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Traversal with SQLAlchemy

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Traversal works most naturally with an object database like ZODB because both are naturally recursive. (I.e., “/a/b” maps naturally to root["a"]["b"].) SQL tables are flat, not recursive. However, it’s possible to use traversal with SQLAlchemy, and it’s becoming increasingly popular. To see how to do this, it helps to consider recursive and non-recursive usage separately.

Non-recursive

A non-recursive use case is where a certain URL maps to a table, and the following component is a record ID. For instance:

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# /persons/123   =>   root["persons"][123]

import myapp.model as model

class Resource(dict):
    def __init__(self, name, parent):
        self.__name__ = name
        self.__parent__ = parent

class Root(Resource):
    """The root resource."""

    def add_resource(self, name, orm_class):
        self[name] = ORMContainer(name, self, self.request, orm_class)

    def __init__(self, request):
        self.request = request
        self.add_resource('persons', model.Person)

root_factory = Root

class ORMContainer(dict):
    """Traversal component tied to a SQLAlchemy ORM class.

    Calling .__getitem__ fetches a record as an ORM instance, adds certain
    attributes to the object, and returns it.
    """
    def __init__(self, name, parent, request, orm_class):
        self.__name__  = name
        self.__parent__ = parent
        self.request = request
        self.orm_class = orm_class

    def __getitem__(self, key):
        try:
            key = int(key)
        except ValueError:
            raise KeyError(key)
        obj = model.DBSession.query(self.orm_class).get(key)
        # If the ORM class has a class method '.get' that performs the
        # query, you could do this:  ``obj = self.orm_class.get(key)``
        if obj is None:
            raise KeyError(key)
        obj.__name__ = key
        obj.__parent__ = self
        return obj

Here, root["persons"] is a container object whose __getitem__ method fetches the specified database record, sets name and parent attribues on it, and returns it. (We’ve verified that SQLAlchemy does not define __name__ or __parent__ attributes in ORM instances.) If the record is not found, raise KeyError to indicate the resource doesn’t exist.

TODO: Describe URL generation, access control lists, and other things needed in a complete application.

One drawback of this approach is that you have to fetch the entire record in order to generate a URL to it. This does not help if you have index views that display links to records, by querying the database directly for the IDs that match a criterion (N most recent records, all records by date, etc). You don’t want to fetch the entire record’s body, or do something silly like asking traversal for the resource at “/persons/123” and then generate the URL – which would be “/persons/123”! There are a few ways to generate URLs in this case:

  • Define a generation-only route; e.g., config.add_route("person", "/persons/{id}", static=True)
  • Instead of returning an ORM instance, return a proxy that lazily fetches the instance when its attributes are accessed. This causes traversal to behave somewhat incorrectly. It should raise KeyError if the record doesn’t exist, but it can’t know whether the record exists without fetching it. If traversal returns a possibly-invalid resource, it puts a burden on the view to check whether its context is valid. Normally the view can just assume it is, otherwise the view wouldn’t have been invoked.

Recursive

The prototypical recursive use case is a content management system, where the user can define URLs arbitrarily deep; e.g., “/a/b/c”. It can also be useful with “canned” data, where you want a small number of views to respond to a large variety of URL hierarchies.

Kotti is the best current example of using traversal with SQLAlchemy recursively. Kotti is a content management system that, yes, lets users define arbitrarily deep URLs. Specifically, Kotti allows users to define a page with subpages; e.g., a “directory” of pages.

Kotti is rather complex and takes some time to study. It uses SQLAlchemy’s polymorphism to make tables “inherit” from other tables. This is an advanced feature which can be complex to grok. On the other hand, if you have the time, it’s a great way to learn how to do recursive traversal and polymorphism.

The main characteristic of a recursive SQL setup is a self-referential table; i.e., table with a foreign key colum pointing to the same table. This allows each record to point to its parent. (The root record has NULL in the parent field.)

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