Cannot delete a credential that have been used in a snapshot

[kedare]

Hello

When trying to delete a credential set that have been used in a previous snapshot (appearing on the device_credential_sets), the user cannot be deleted and the logs are showing this error

org.postgresql.util.PSQLException: ERROR: update or delete on table "device_credential_set" violates foreign key constraint "FK_e1p7bmak3fa8yhqns9y4ttofg" on table "device_credential_sets"
  Detail: Key (id)=(2) is still referenced from table "device_credential_sets".
        at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:2553)
        at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:2285)
        at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:323)
        at org.postgresql.jdbc.PgStatement.executeInternal(PgStatement.java:481)
        at org.postgresql.jdbc.PgStatement.execute(PgStatement.java:401)
        at org.postgresql.jdbc.PgPreparedStatement.executeWithFlags(PgPreparedStatement.java:164)
        at org.postgresql.jdbc.PgPreparedStatement.executeUpdate(PgPreparedStatement.java:130)
        at com.mchange.v2.c3p0.impl.NewProxyPreparedStatement.executeUpdate(NewProxyPreparedStatement.java:1502)
        at org.hibernate.engine.jdbc.internal.ResultSetReturnImpl.executeUpdate(ResultSetReturnImpl.java:197)
        ... 55 common frames omitted

I think putting an on delete set null on the foreign key should allow to delete the credential (but may cause errors on the backend if this is not a supported behaviour?)

[SCadilhac]

If you edit the device(s) which made use of the credential set, and uncheck it, you should then be able to remove the credential set. But we might change this to delete even used...