How to parse msg to json object instead of text?

[fly7632785]

Hello
I got a problem
My msg is json String like "{"a":"b","c":"d"}"
The type of the field msg in elasticsearch is text

I want the data to be parsed to json in elasticsearch, so I can filter by the params
I tried to create index template and set msg type "Flattened" or "Nested". But it didn't work

please help, than you !