test(schema): cover SET defaults through server round trips

Add server-side regression tests to ensure that Cardinality.SET
default values (which deserialize from JSON as String ArrayLists)
are correctly normalized to a Set and duplicates are collapsed.

Author: dpol1

hugegraph-server/hugegraph-test/src/main/java/org/apache/hugegraph/core/VertexCoreTest.java

@@ -19,6 +19,7 @@
 
 import java.math.BigDecimal;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.Date;
 import java.util.HashSet;
@@ -854,6 +855,36 @@ public void testAddVertexWithDateDefaultValue() {
         Assert.assertEquals(joined, value);
     }
 
+    @Test
+    public void testAddVertexWithDateSetDefaultValue() {
+        SchemaManager schema = graph().schema();
+
+        String dateStr = "2026-05-14 10:11:12.345";
+        Date expected = DateUtil.parse(dateStr);
+
+        // Simulate JSON-deserialized default: ArrayList of Strings with duplicates
+        schema.propertyKey("joinDates").asDate().valueSet()
+              .userdata(Userdata.DEFAULT_VALUE, Arrays.asList(dateStr, dateStr))
+              .create();
+        schema.vertexLabel("person")
+              .properties("joinDates")
+              .nullableKeys("joinDates").append();
+
+        Vertex vertex = graph().addVertex(T.label, "person",
+                                          "name", "Baby", "city", "Shanghai");
+        this.commitTx();
+
+        vertex = graph().vertex(vertex.id());
+        Object raw = vertex.value("joinDates");
+
+        Assert.assertTrue("joinDates should be a Set, was " +
+                          (raw == null ? "null" : raw.getClass()),
+                          raw instanceof Set);
+        Set<?> values = (Set<?>) raw;
+        Assert.assertEquals("duplicates must be collapsed", 1, values.size());
+        Assert.assertTrue(values.contains(expected));
+    }
+
     @Test
     public void testAddVertexWithNotExistsVertexPropKey() {
         HugeGraph graph = graph();

hugegraph-server/hugegraph-test/src/main/java/org/apache/hugegraph/unit/serializer/BinarySerializerTest.java

@@ -17,7 +17,9 @@
 
 package org.apache.hugegraph.unit.serializer;
 
+import java.util.Arrays;
 import java.util.Date;
+import java.util.Set;
 
 import org.apache.hugegraph.backend.id.IdGenerator;
 import org.apache.hugegraph.backend.serializer.BinarySerializer;
@@ -29,6 +31,7 @@
 import org.apache.hugegraph.structure.HugeVertex;
 import org.apache.hugegraph.testutil.Assert;
 import org.apache.hugegraph.testutil.Whitebox;
+import org.apache.hugegraph.type.define.Cardinality;
 import org.apache.hugegraph.type.define.DataType;
 import org.apache.hugegraph.unit.BaseUnitTest;
 import org.apache.hugegraph.unit.FakeObjects;
@@ -154,6 +157,33 @@ public void testPropertyKeyDefaultValueRoundTripsAsDate() {
         Assert.assertEquals(defaultValue, value);
     }
 
+    @Test
+    public void testPropertyKeySetDefaultValueRoundTripsAsDate() {
+        HugeConfig config = FakeObjects.newConfig();
+        BinarySerializer ser = new BinarySerializer(config);
+
+        FakeObjects objects = new FakeObjects();
+        PropertyKey original = objects.newPropertyKey(IdGenerator.of(2L),
+                                                      "tags", DataType.DATE);
+        original.cardinality(Cardinality.SET);
+
+        String dateStr = "2026-05-14 10:11:12.345";
+        Date expected = DateUtil.parse(dateStr);
+        // ArrayList<String> with duplicates — what JSON deserialization produces
+        original.userdata(Userdata.DEFAULT_VALUE, Arrays.asList(dateStr, dateStr));
+
+        BackendEntry entry = ser.writePropertyKey(original);
+        PropertyKey reloaded = ser.readPropertyKey(objects.graph(), entry);
+
+        Object value = reloaded.defaultValue();
+        Assert.assertTrue("DEFAULT_VALUE should be a Set after round-trip, was " +
+                          (value == null ? "null" : value.getClass()),
+                          value instanceof Set);
+        Set<?> values = (Set<?>) value;
+        Assert.assertEquals("duplicates must be collapsed", 1, values.size());
+        Assert.assertTrue(values.contains(expected));
+    }
+
     @Test
     public void testEdgeForPartition() {
         BinarySerializer ser = new BinarySerializer(true, true, true);

hugegraph-server/hugegraph-test/src/main/java/org/apache/hugegraph/unit/serializer/TextSerializerTest.java

@@ -17,7 +17,9 @@
 
 package org.apache.hugegraph.unit.serializer;
 
+import java.util.Arrays;
 import java.util.Date;
+import java.util.Set;
 
 import org.apache.hugegraph.backend.id.IdGenerator;
 import org.apache.hugegraph.backend.serializer.TextSerializer;
@@ -26,6 +28,7 @@
 import org.apache.hugegraph.schema.PropertyKey;
 import org.apache.hugegraph.schema.Userdata;
 import org.apache.hugegraph.testutil.Assert;
+import org.apache.hugegraph.type.define.Cardinality;
 import org.apache.hugegraph.type.define.DataType;
 import org.apache.hugegraph.unit.BaseUnitTest;
 import org.apache.hugegraph.unit.FakeObjects;
@@ -75,4 +78,30 @@ public void testPropertyKeyDefaultValueRoundTripsAsDate() {
                           value instanceof Date);
         Assert.assertEquals(defaultValue, value);
     }
+
+    @Test
+    public void testPropertyKeySetDefaultValueRoundTripsAsDate() {
+        HugeConfig config = FakeObjects.newConfig();
+        TextSerializer ser = new TextSerializer(config);
+
+        FakeObjects objects = new FakeObjects();
+        PropertyKey original = objects.newPropertyKey(IdGenerator.of(2L),
+                                                      "tags", DataType.DATE);
+        original.cardinality(Cardinality.SET);
+
+        String dateStr = "2026-05-14 10:11:12.345";
+        Date expected = DateUtil.parse(dateStr);
+        original.userdata(Userdata.DEFAULT_VALUE, Arrays.asList(dateStr, dateStr));
+
+        BackendEntry entry = ser.writePropertyKey(original);
+        PropertyKey reloaded = ser.readPropertyKey(objects.graph(), entry);
+
+        Object value = reloaded.defaultValue();
+        Assert.assertTrue("DEFAULT_VALUE should be a Set after round-trip, was " +
+                          (value == null ? "null" : value.getClass()),
+                          value instanceof Set);
+        Set<?> values = (Set<?>) value;
+        Assert.assertEquals("duplicates must be collapsed", 1, values.size());
+        Assert.assertTrue(values.contains(expected));
+    }
 }