fix: add util module, w/ model_transaction_query()

just a basic implementation, will have to improve later

src/wutta_continuum/conf.py

@@ -45,7 +45,28 @@ class WuttaContinuumConfigExtension(WuttaConfigExtension):
     key = "wutta_continuum"
 
     def startup(self, config):  # pylint: disable=empty-docstring
-        """ """
+        """
+        Perform final configuration setup for app startup.
+
+        This will do nothing at all, unless config enables the
+        versioning feature.  This must be done in config file and not
+        in DB settings table:
+
+        .. code-block:: ini
+
+           [wutta_continuum]
+           enable_versioning = true
+
+        Once enabled, this method will configure the integration, via
+        these steps:
+
+        1. call :func:`sqlalchemy-continuum:sqlalchemy_continuum.make_versioned()`
+        2. call :meth:`wuttjamaican:wuttjamaican.app.AppHandler.get_model()`
+        3. call :func:`sqlalchemy:sqlalchemy.orm.configure_mappers()`
+
+        For more about SQLAlchemy-Continuum see
+        :doc:`sqlalchemy-continuum:intro`.
+        """
         # only do this if config enables it
         if not config.get_bool(
             "wutta_continuum.enable_versioning", usedb=False, default=False
@@ -60,14 +81,17 @@ class WuttaContinuumConfigExtension(WuttaConfigExtension):
         )
         plugin = load_object(spec)
 
-        # tell sqlalchemy-continuum to do its thing
+        app = config.get_app()
+        if "model" in app.__dict__:
+            raise RuntimeError("something not right, app already has model")
+
+        # let sqlalchemy-continuum do its thing
         make_versioned(plugins=[plugin()])
 
-        # nb. must load the model before configuring mappers
-        app = config.get_app()
-        model = app.model  # pylint: disable=unused-variable
+        # must load model *between* prev and next calls
+        app.get_model()
 
-        # tell sqlalchemy to do its thing
+        # let sqlalchemy do its thing
         configure_mappers()