Make sure _id gets written first when serializing BSON.

lib/mongo/util/bson.rb

@@ -333,7 +333,20 @@ class BSON
   def serialize_object_element(buf, key, val, opcode=OBJECT)
     buf.put(opcode)
     self.class.serialize_cstr(buf, key)
-    buf.put_array(BSON.new.serialize(val).to_a)
+    buf.put_array(BSON.new.serialize(put_id_first(val)).to_a)
+  end
+
+  # For internal use only. Looks for '_id' or :_id key of val. If there is
+  # one, returns an OrderedHash where '_id' is first. If not, returns val.
+  def put_id_first(val)
+    oid = val.delete('_id') || val.delete(:_id)
+    if oid
+      h = OrderedHash.new
+      h['_id'] = oid
+      val.each { |k, v| h[k] = v }
+      val = h
+    end
+    val
   end
 
   def serialize_array_element(buf, key, val)

tests/test_bson.rb

@@ -132,4 +132,20 @@ class BSONTest < Test::Unit::TestCase
     assert_kind_of Undefined, doc2['undef']
   end
 
+  def test_put_id_first
+    val = {'a' => 'foo'}
+    assert_same val, @b.put_id_first(val)
+
+    val = OrderedHash.new
+    val['not_id'] = 1
+    val['_id'] = 2
+    id_first = @b.put_id_first(val)
+    assert_equal ['_id', 'not_id'], id_first.keys
+
+    val = {'a' => 'foo', 'b' => 'bar', :_id => 42, 'z' => 'hello'}
+    id_first = @b.put_id_first(val)
+    assert id_first.keys.include?('_id')
+    assert !id_first.keys.include?(:_id)
+  end
+
 end