[dholbach]

Problem

Posting a new session log raised MultipleObjectsReturned: get() returned more than one Session -- it returned 2! for a client who had been rescheduled: the original event was cancelled and a new one booked at the same date and time, leaving two Session rows (one cancelled) on that date. The calendar import keys sessions on date + time, so several sessions per date are legitimate, but two sites looked them up by date only via get_or_create:

• session_log_create (views/clinical_views.py)
• InvoiceFormsetMixin.attach_sessions_to_formset (views/crud_mixins.py), the same crash when saving an invoice item dated on such a day

Fix

Both sites now:

• choose from the day's live (non-cancelled) sessions when there are any, falling back to all of the day's sessions;
• prefer one not yet logged (session log) or not yet billed (invoice item), ordered by time;
• create a Session only when none exists for that date.

The log view still redirects to editing the existing log when every candidate already has one. An invoice item already linked to a session on the same date keeps its link on edit.

Tests

Six new tests in test_clinical.py and test_views_invoice_billing.py:

• two sessions on the same day
• every session that day already logged
• the cancelled leftover is skipped, for both logs and invoices
• a logged live session that has a cancelled leftover beside it

./dev.py test --fast passes.

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