Sounds like a good plan. Thanks for your thoughts. On Oct 6, 12:18 am, Nick Johnson <nickjohn...@google.com> wrote: > On Thu, Oct 6, 2011 at 1:34 AM, PatrickCD <patrick.do...@gmail.com> wrote: > > Nick, > > > > It _is_ a hack. A better approach would be to use a single class with > > > appropriate properties, or create a new instance of a new class and > > discard > > > the old one at such time as you need to make the change. > > > Thanks for your reply. I'm sure you're bored of this thread so I'll > > try to summarise. > > > To take another example, SQLAlchemy supports what it calls "Single > > Table Inheritance" - > >http://www.sqlalchemy.org/docs/orm/inheritance.html#single-table-inhe... > > . In this approach, there is a field "type" that SQLAlchemy uses to > > determine what class the row should be mapped to. So if you change the > > 'type' you change the class. > > This is true, but I think if you ask the SQLAlchemy authors, they would also > say that they don't expect this to change after a row is created. > > > > > You seem to be saying that each record in the datastore should map to > > one and only one Python class, so a similar approach doesn't work. > > No, you can map datastore entities to objects any way you like. I'm just > pointing out that the thing you're trying to do is a hack, and violates some > basic expectations OO systems make, and the way you're trying to do it is > unsupported and may break. > > > > > Polymodel Objects in the GAE Datastore have a property "class" which > > stores a list of super classes. This property appears to be used by > > the DataStore API in a similar way to how "type" is used by > > SQLAlchemy. It's currently possible to change the value of this field > > by setting the __class__ attribute of an instance and saving it back > > to the DataStore. However, you state: > > > "You're breaking any number of invariants by doing this, so things may > > break without notice at any time. " > > > Somewhat cryptic, but I'll take your word for it. > > Invariants like "the set of superclasses of an instance will remain constant > after an object is created". It's possible that not just us, but also other > Python libraries expect that invariant, so once you start messing with it, > you may get unexpected problems elsewhere. > > > > > So perhaps the question boils down to whether or not there _is_ a > > reliable way to change the value of the class property for polymodel > > records? I'm guessing the answer is "no", but I'd like to check. > > You could use the low level datastore API to modify the class property in > the datastore. > > If I understand correctly, you want the behavior of a class's methods to > depend on the 'type' of entity being handled, and that 'type' may change > over the lifetime of an entity. Let me suggest an alternative way to do what > you're trying to do: > > - Create a "DeveloperBehavior" base class, which encapsulates the behaviors > that change between types > - Create subclasses for each type of behavior. > - Create a 'behaviour' property on your model class, which fetches or > instantiates the appropriate behavior for the current entity. > - Either call the behavior methods directly, or wrap them with methods on > your entity. > > This will allow you to change the behavior at runtime, without messing with > Python and DB internals. > > -Nick Johnson > > > > > Thanks > > > Patrick > > > -- > > You received this message because you are subscribed to the Google Groups > > "Google App Engine" group. > > To post to this group, send email to google-appengine@googlegroups.com. > > To unsubscribe from this group, send email to > > google-appengine+unsubscr...@googlegroups.com. > > For more options, visit this group at > >http://groups.google.com/group/google-appengine?hl=en. > > -- > Nick Johnson, Developer Programs Engineer, App Engine
-- You received this message because you are subscribed to the Google Groups "Google App Engine" group. To post to this group, send email to google-appengine@googlegroups.com. To unsubscribe from this group, send email to google-appengine+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en.
