Create Access Rule on entity
Create a XPATH constraint on Mydata.
Click on button owner.
Make an association to the user entity. (MyData_User)
Create a After create event on MyData that fills this association (retrieve current user, change object)
Create Access rule. Put an XPATH constraint on the MyData entity. Click on path to user.
With some extension can this method be used if groups of users can see each data, but not all.