This example demonstrates the use of version 5 UUIDs for both discovery of aggregate IDs and also to implement an application-wide rule (or "invariant"), the use of the declarative syntax for domain models with a "non-trivial" command method, automatic snapshotting, automatic setting of a common attribute on all events without needing to mention this attribute in the command methods, and a recipe for an event-sourced log.
In the domain model below, the Page aggregate has a base class Event which is defined with a user_id dataclass field that is defined not to be included in its __init__ method, and so does not need to be matched by parameters in the command method signatures. It has a default factory which gets the event attribute value from a Python context variable. This base aggregate event class is inherited by all its concrete aggregate event classes.
The update_body() command method does a "non-trival" amount of work before the BodyUpdated event is triggered, by creating a "diff" of the current version of the body and the new version. It then triggers an event, which contains the diff. The event is applied to the body by "patching" the current version of the body with this diff.
The Index aggregate has a version 5 UUID which is a function of a slug. The Index and Page aggregates are used in combination to maintain editable pages of text, with editable titles, and with editable "slugs" that can be used in page URLs.
A PageLogged event is also defined, and used to define a "page log" in the application.
../../../eventsourcing/examples/wiki/domainmodel.py
The create_diff() and apply_patch() functions use the Unix command line tools patch and diff.
../../../eventsourcing/examples/wiki/utils.py
The application provides methods to create a new page, get the details for a page by its slug, update the title of a page referenced by its slug, update the body of a page, and change the page slug. Please note that none of these methods mention a user_id argument. To get to a page, the slug is used to identify an index, and the index is used to get the page ID, and then the page ID is used to get the body and title of the page. To change a slug, the index objects for the old and the new are identified, the page ID is removed as the reference from the old index and set as the reference on the new index. The indexes are also used to implement a application-wide rule (or "invariant") that a slug can be used by only one page, such that if an attempt is made to change the slug of one page to a slug that is already being used by another page, then a SlugConflictError will be raised, and no changes made.
The application also demonstrates the "event-sourced log" recipe, by showing how all the IDs of the Page aggregates can be listed, by logging the IDs when a new page is created, in a sequence of stored events, and then selecting from this sequence when presenting a list of pages.
../../../eventsourcing/examples/wiki/application.py
The test case below sets a user ID in the context variable. A page is created and updated in various ways. At the end, all the page events are checked to make sure they all have the user ID that was set in the context variable.
../../../eventsourcing/examples/wiki/test.py