Support I do the following:

Reservation.all.includes(:customer).each do |reservation|
  # do something
end

If I have 100,000 Reservations, this will:

1. Load all 100,000 reservations into Ruby memory, in batches of 1000 using GETMORE
2. Eager load all their 100,000 customers into Ruby memory
3. Execute the loop

This will usually cause "out-of-memory" errors. A better way is:

1. Load the first 1000 Reservations
2. Eager load the Customers for the first 1000 Reservations
3. Execute the block for the first 1000 Reservations + Customers
4. (The Reservations/Customers from step 1/2 will be garbage collected)
5. Load the next 1000 Reservations using GET MORE
6. (step 2), etc.

See also MONGOID-5580 for a multi-threaded take on this logic.