From 30da7dabe09f81802a4ae9161c2c24ab03ff8d0f Mon Sep 17 00:00:00 2001
From: Michael Sokolov <sokolov@falutin.net>
Date: Mon, 6 May 2024 12:53:13 -0400
Subject: [PATCH] gh-13340: Allow adding a parent field to an index with no
 fields (#13341)

---
 .../org/apache/lucene/index/IndexWriter.java  |  1 +
 .../apache/lucene/index/TestIndexWriter.java  | 19 ++++++++++++++++++-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java b/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java
index 7227ed56f34..3cbf433e174 100644
--- a/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java
+++ b/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java
@@ -1122,6 +1122,7 @@ public class IndexWriter
       globalFieldNumberMap = getFieldNumberMap();
       if (create == false
           && conf.getParentField() != null
+          && globalFieldNumberMap.getFieldNames().isEmpty() == false
           && globalFieldNumberMap.getFieldNames().contains(conf.getParentField()) == false) {
         throw new IllegalArgumentException(
             "can't add a parent field to an already existing index without a parent field");
diff --git a/lucene/core/src/test/org/apache/lucene/index/TestIndexWriter.java b/lucene/core/src/test/org/apache/lucene/index/TestIndexWriter.java
index 4cee1635c42..163bb21d67d 100644
--- a/lucene/core/src/test/org/apache/lucene/index/TestIndexWriter.java
+++ b/lucene/core/src/test/org/apache/lucene/index/TestIndexWriter.java
@@ -4851,7 +4851,9 @@ public class TestIndexWriter extends LuceneTestCase {
     try (Directory dir = newDirectory()) {
       IndexWriterConfig iwc = new IndexWriterConfig(new MockAnalyzer(random()));
       try (IndexWriter writer = new IndexWriter(dir, iwc)) {
-        writer.addDocument(new Document());
+        Document d = new Document();
+        d.add(new TextField("f", "a", Field.Store.NO));
+        writer.addDocument(d);
       }
       IllegalArgumentException iae =
           expectThrows(
@@ -4959,4 +4961,19 @@ public class TestIndexWriter extends LuceneTestCase {
           iae.getMessage());
     }
   }
+
+  public void testParentFieldEmptyIndex() throws IOException {
+    try (Directory dir = newMockDirectory()) {
+      IndexWriterConfig iwc = new IndexWriterConfig(new MockAnalyzer(random()));
+      iwc.setParentField("parent");
+      try (IndexWriter writer = new IndexWriter(dir, iwc)) {
+        writer.commit();
+      }
+      IndexWriterConfig iwc2 = new IndexWriterConfig(new MockAnalyzer(random()));
+      iwc2.setParentField("parent");
+      try (IndexWriter writer = new IndexWriter(dir, iwc2)) {
+        writer.commit();
+      }
+    }
+  }
 }