JAVA-1167: Backing out this change after deciding that it's too risky, as users may be relying on the current behaviour.

Author: jyemin

src/main/com/mongodb/DBCollection.java

@@ -905,13 +905,15 @@ public Object apply( DBObject o ){
      * @return the modified object {@code o}
      */
     public Object apply( DBObject jo , boolean ensureID ){
-        if ( ensureID && !jo.containsField("_id") ){
-            jo.put( "_id" , ObjectId.get() );
+        Object id = jo.get("_id");
+        if (ensureID && id == null) {
+            id = ObjectId.get();
+            jo.put("_id", id);
         }
 
-        doapply( jo );
+        doapply(jo);
 
-        return jo.get("_id");
+        return id;
     }
 
     /**
@@ -954,20 +956,25 @@ public WriteResult save( DBObject jo, WriteConcern concern ){
 
         Object id = jo.get( "_id" );
 
-        if ((id == null && !jo.containsField("_id")) || (id instanceof ObjectId && ((ObjectId) id).isNew())) {
+        if (id == null || (id instanceof ObjectId && ((ObjectId) id).isNew())) {
+            if (id != null) {
+                ((ObjectId) id).notNew();
+            }
             if (concern == null) {
                 return insert(jo);
             } else {
                 return insert(jo, concern);
             }
         }
 
-        DBObject q = new BasicDBObject("_id" , id);
+        DBObject q = new BasicDBObject();
+        q.put("_id", id);
         if (concern == null) {
             return update(q, jo, true, false);
         } else {
             return update(q, jo, true, false, concern);
         }
+
     }
 
     // ---- DB COMMANDS ----

src/test/com/mongodb/DBCollectionTest.java

@@ -31,7 +31,6 @@
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNotNull;
-import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
 import static org.junit.Assume.assumeFalse;
@@ -43,24 +42,32 @@ public class DBCollectionTest extends TestCase {
     public void shouldCreateIdOnInsertIfThereIsNone() {
         BasicDBObject document = new BasicDBObject();
         collection.insert(document);
+        assertEquals(ObjectId.class, document.get("_id").getClass());
         assertEquals(document, collection.findOne());
     }
 
     @Test
-    public void shouldPreserveNullIdOnInsert() {
+    public void shouldCreateIdOnInsertIfTheValueIsNull() {
         BasicDBObject document = new BasicDBObject("_id", null);
         collection.insert(document);
-        assertNull(document.get("_id"));
+        assertEquals(ObjectId.class, document.get("_id").getClass());
         assertEquals(document, collection.findOne());
     }
 
     @Test
-    public void saveShouldUpdateAnExistingDocumentWithNullId() {
+    public void saveShouldInsertADocumentWithNullId() {
         BasicDBObject document = new BasicDBObject("_id", null);
-        collection.insert(document);
-        document.put("x", 1);
         collection.save(document);
-        assertNull(document.get("_id"));
+        assertEquals(ObjectId.class, document.get("_id").getClass());
+        assertEquals(document, collection.findOne());
+    }
+
+    @Test
+    public void saveShouldInsertADocumentWithNewObjectId() {
+        ObjectId newObjectId = new ObjectId();
+        BasicDBObject document = new BasicDBObject("_id", newObjectId);
+        collection.save(document);
+        assertEquals(newObjectId, document.get("_id"));
         assertEquals(document, collection.findOne());
     }