[web2py] define_table - how to do this correctly (best practice?)

[lyn2py]

I have tables with individual properties,

db.define_table('article',
  Field('title', 'string', length=255,required=True),
  Field('description', 'text',required=True),
)
db.define_table('post',
  Field('title', 'string', length=255,required=True),
  Field('description', 'text',required=True),
)
db.define_table('page',
  Field('title', 'string', length=255,required=True),
  Field('description', 'text',required=True),
)

And I have a table with many-to-many relationship (with different
tables):

db.define_table('action',
  Field('user_id','reference
auth_user',default=auth.user_id,required=True),
 
Field('action','string',requires=IS_IN_SET(('fav','subscribe','report','follow')),required=True),
  Field('table',requires=???,required=True),
  Field('table_id','integer',default=???,required=True),
)

My questions:
1.. The field, "table" is actually one of the tables, article/post/
page. Is the only correct way to use
requires=IS_IN_SET(('article','post','page')) ?

2.. "table_id" is actually dependent on the "table" selected. what
should I put for "default=???"

3.. Can I set a different set of IS_IN_SET for "action" field,
depending on the "table" field chosen?

4.. Finally, if the above is the wrong approach, how should I do it
correctly (best practice?)

Thanks!