diff --git a/lucene/facet/src/test/org/apache/lucene/facet/simple/AssertingSubDocsAtOnceCollector.java b/lucene/facet/src/test/org/apache/lucene/facet/simple/AssertingSubDocsAtOnceCollector.java
new file mode 100644
index 00000000000..89f0ab00a68
--- /dev/null
+++ b/lucene/facet/src/test/org/apache/lucene/facet/simple/AssertingSubDocsAtOnceCollector.java
@@ -0,0 +1,67 @@
+package org.apache.lucene.facet.simple;
+
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import java.util.ArrayList;
+import java.util.List;
+
+import org.apache.lucene.index.AtomicReaderContext;
+import org.apache.lucene.search.Collector;
+import org.apache.lucene.search.Scorer.ChildScorer;
+import org.apache.lucene.search.Scorer;
+
+/** Verifies in collect() that all child subScorers are on
+ *  the collected doc. */
+class AssertingSubDocsAtOnceCollector extends Collector {
+
+  // TODO: allow wrapping another Collector
+
+  List<Scorer> allScorers;
+
+  @Override
+  public void setScorer(Scorer s) {
+    // Gathers all scorers, including s and "under":
+    allScorers = new ArrayList<Scorer>();
+    allScorers.add(s);
+    int upto = 0;
+    while(upto < allScorers.size()) {
+      s = allScorers.get(upto++);
+      for (ChildScorer sub : s.getChildren()) {
+        allScorers.add(sub.child);
+      }
+    }
+  }
+
+  @Override
+  public void collect(int docID) {
+    for(Scorer s : allScorers) {
+      if (docID != s.docID()) {
+        throw new IllegalStateException("subScorer=" + s + " has docID=" + s.docID() + " != collected docID=" + docID);
+      }
+    }
+  }
+
+  @Override
+  public void setNextReader(AtomicReaderContext context) {
+  }
+
+  @Override
+  public boolean acceptsDocsOutOfOrder() {
+    return false;
+  }
+}