Assumptions on our ER diagram
1.
We allow as many users to join our website. Each user is identified by key u_id.
2.
Single user can create only one community and becomes a master.
3.
Single user can join multiple communities and become members. Weak entity
‘member’ has discriminator key ‘c_id’ which constraints single user joining
multiple times to single community. Weak entity member is identified using
identifying entity set ‘user’.
4.
Both master and member can upload files to file entity set. Each file can be
differentiated by attributes of this set. Master has special privilege to delete,
and update files. File entity set’s primary key is f_id.
5.
Each community has multiple boards to write messages. And each board is
identified using weak entity set ‘board’ discriminator key ‘index’.
6.
Community must have only single master managing the community.
7.
Member can write to the community board.
8.
Participation of member in relationship set ‘become’ is total.