Index: src/main/java/org/apache/jackrabbit/mongomk/impl/command/CommitCommandNew.java
===================================================================
--- src/main/java/org/apache/jackrabbit/mongomk/impl/command/CommitCommandNew.java	(revision 1433907)
+++ src/main/java/org/apache/jackrabbit/mongomk/impl/command/CommitCommandNew.java	(working copy)
@@ -29,6 +29,7 @@
 import org.apache.jackrabbit.mongomk.api.model.Commit;
 import org.apache.jackrabbit.mongomk.impl.MongoNodeStore;
 import org.apache.jackrabbit.mongomk.impl.action.FetchCommitAction;
+import org.apache.jackrabbit.mongomk.impl.action.FetchHeadRevisionIdAction;
 import org.apache.jackrabbit.mongomk.impl.action.ReadAndIncHeadRevisionAction;
 import org.apache.jackrabbit.mongomk.impl.action.SaveAndSetHeadRevisionAction;
 import org.apache.jackrabbit.mongomk.impl.action.SaveCommitAction;
@@ -96,8 +97,14 @@
             readBranchIdFromBaseCommit();
             createMongoNodes();
             prepareCommit();
-            readExistingNodes();
-            mergeNodes();
+            // If base revision is older than the head revision, need to read
+            // and merge nodes at the head revision.
+            FetchHeadRevisionIdAction action = new FetchHeadRevisionIdAction(nodeStore, branchId);
+            long headRevisionId = action.execute();
+            if (baseRevisionId < headRevisionId) {
+                readExistingNodes();
+                mergeNodes();
+            }
             prepareMongoNodes();
             new SaveNodesAction(nodeStore, nodes.values()).execute();
             new SaveCommitAction(nodeStore, commit).execute();
Index: src/main/java/org/apache/jackrabbit/mongomk/impl/action/FetchHeadRevisionIdAction.java
===================================================================
--- src/main/java/org/apache/jackrabbit/mongomk/impl/action/FetchHeadRevisionIdAction.java	(revision 1433907)
+++ src/main/java/org/apache/jackrabbit/mongomk/impl/action/FetchHeadRevisionIdAction.java	(working copy)
@@ -31,13 +31,26 @@
  */
 public class FetchHeadRevisionIdAction extends BaseAction<Long> {
 
+    private final String branchId;
+
     /**
      * Constructs a new {@code FetchHeadRevisionIdAction}.
      *
      * @param nodeStore Node store.
      */
     public FetchHeadRevisionIdAction(MongoNodeStore nodeStore) {
+        this(nodeStore, null);
+    }
+
+    /**
+     * Constructs a new {@code FetchHeadRevisionIdAction}.
+     *
+     * @param nodeStore Node store.
+     * @param branchId Branch id.
+     */
+    public FetchHeadRevisionIdAction(MongoNodeStore nodeStore, String branchId) {
         super(nodeStore);
+        this.branchId = branchId;
     }
 
     @Override
@@ -46,15 +59,19 @@
         MongoSync syncMongo = (MongoSync)headCollection.findOne();
         long headRevisionId = syncMongo.getHeadRevisionId();
 
-        // Find the first revision id that's not part of a branch.
         DBCollection collection = nodeStore.getCommitCollection();
-        DBObject query = QueryBuilder.start(MongoCommit.KEY_FAILED).notEquals(Boolean.TRUE)
-                .and(MongoCommit.KEY_REVISION_ID).lessThanEquals(headRevisionId)
-                .and(new BasicDBObject(MongoNode.KEY_BRANCH_ID, new BasicDBObject("$exists", false)))
-                .get();
+        QueryBuilder qb = QueryBuilder.start(MongoCommit.KEY_FAILED).notEquals(Boolean.TRUE)
+                .and(MongoCommit.KEY_REVISION_ID).lessThanEquals(headRevisionId);
+        if (branchId == null) {
+            qb = qb.and(new BasicDBObject(MongoNode.KEY_BRANCH_ID, new BasicDBObject("$exists", false)));
+        } else {
+            qb = qb.and(MongoNode.KEY_BRANCH_ID).is(branchId);
+        }
+
+        DBObject query = qb.get();
         DBObject fields = new BasicDBObject(MongoCommit.KEY_REVISION_ID, 1);
         DBObject orderBy = new BasicDBObject(MongoCommit.KEY_REVISION_ID, -1);
         MongoCommit commit = (MongoCommit)collection.findOne(query, fields, orderBy);
-        return commit.getRevisionId();
+        return commit != null? commit.getRevisionId() : 0L;
     }
 }
Index: src/test/java/org/apache/jackrabbit/mongomk/impl/MongoMKCommitAddTest.java
===================================================================
--- src/test/java/org/apache/jackrabbit/mongomk/impl/MongoMKCommitAddTest.java	(revision 1433907)
+++ src/test/java/org/apache/jackrabbit/mongomk/impl/MongoMKCommitAddTest.java	(working copy)
@@ -175,4 +175,21 @@
         }
         if (debug) System.out.println("Final Result:" + commitMonitor);
     }
+
+    @Test
+    public void existingNodesMerged() throws Exception {
+        String rev = mk.commit("/", "+\"a\" : {}", null, null);
+        mk.commit("/", "+\"a/b\" : {}", null, null);
+        mk.commit("/", "^\"a/key1\" : \"value1\"", null, null);
+
+        // Commit to rev before key1 and b were added
+        mk.commit("/", "^\"a/key2\" : \"value2\"", rev, null);
+
+        // Check that key1 and b were merged
+        String nodes = mk.getNodes("/", null, 1 /*depth*/, 0 /*offset*/, -1 /*maxChildNodes*/, null /*filter*/);
+        JSONObject obj = parseJSONObject(nodes);
+        assertPropertyValue(obj, ":childNodeCount", 1L);
+        assertPropertyValue(obj, "a/key1", "value1");
+        assertPropertyValue(obj, "a/key2", "value2");
+    }
 }
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