Context Navigation

Changes between Version 15 and Version 17 of Ticket #11648

v15	v17
1	1	The `Changeset`, `Milestone` and `Node` classes have a `resource` read-only property while the `Attachment`, `Ticket`, `Repository` and `WikiPage` classes create the `resource` attribute in their `__init__` method.
2	2
3		Having `resource` as a read-only property seems simpler since we don't have to worry about updating the `resource` attribute as the object changes. For example: [browser:/tags/trac-1.0.1/trac/ticket/model.py@:252#L236]. As a read-only property, `resource` will be implicitly updated as the object changes. This change ~~would avoids~~ defects like the one we saw in #11138.
	3	Having `resource` as a read-only property seems simpler since we don't have to worry about updating the `resource` attribute as the object changes. For example: [browser:/tags/trac-1.0.1/trac/ticket/model.py@:252#L236]. As a read-only property, `resource` will be implicitly updated as the object changes. This change prevent defects like the one we saw in #11138.
4	4
5	5	With this change and the one in #11609, if we ever find it worthwhile to create a `ModelBase` class it will be clear that `resource` and `realm` should be abstract properties (decorated with `abc.abstractproperty`). I also wanted to make this change ahead of modifying the `Component` and `Version` classes in #1233 so that the pattern to follow is clear.