fix json externalization problem

dashjoin-core/src/main/java/org/dashjoin/service/JSONDatabase.java

@@ -166,8 +166,12 @@ public boolean update(Table s, Map<String, Object> search, Map<String, Object> o
     for (Entry<String, Object> e : object.entrySet()) {
       if (e.getValue() == null)
         r.remove(e.getKey());
-      else
+      else {
+        // when nested objects are overwritten and the old value contains a pointer,
+        // make sure to re-use the pointer
+        copyPointers(r.get(e.getKey()), e.getValue());
         r.put(e.getKey(), e.getValue());
+      }
     }
     return update(s, r);
   }
@@ -304,4 +308,29 @@ public static String toJsonString(Map<String, Object> obj) throws JsonProcessing
   public static Map<String, Object> fromJsonString(String json) throws JsonProcessingException {
     return objectMapper.readValue(json, JSONDatabase.tr);
   }
+
+  /**
+   * copy -pointer fields from source to dest
+   */
+  @SuppressWarnings("unchecked")
+  static void copyPointers(Object _from, Object _to) {
+    if (_from instanceof Map && _to instanceof Map) {
+      Map<String, Object> from = (Map<String, Object>) _from;
+      Map<String, Object> to = (Map<String, Object>) _to;
+      for (Entry<String, Object> e : from.entrySet()) {
+        Object toValue = to.get(e.getKey());
+        if (e.getKey().endsWith("-pointer")) {
+          String key = e.getKey().substring(0, e.getKey().length() - "-pointer".length());
+          if (from.containsKey(key))
+            if (to.containsKey(key))
+              if (toValue == null)
+                // from and to contain externalized value
+                // only from contains pointer
+                // re-use the from pointer
+                to.put(e.getKey(), e.getValue());
+        }
+        copyPointers(e.getValue(), toValue);
+      }
+    }
+  }
 }

dashjoin-core/src/test/java/org/dashjoin/service/JSONDatabaseTest.java

@@ -428,4 +428,45 @@ public void testExtraField() throws Exception {
     Payload p = JSONDatabase.fromMap(MapUtil.of("s", "hello world", "unknown", 42), Payload.class);
     Assertions.assertEquals(p.s, "hello world");
   }
+
+  @SuppressWarnings("unchecked")
+  @Test
+  public void testCopyPointer() {
+    JSONDatabase.copyPointers(null, null);
+    JSONDatabase.copyPointers(1, null);
+    JSONDatabase.copyPointers("test", null);
+    JSONDatabase.copyPointers(null, 1);
+    JSONDatabase.copyPointers(null, "test");
+    JSONDatabase.copyPointers(null, Map.of("x", 1));
+    JSONDatabase.copyPointers(Map.of("x", 1), null);
+
+    Map<String, Object> from = MapUtil.of("x", "code", "x-pointer", "0.txt");
+    Map<String, Object> to = MapUtil.of("x", "new code");
+    JSONDatabase.copyPointers(from, to);
+    Assertions.assertEquals("0.txt", to.get("x-pointer"));
+
+    from = Map.of("n", MapUtil.of("x", "code", "x-pointer", "0.txt"));
+    to = Map.of("n", MapUtil.of("x", "new code"));
+    JSONDatabase.copyPointers(from, to);
+    Assertions.assertEquals("0.txt", ((Map<String, Object>) to.get("n")).get("x-pointer"));
+
+    from = Map.of("n", MapUtil.of("x", "code", "x-pointer", "0.txt"));
+    to = Map.of();
+    JSONDatabase.copyPointers(from, to);
+
+    from = MapUtil.of("x", "code", "x-pointer", "0.txt");
+    to = MapUtil.of();
+    JSONDatabase.copyPointers(from, to);
+    Assertions.assertNull(to.get("x-pointer"));
+
+    from = MapUtil.of("x-pointer", "0.txt");
+    to = MapUtil.of("x", "code");
+    JSONDatabase.copyPointers(from, to);
+    Assertions.assertNull(to.get("x-pointer"));
+
+    from = MapUtil.of("x", "code", "x-pointer", "0.txt");
+    to = MapUtil.of("x", "code", "x-pointer", "1.txt");
+    JSONDatabase.copyPointers(from, to);
+    Assertions.assertEquals("1.txt", to.get("x-pointer"));
+  }
 }