Add missing type for MapVirtualColumn (#5598)

extensions-contrib/virtual-columns/src/main/java/io/druid/segment/DruidVirtualColumnsModule.java

@@ -20,6 +20,7 @@
 package io.druid.segment;
 
 import com.fasterxml.jackson.databind.Module;
+import com.fasterxml.jackson.databind.jsontype.NamedType;
 import com.fasterxml.jackson.databind.module.SimpleModule;
 import com.google.common.collect.ImmutableList;
 import com.google.inject.Binder;
@@ -34,7 +35,12 @@ public class DruidVirtualColumnsModule implements DruidModule
   @Override
   public List<? extends Module> getJacksonModules()
   {
-    return ImmutableList.of(new SimpleModule().registerSubtypes(MapVirtualColumn.class));
+    return ImmutableList.of(
+        new SimpleModule(getClass().getSimpleName())
+            .registerSubtypes(
+                new NamedType(MapVirtualColumn.class, "map")
+            )
+    );
   }
 
   @Override

extensions-contrib/virtual-columns/src/test/java/io/druid/segment/MapVirtualColumnTest.java

@@ -19,6 +19,7 @@
 
 package io.druid.segment;
 
+import com.fasterxml.jackson.databind.ObjectMapper;
 import com.google.common.base.Supplier;
 import com.google.common.base.Suppliers;
 import com.google.common.collect.ImmutableMap;
@@ -140,6 +141,18 @@ public class MapVirtualColumnTest
                  .pagingSpec(new PagingSpec(null, 3));
   }
 
+  @Test
+  public void testSerde() throws IOException
+  {
+    final ObjectMapper mapper = new DefaultObjectMapper();
+    new DruidVirtualColumnsModule().getJacksonModules().forEach(mapper::registerModule);
+
+    final MapVirtualColumn column = new MapVirtualColumn("keys", "values", "params");
+    final String json = mapper.writeValueAsString(column);
+    final VirtualColumn fromJson = mapper.readValue(json, VirtualColumn.class);
+    Assert.assertEquals(column, fromJson);
+  }
+
   @Test
   public void testBasic()
   {