sub aggregation with elasticsearch
Here is the correct aggregation query (hint: you need to jump out of the nested context back into the parent one using reverse_nested)
{ "aggs": { "members": { "nested": { "path": "members" }, "aggs": { "language": { "terms": { "field": "members.language" }, "aggs": { "parent": { "reverse_nested": {}, "aggs": { "date": { "date_histogram": { "field": "signupdate", "interval": "month" } } } } } } } } }}