I have three database tables:

• users
• emails
• invitations

Emails are linked to users by a user_id field.

Invitations are also linked to users by a user_id field

Emails can be created without an invitation, but every invitation must have an email.

I would like to link the emails and invitations tables so it is possible to find the email for a particular invitation.

However this creates a circular reference, both an invitation and an email record hold the id for the same user.

Is this bad design and if so, how could I improve it?

My feeling is that with use of foreign keys and good business logic, it is fine.