Fix bug that results in AttributeError - 'OccurrencePreview' object has no attribute 'object'

[fatboystring]

Fixes issues #301 and #554

A couple of issues identified with the OccurrencePreview view - as follows:

• The MRO of the OccurrencePreview class means that the ProcessFormView.get method is called first and foremost. ProcessFormView.get does not invoke the get_object method meaning the object is never fetched from the database.
• The model attribute defined on the view class is Occurrence. The pk_url_kwarg attribute on the view class is occurrence_id. The url pattern that this view class is bound to does not define an occurrence_id pattern meaning the get_object method call would fail if the default implementation of get_object were used

As a result of the above, when get_context_data is called, the object attribute of the view has not been set, resulting in an AttributeError.

The fix involves:

1. using the event_id from the url pattern to fetch the event from the db
2. getting the associated occurrence by constructing the start/end dates from the url parameters and using the Period class to get occurrences for that point in time
3. returning the occurrence if found
4. raising an http 404 if the event could not be found, or no occurrences exist for the specified point in time

Thanks for the library. Please let me know if you'd like anything changed in the pull request

[coveralls]

coverage: 80.36% (+0.4%) from 79.986% when pulling f6479c2 on fatboystring:bugfix/301 into 8aa6f87 on llazzaro:develop.

[llazzaro]

@fatboystring One check is failing, can you fix it? https://github.com/llazzaro/django-scheduler/actions/runs/5405303603/jobs/9970975750?pr=555

Regards

[fatboystring]

@llazzaro should be sorted now. Thanks

[h3x4git]

I don't think this is compatible with #488