Enforcing existence of records while avoiding circular references I'm having issues designing a satisfactory solution for the following problem. 

There are two main entities:


Customer
User


A customer can have many users associated to it. A user can be associated with many customers. Definitely a many-to-many relation for which a junction table should exist.

A customer must have at least one admin user. This is absolutely critical.

For several reasons it would be nice to enforce that the combination of customer &amp; corresponding admin account exists in the junction table. For example: when listing the customer's users, it would be nice to not have to manually include the admin user.

In the following design it would be possible for the combination of customer &amp; corresponding admin to not exist in the junction table:



(ignore the erroneously placed crow's foot at 'User')

Is there a good way to solve this problem?

The solutions I came up with so far involved introducing circular references, which I think are undesirable. Another option might be to go with this design and use a VIEW for the user listing. Feels like a workaround.

Is there a flaw in my design or is this a limitation of relational databases?
I think that your problem could be solved by imposing a new Foreign Key constraints in your tables:

User (UserId, UserOther) PK(UserId)
Customer (CustomerId, CustomerOther, UserAdminId) PK(CustomerId)
Customer_User (CustomerId, UserId), PK(CustomerId, UserId)

In Customer_User:
  FK(CustomerId) REFERENCES Customer
  FK(UserID) REFERENCES User

In Customer:
  FK(UserAdminId) REFERENCES User
  FK(CustomerId, UseAdminId) REFERENCES PK(Customer_User)


In this way you are guaranteed that a Customer has a valid User as admin, and that there is a tuple in Customer_User that corresponds to a the admin of a certain Customer.