How to design functionality to comment about a user joining a Facebook group? User A joined the group DBA


The table responsible for users x groups is "Following" table.


    user_id      group_id
       1             1
       1             2
       2             1


And in my app I have a table Comments. the foreign key of it is polimorphic, because comments can happen inside posts, photos, etc..


      text       parent_type      parent_id  
     "cool"         Post             1
     "nice pic"     Photo            2
     "welcome!"     Following        3
 

It is a good schema? The following table has_many comments?

And a more complex one would be: "How to comment on a set of people joining?"
I think if you want to be able to comment on someone joining a group, then "join a group" must be an entity that can be referenced by other records. You might want to have another "parent_type" called "event" and one of the event types would be "joined a group". That would let you reference it from your comments table.