Thanks for the link. I understand the changes, but at the same time I see no solution unless logstash is enhanced with the ability to set the _parent meta-field in the output.
I reverted to ES 1.7.3 now, where I can pass the _parent field in the document body and in combination with routing the child to the right parent shard, it appears to work. Feels like a hack, however. 
On a side note, is it possible to have logstash generate a nested field from the child tables while reading the parent table using JDBC?