Updated save to actually check if id exists in the collection.

lib/mongo/collection.rb

@@ -72,13 +72,14 @@ module XGen
           cursor = find(selector, h)
           cursor.next_object    # don't need to explicitly close b/c of limit
         end
-
+        
         # Save an updated +object+ to the collection, or insert it if it doesn't exist already.
         def save(object)
-          if id = object[:_id] || object['_id']
-            modify({:_id => id}, object)
-          else
+          id = object[:_id] || object['_id']
+          if id.nil? || find_first({'_id' => id}).nil?
             insert(object)
+          else
+            modify({:_id => id}, object)
           end
         end
 

tests/test_db_api.rb

@@ -612,6 +612,14 @@ class DBAPITest < Test::Unit::TestCase
     @@coll.save(a)
     assert_equal 2, @@coll.count
   end
+  
+  def test_save_with_object_that_has_id_but_does_not_actually_exist_in_collection
+    @@coll.clear
+    
+    a = {'_id' => '1', 'hello' => 'world'}
+    @@coll.save(a)    
+    assert_equal(1, @@coll.count)
+  end
 
 # TODO this test fails with error message "Undefed Before end of object"
 # That is a database error. The undefined type may go away.