Hi John,
I believe this is an oversight in the API.  I think we should probably open up a JIRA and try to address it.  

Depending on the use-case you might be able to write a map as list-of-struct<key, value> and replace the metadata at some point, but I'm not sure if this is possible in the Java library, so I don't have any examples. 

Thanks,
Micah

On Fri, Jan 1, 2021 at 10:02 PM John Peterson <donttrytocontactme2@gmail.com> wrote:
Hi All,

I’m attempting to convert the following data structure into vectors for Arrow:

Map<String, Map<String, Integer>> 

Using the UnionMapWriter, it seems that I can’t grab a map writer for the value of the map, but a nested complex type such as a list is supported:

writer.value().list(); // works, get a ListWriter
writer.value().map(); // doesn’t work, no such method

Is this a known limitation, or is there a proper way to translate nested map structures to Arrow? I’m planning on supporting a Map as both a key and a value of a map. If there is a way to do this (even without the writers), could you please point me to an example?

Thanks in advance for your help with this. 

John