ascade-destroy occurrences; support events without a fixed start time

- Event#occurrences is now dependent: :destroy, so destroying an Event
  (directly, or via Node's cascade) removes its Occurrence rows instead
  of orphaning them.
- Renamed generate_occurences -> generate_occurrences (typo fix);
  callback and method updated together.
- Occurrence.generate returns early when event.start_time is nil, so a
  chapter with no sourced time produces zero occurrences instead of a
  guessed one.
- seed_chapter no longer falls back to 19:00 when start_time is absent;
  base_time/end_time are nil in that case.

Database-level foreign key cascade on occurrences.event_id is still
pending.

1 files changed, 1 insertions, 0 deletions

diff --git a/app/models/occurrence.rb b/app/models/occurrence.rb
index 41ef4cb..c6a2380 100644
--- a/app/models/occurrence.rb
+++ b/app/models/occurrence.rb
@@ -27,6 +27,7 @@ class Occurrence < ApplicationRecord
   # event are then calculated and created.
   def self.generate event
     self.where(:event_id => event.id).delete_all
+    return if event.start_time.nil?
     
     node        = event.node
     duration    = (event.end_time - event.start_time)