pig/src/main/java/com/twitter/elephantbird/pig/util/ProtobufToPig.java
private FieldSchema messageToFieldSchema(FieldDescriptor fieldDescriptor) throws FrontendException {
    assert fieldDescriptor.getType() == FieldDescriptor.Type.MESSAGE : "messageToFieldSchema called with field of type " + fieldDescriptor.getType();

    Schema innerSchema = toSchema(fieldDescriptor.getMessageType());

    if (fieldDescriptor.isRepeated()) {
      Schema tupleSchema = new Schema();
      tupleSchema.add(new FieldSchema(fieldDescriptor.getName() + "_tuple", innerSchema, DataType.TUPLE));
      return new FieldSchema(fieldDescriptor.getName(), tupleSchema, DataType.BAG);
    } else {
      return new FieldSchema(fieldDescriptor.getName(), innerSchema, DataType.TUPLE);
    }
  }
private FieldSchema messageToFieldSchema(FieldDescriptor fieldDescriptor) throws FrontendException {
    assert fieldDescriptor.getType() == FieldDescriptor.Type.MESSAGE : "messageToFieldSchema called with field of type " + fieldDescriptor.getType();

    Schema innerSchema = toSchema(fieldDescriptor.getMessageType());

    if (fieldDescriptor.isRepeated()) {
      Schema tupleSchema = new Schema();
      tupleSchema.add(new FieldSchema(fieldDescriptor.getName() + "_tuple", innerSchema, DataType.TUPLE));
      return new FieldSchema(fieldDescriptor.getName(), tupleSchema, DataType.BAG);
    } else {
      return new FieldSchema(fieldDescriptor.getName(), innerSchema, DataType.TUPLE);
    }
  }
Turn a nested message into a Schema object.  For repeated nested messages, it generates a schema for a bag of
tuples.  For non-repeated nested messages, it just generates a schema for the tuple itself.
@param fieldDescriptor the descriptor object for the given field.
@return the Schema for the nested message.
@throws FrontendException if Pig decides to.