I've been unable to use the cursor option with the aggregation pipeline when using a replica set connection. An exception is raised when send_get_more runs because @pool is nil, although I don't understand enough about the aggregation pipeline to know why.

I've included a sample that reproduces the issue - this is basically lifted from the test_aggregation_cursor test. If I swap the connection for just Mongo::MongoClient then the snippet runs successfully

require 'mongo'
client = Mongo::MongoReplicaSetClient.new ['127.0.0.1'], :logger => logger
db = client.db 'agg_test'
test = db.collection("test")

size = 10
test.drop
size.times {|i| test.insert({ :_id => i }) }
expected_sum = size.times.reduce(:+)
cursor = test.aggregate(
  [{ :$project => {:_id => '$_id'}} ],
  :cursor => {:batchSize => 1}
)

cursor_sum = cursor.reduce(0) do |sum, doc|
  sum += doc['_id']
end

The backtrace is

NoMethodError: undefined method `checkout' for nil:NilClass
	from /usr/local/rvm/gems/ruby-2.1.3/gems/mongo-1.11.1/lib/mongo/cursor.rb:598:in `send_get_more'
	from /usr/local/rvm/gems/ruby-2.1.3/gems/mongo-1.11.1/lib/mongo/cursor.rb:531:in `refresh'
	from /usr/local/rvm/gems/ruby-2.1.3/gems/mongo-1.11.1/lib/mongo/cursor.rb:136:in `next'
	from /usr/local/rvm/gems/ruby-2.1.3/gems/mongo-1.11.1/lib/mongo/cursor.rb:339:in `each'
{

}