mongo-ruby-driver/lib/mongo/cursor.rb

# --
# Copyright (C) 2008-2009 10gen Inc.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
# ++

require 'mongo/message'
require 'mongo/util/byte_buffer'
require 'mongo/util/bson'

module XGen
  module Mongo
    module Driver

      # A cursor over query results. Returned objects are hashes.
      class Cursor

        include Enumerable

        RESPONSE_HEADER_SIZE = 20

        attr_reader :db, :collection, :query

        def initialize(db, collection, query)
          @db, @collection, @query = db, collection, query
          @num_to_return = @query.number_to_return || 0
          @cache = []
          @closed = false
          @can_call_to_a = true
          @query_run = false
          @rows = nil
        end

        def closed?; @closed; end

        # Internal method, not for general use. Return +true+ if there are
        # more records to retrieve. We do not check @num_to_return; #each is
        # responsible for doing that.
        def more?
          num_remaining > 0
        end

        # Return the next object or nil if there are no more. Raises an error
        # if necessary.
        def next_object
          refill_via_get_more if num_remaining == 0
          o = @cache.shift

          if o && o['$err']
            err = o['$err']

            # If the server has stopped being the master (e.g., it's one of a
            # pair but it has died or something like that) then we close that
            # connection. If the db has auto connect option and a pair of
            # servers, next request will re-open on master server.
            @db.close if err == "not master"

            raise err
          end

          o
        end

        # Iterate over each object, yielding it to the given block. At most
        # @num_to_return records are returned (or all of them, if
        # @num_to_return is 0).
        #
        # If #to_a has already been called then this method uses the array
        # that we store internally. In that case, #each can be called multiple
        # times because it re-uses that array.
        #
        # You can call #each after calling #to_a (multiple times even, because
        # it will use the internally-stored array), but you can't call #to_a
        # after calling #each unless you also called it before calling #each.
        # If you try to do that, an error will be raised.
        def each
          if @rows              # Already turned into an array
            @rows.each { |row| yield row }
          else
            num_returned = 0
            while more? && (@num_to_return <= 0 || num_returned < @num_to_return)
              yield next_object()
              num_returned += 1
            end
            @can_call_to_a = false
          end
        end

        # Return all of the rows (up to the +num_to_return+ value specified in
        # #new) as an array. Calling this multiple times will work fine; it
        # always returns the same array.
        #
        # Don't use this if you're expecting large amounts of data, of course.
        # All of the returned rows are kept in an array stored in this object
        # so it can be reused.
        #
        # You can call #each after calling #to_a (multiple times even, because
        # it will use the internally-stored array), but you can't call #to_a
        # after calling #each unless you also called it before calling #each.
        # If you try to do that, an error will be raised.
        def to_a
          return @rows if @rows
          raise "can't call Cursor#to_a after calling Cursor#each" unless @can_call_to_a
          @rows = []
          num_returned = 0
          while more? && (@num_to_return <= 0 || num_returned < @num_to_return)
            @rows << next_object()
            num_returned += 1
          end
          @rows
        end

        # Returns an explain plan record.
        def explain
          old_val = @query.explain
          @query.explain = true

          c = Cursor.new(@db, @collection, @query)
          explanation = c.next_object
          c.close

          @query.explain = old_val
          explanation
        end

        # Close the cursor.
        #
        # Note: if a cursor is read until exhausted (read until OP_QUERY or
        # OP_GETMORE returns zero for the cursor id), there is no need to
        # close it by calling this method.
        def close
          @db.send_to_db(KillCursorsMessage.new(@cursor_id)) if @cursor_id
          @cache = []
          @cursor_id = 0
          @closed = true
        end

        protected

        def read_all
          read_message_header
          read_response_header
          read_objects_off_wire
        end

        def read_objects_off_wire
          while doc = next_object_on_wire
            @cache << doc
          end
        end

        def read_message_header
          MessageHeader.new.read_header(@db)
        end

        def read_response_header
          header_buf = ByteBuffer.new
          header_buf.put_array(@db.receive_full(RESPONSE_HEADER_SIZE).unpack("C*"))
          raise "Short read for DB response header; expected #{RESPONSE_HEADER_SIZE} bytes, saw #{header_buf.length}" unless header_buf.length == RESPONSE_HEADER_SIZE
          header_buf.rewind
          @result_flags = header_buf.get_int
          @cursor_id = header_buf.get_long
          @starting_from = header_buf.get_int
          @n_returned = header_buf.get_int
          @n_remaining = @n_returned
        end

        def num_remaining
          refill_via_get_more if @cache.length == 0
          @cache.length
        end

        private

        def next_object_on_wire
          send_query_if_needed
          # if @n_remaining is 0 but we have a non-zero cursor, there are more
          # to fetch, so do a GetMore operation, but don't do it here - do it
          # when someone pulls an object out of the cache and it's empty
          return nil if @n_remaining == 0
          object_from_stream
        end

        def refill_via_get_more
          send_query_if_needed
          return if @cursor_id == 0
          @db.send_to_db(GetMoreMessage.new(@db.name, @collection.name, @cursor_id))
          read_all
        end

        def object_from_stream
          buf = ByteBuffer.new
          buf.put_array(@db.receive_full(4).unpack("C*"))
          buf.rewind
          size = buf.get_int
          buf.put_array(@db.receive_full(size - 4).unpack("C*"), 4)
          @n_remaining -= 1
          buf.rewind
          BSON.new(@db).deserialize(buf)
        end

        def send_query_if_needed
          # Run query first time we request an object from the wire
          unless @query_run
            @db.send_query_message(QueryMessage.new(@db.name, @collection.name, @query))
            @query_run = true
            read_all
          end
        end

        def to_s
          "DBResponse(flags=#@result_flags, cursor_id=#@cursor_id, start=#@starting_from, n_returned=#@n_returned)"
        end
      end
    end
  end
end