Logstash mutate - add field with dot not working

Question

According to elastic convention we use dot for grouping. However when I try to use a dot in add_field it stops working.

Working:

 mutate {
   add_field => {"foo" => "baz"}
 }

Not working:

 mutate {
   add_field => {"foo.bar" => "baz"}
 }

Not working:

 mutate {
   add_field => {"[foo][bar]" => "baz"}
 }

So how do I make it work with dot in field's name?

Answer 1

I think I've found a solution.

This is working for me (for some unexplained reason):

 mutate {
   add_field => { "[][foo][bar]" => "baz" }
 }