How to set the subreport to read whatever the dataset/datasource of the main report?

Let me describe the situation.
I design main report and setup the database connection.
Then I insert the subreport, setup the database connection and link it with the main report.

However, we change the database connection of the main report, the subreport does not read from the old connection, it does not read from the new connection of the main report.

So how to make the subreport connect to whatever the data source/data file the main report connect to, automatically without manually change it?

Hopefully you got my point. It has been several days but if you can help me will be great!! Thanks!