going somewhere.

migrations/create_tables_001.sql

@@ -65,5 +65,6 @@ CREATE TABLE IF NOT EXISTS cashed_cheques
 
     FOREIGN KEY (underlying_security) REFERENCES securities (id),
     FOREIGN KEY (drawer_user_id) REFERENCES users (id),
-    FOREIGN KEY (drawer_account_id) REFERENCES accounts (id)
+    FOREIGN KEY (drawer_account_id) REFERENCES accounts (id),
+    CONSTRAINT double_spending UNIQUE (drawer_user_id, cheque_number) -- no double spending
 );