:fk

The foreign key constraint allows you to ensure referenced rows always exist.

Like other constraints, :fk can be applied to any query, not just tables.

(def OrderMustReferToACustomer
  [[:from :Order] [:fk :Customer {:customer-id :customer-id}]])
  
(def db (rel/materialize {} OrderMustReferToACustomer))

;; this is fine
(rel/transact db [:insert :Customer {:customer-id 42}] [:insert :Order {:customer-id 42}])

;; this is also fine, you can insert out-of-order.
(rel/transact db [:insert :Order {:customer-id 42}] [:insert :Customer {:customer-id 42}])

;; BOOM! Foreign key violation
(rel/transact db [:insert :Order {:customer-id "woops"}])

Cascading deletes

Like in SQL you can specify a :cascade option to unwind references that are invalidated by transactions.

The below example would cause :Order rows to be themselves be deleted if a transaction deletes the customers they point to.

[[:from :Order]
 [:fk :Customer {:customer-id :customer-id} {:cascade :delete}]]

:cascade only works if the left-side is a table (at the moment!).