Fix serialization of union types at root level (fixes #168)

avro/src/main/java/com/fasterxml/jackson/dataformat/avro/ser/AvroWriteContext.java

@@ -163,6 +163,28 @@ public final String toString()
 
     // // // Shared helper methods
 
+    protected GenericRecord _createRecord(Schema schema, Object currValue) throws JsonMappingException
+    {
+        Type type = schema.getType();
+        if (type == Schema.Type.UNION) {
+            try {
+                schema = resolveUnionSchema(schema, currValue);
+            } catch (UnresolvedUnionException e) {
+                // couldn't find an exact match
+                schema = _recordOrMapFromUnion(schema);
+            }
+        }
+        if (type == Schema.Type.MAP) {
+            throw new IllegalStateException("_createRecord should never be called for elements of type MAP");
+        }
+        try {
+            return new GenericData.Record(schema);
+        } catch (RuntimeException e) {
+            // alas, generator not passed to us
+            throw new JsonMappingException(null, "Failed to create Record type from "+type, e);
+        }
+    }
+
     protected GenericRecord _createRecord(Schema schema) throws JsonMappingException
     {
         // Quick check: if type is Union, need to find actual record type...

avro/src/main/java/com/fasterxml/jackson/dataformat/avro/ser/RootContext.java

@@ -62,7 +62,7 @@ public final AvroWriteContext createChildObjectContext(Object currValue) throws
         case RECORD:
         case UNION: // maybe
             {
-                GenericRecord rec = _createRecord(_schema);
+                GenericRecord rec = _createRecord(_schema, currValue);
                 _rootValue = rec;
                 return new ObjectWriteContext(this, _generator, rec, currValue);
             }

avro/src/test/java/com/fasterxml/jackson/dataformat/avro/interop/annotations/UnionTest.java

@@ -109,6 +109,15 @@ public boolean equals(Object o) {
         }
     }
 
+    @Test
+    public void testRootUnionWithAnimal() throws IOException {
+        Cat cat = new Cat("meow");
+        //
+        Cat result = roundTrip(Animal.class, cat);
+        //
+        assertThat(result).isEqualTo(cat);
+    }
+
     @Test
     public void testInterfaceUnionWithCat() throws IOException {
         Cage cage = new Cage(new Cat("test"));