add Cursor#count method

lib/mongo/cursor.rb

@@ -71,6 +71,20 @@ module XGen
           o
         end
 
+        # Get the size of the results set for this query.
+        #
+        # Returns the number of objects in the results set for this query. Does
+        # not take limit and skip into account. Raises OperationFailure on a
+        # database error.
+        def count
+          command = OrderedHash["count", @collection.name,
+                                "query", @query.selector]
+          response = @db.db_command(command)
+          return response['n'].to_i if response['ok'] == 1
+          return 0 if response['errmsg'] == "ns missing"
+          raise OperationFailure, "Count failed: #{response['errmsg']}"
+        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).

test/test_cursor.rb

@@ -31,6 +31,33 @@ class CursorTest < Test::Unit::TestCase
     assert_kind_of Numeric, explaination['nscanned']
   end
 
+  def test_count
+    @@coll.clear
+
+    assert_equal 0, @@coll.find().count()
+
+    10.times do |i|
+      @@coll.save("x" => i)
+    end
+
+    assert_equal 10, @@coll.find().count()
+    assert_kind_of Integer, @@coll.find().count()
+    assert_equal 10, @@coll.find({}, :limit => 5).count()
+    assert_equal 10, @@coll.find({}, :offset => 5).count()
+
+    assert_equal 1, @@coll.find({"x" => 1}).count()
+    assert_equal 5, @@coll.find({"x" => {"$lt" => 5}}).count()
+
+    a = @@coll.find()
+    b = a.count()
+    a.each do |doc|
+      break
+    end
+    assert_equal b, a.count()
+
+    assert_equal 0, @@db['acollectionthatdoesn'].count()
+  end
+
   def test_close_no_query_sent
     begin
       cursor = @@coll.find('a' => 1)