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I am developing an application which allows 'users' to create 'lists'. I have a many-to-many relationship between 'users' and 'lists' (this might not be right). Additionally 'Lists' have many 'Tasks'.

What I would like to do is extend this model to include the idea of 'invitations'. I would like users to be able to invite one another to other lists. A user can create many invitations. A user can create an 'invitation' and the invitation would have one or many 'invitees' which, in turn, would be 'Users'. So am confused about how this should be organized in a relational db.

I think the core problem is: users 'own' invitations but also can be the recipient of an invitation. Clear as mud? :)

I was hoping someone could offer some advice on how this might be accomplished. Any example ERDs would be useful. If my question requires further clarification I can provide it.


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up vote 2 down vote accepted

i will recapitulate your description:

  • A user can create lists.
  • A list tackles some tasks.
  • Users can join some lists.
  • A user can send an invitation to some users and propose to join some lists

Based on this description I would propose the following ERD


From this I would deduce the following relations


Primary keys are between apostrophes. Foreign keys can be deduced by the names if the fields (so user_id in lists references user_id in user) The word in parantheses after a foreign_key describes from which relation in the ERD this foreign key is derrived.

I did not care abou the names of the id-columns. Perhaps an id is a surogate key (an artificial key) or some attribue of the entity. also i did not add additional attributes which can be easily added. The characterisation o task as a weak entity is rather arbitrary.

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This is great. Thanks for the diagram it really helps. – Nick Aug 17 '12 at 15:48

You could have a pretty simple schema for this with a "trick": treat all user -> list associations as "invitations", including the ownership relation.

You'd have one table for the users (with say a userid column). Then a table for the lists (with a listid).
The relation between the users and lists tables could be:

userid | listid    | role
bob    | dbas      | owner
alice  | dbas      | invitee
alice  | sysadmins | owner
bob    | sysadmins | invitee
eve    | sysadmins | invitee

An invitation is simply another line in that table.
This allows you to have different levels of access to the lists. A user could "invite" another to have full-control of a list, just read access, or something in between - just define the appropriate set of roles for your use-case.

You might want to add things like "creator" information, last modification information to the lists themselves.

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squeezing all user list relations in one table seems not to be a good idea to me. You are ignoring all relational design principles (normal forms) – miracle173 Aug 17 '12 at 15:12
I'm not "squeezing all the list relations in one table", I'm creating a table for the one (and only) relation I'm considering (which is "user has access to list" essentially). – Mat Aug 18 '12 at 8:45
From your statement 'treat all user -> list associations as "invitations"' I concluded that you want to store all user-list relations in one table. I see now that you do not put the 'creator' information in the invitation table (using a "createor" role). It is possible to merge some tables respresenting the relations between two entities into one table that contains a third column that holds a value to discriminate the relations. In contrast to my former statement I think this will not conflict to some normal forms. Nevertheless I do not like such an approach. – miracle173 Aug 21 '12 at 15:40

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