the selection :

4
3
2
1

Take only the two first values :

4
3

And finally revert the sorting :

3
4

To avoid using aggregation, another solution was possible by sorting first with mongo and the reverse the records to display in the correct order
Another possible solution was to do the same in pure Ruby :

view.sort[scroll_field] = -scroll_direction if cursor.previous
records = Mongo::Collection::View.new(
  view.collection,
  view.selector.merge(cursor.criteria),
  sort: (view.sort || {}).merge(_id: -scroll_direction),
  skip: skip,
  limit: limit
)
records.to_a.sort { |h1, h2| h1[scroll_field] <=> h2[scroll_field] }.each do |record|
  # ...
end

Even if this version is simpler, I chose to use aggregates for performance reason.

[GCorbel]

tokens changed because previous: false is encoded.

[GCorbel]

I'm not sure about the naming of the previous flag.

[GCorbel]

I opened mongoid#38. Let me know if you want to close this PR in favor of the other one.