don't set _id to default value for hash

ext/cbson/cbson.c

@@ -377,17 +377,16 @@ static void write_doc(bson_buffer* buffer, VALUE hash) {
     int length_location = buffer_save_bytes(buffer, 4);
 
     VALUE key = rb_str_new2("_id");
-    VALUE id = rb_hash_aref(hash, key);
-    if (TYPE(id) != T_NIL) {
+    if (rb_funcall(hash, rb_intern("has_key?"), 1, key) == Qtrue) {
+        VALUE id = rb_hash_aref(hash, key);
         write_element_allow_id(key, id, (VALUE)buffer, 1);
     }
     key = ID2SYM(rb_intern("_id"));
-    id = rb_hash_aref(hash, key);
-    if (TYPE(id) != T_NIL) {
+    if (rb_funcall(hash, rb_intern("has_key?"), 1, key) == Qtrue) {
+        VALUE id = rb_hash_aref(hash, key);
         write_element_allow_id(key, id, (VALUE)buffer, 1);
     }
 
-
     // we have to check for an OrderedHash and handle that specially
     if (strcmp(rb_class2name(RBASIC(hash)->klass), "OrderedHash") == 0) {
         VALUE keys = rb_funcall(hash, rb_intern("keys"), 0);

lib/mongo/util/bson.rb

@@ -85,7 +85,13 @@ def serialize(obj)
       @buf.put_int(0)
 
       # Write key/value pairs. Always write _id first if it exists.
-      oid = obj['_id'] || obj[:_id]
+      if obj.has_key? '_id'
+        oid = obj['_id']
+      elsif obj.has_key? :_id
+        oid = obj[:_id]
+      else
+        oid = false
+      end
       serialize_key_value('_id', oid) if oid
       obj.each {|k, v| serialize_key_value(k, v) unless k == '_id' || k == :_id }
 

tests/test_db_api.rb

@@ -522,6 +522,14 @@ def test_hint
     end
   end
 
+  def test_hash_default_value_id
+    val = Hash.new(0)
+    val["x"] = 5
+    @@coll.insert val
+    id = @@coll.find_first("x" => 5)["_id"]
+    assert id != 0
+  end
+
 # TODO this test fails with error message "Undefed Before end of object"
 # That is a database error. The undefined type may go away.