ARROW-326: Initialize nested writers in MapWriter based on the underlying MapVector's field

Closes #163

Author: StevenMPhillips

java/vector/src/main/codegen/templates/MapWriters.java

@@ -56,6 +56,28 @@ public class ${mode}MapWriter extends AbstractFieldWriter {
     }
     </#if>
     this.container = container;
+    for (Field child : container.getField().getChildren()) {
+      switch (Types.getMinorTypeForArrowType(child.getType())) {
+      case MAP:
+        map(child.getName());
+        break;
+      case LIST:
+        list(child.getName());
+        break;
+      case UNION:
+        UnionWriter writer = new UnionWriter(container.addOrGet(child.getName(), MinorType.UNION, UnionVector.class));
+        fields.put(child.getName().toLowerCase(), writer);
+        break;
+<#list vv.types as type><#list type.minor as minor>
+<#assign lowerName = minor.class?uncap_first />
+<#if lowerName == "int" ><#assign lowerName = "integer" /></#if>
+<#assign upperName = minor.class?upper_case />
+      case ${upperName}:
+        ${lowerName}(child.getName());
+        break;
+</#list></#list>
+      }
+    }
   }
 
   @Override

java/vector/src/test/java/org/apache/arrow/vector/complex/impl/TestPromotableWriter.java

@@ -21,13 +21,16 @@
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 
+import org.apache.arrow.flatbuf.Type;
 import org.apache.arrow.memory.BufferAllocator;
 import org.apache.arrow.vector.DirtyRootAllocator;
 import org.apache.arrow.vector.complex.AbstractMapVector;
 import org.apache.arrow.vector.complex.MapVector;
 import org.apache.arrow.vector.complex.NullableMapVector;
 import org.apache.arrow.vector.complex.UnionVector;
+import org.apache.arrow.vector.complex.writer.BaseWriter.MapWriter;
 import org.apache.arrow.vector.types.Types.MinorType;
+import org.apache.arrow.vector.types.pojo.Field;
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
@@ -50,7 +53,7 @@ public void terminate() throws Exception {
   @Test
   public void testPromoteToUnion() throws Exception {
 
-    try (final AbstractMapVector container = new MapVector(EMPTY_SCHEMA_PATH, allocator, null);
+    try (final MapVector container = new MapVector(EMPTY_SCHEMA_PATH, allocator, null);
          final NullableMapVector v = container.addOrGet("test", MinorType.MAP, NullableMapVector.class);
          final PromotableWriter writer = new PromotableWriter(v, container)) {
 
@@ -92,6 +95,22 @@ public void testPromoteToUnion() throws Exception {
 
       assertFalse("4 shouldn't be null", accessor.isNull(4));
       assertEquals(100, accessor.getObject(4));
+
+      container.clear();
+      container.allocateNew();
+
+      ComplexWriterImpl newWriter = new ComplexWriterImpl(EMPTY_SCHEMA_PATH, container);
+
+      MapWriter newMapWriter = newWriter.rootAsMap();
+
+      newMapWriter.start();
+
+      newMapWriter.setPosition(2);
+      newMapWriter.integer("A").writeInt(10);
+
+      Field childField = container.getField().getChildren().get(0).getChildren().get(0);
+      assertEquals("Child field should be union type: " + childField.getName(), Type.Union, childField.getType().getTypeType());
+
     }
   }
 }