4.6. Interfaces and Adapters

Interfaces are the same thing as object-oriented programming interfaces. Adapter refers to a well-known adapter design pattern that helps separating concerns in object oriented applications.

In CubicWeb adapters provide logical functionalities to entity types.

Definition of an adapter is quite trivial. An excerpt from cubicweb itself (found in cubicweb.entities.adapters):

class ITreeAdapter(EntityAdapter):
    """This adapter has to be overriden to be configured using the
    tree_relation, child_role and parent_role class attributes to
    benefit from this default implementation
    """
    __regid__ = 'ITree'

    child_role = 'subject'
    parent_role = 'object'

    def children_rql(self):
        """returns RQL to get children """
        return self.entity.cw_related_rql(self.tree_relation, self.parent_role)

The adapter object has self.entity attribute which represents the entity being adapted.

Note

Adapters came with the notion of service identified by the registry identifier of an adapters, hence dropping the need for explicit interface and the cubicweb.predicates.implements selector. You should instead use cubicweb.predicates.is_instance when you want to select on an entity type, or cubicweb.predicates.adaptable when you want to select on a service.

4.6.1. Specializing and binding an adapter

from cubicweb.entities.adapters import ITreeAdapter

class MyEntityITreeAdapter(ITreeAdapter):
    __select__ = is_instance('MyEntity')
    tree_relation = 'filed_under'

The ITreeAdapter here provides a default implementation. The tree_relation class attribute is actually used by this implementation to help implement correct behaviour.

Here we provide a specific implementation which will be bound for MyEntity entity type (the adaptee).

4.6.2. Converting code from Interfaces/Mixins to Adapters

Here we go with a small example. Before:

from cubicweb.predicates import implements
from cubicweb.interfaces import ITree
from cubicweb.mixins import ITreeMixIn

class MyEntity(ITreeMixIn, AnyEntity):
    __implements__ = AnyEntity.__implements__ + (ITree,)


class ITreeView(EntityView):
    __select__ = implements('ITree')
    def cell_call(self, row, col):
        entity = self.cw_rset.get_entity(row, col)
        children = entity.children()

After:

from cubicweb.predicates import adaptable, is_instance
from cubicweb.entities.adapters import ITreeAdapter

class MyEntityITreeAdapter(ITreeAdapter):
    __select__ = is_instance('MyEntity')

class ITreeView(EntityView):
    __select__ = adaptable('ITree')
    def cell_call(self, row, col):
        entity = self.cw_rset.get_entity(row, col)
        itree = entity.cw_adapt_to('ITree')
        children = itree.children()

As we can see, the interface/mixin duality disappears and the entity class itself is completely freed from these concerns. When you want to use the ITree interface of an entity, call its cw_adapt_to method to get an adapter for this interface, then access to members of the interface on the adapter

Let’s look at an example where we defined everything ourselves. We start from:

class IFoo(Interface):
    def bar(self, *args):
        raise NotImplementedError

class MyEntity(AnyEntity):
    __regid__ = 'MyEntity'
    __implements__ = AnyEntity.__implements__ + (IFoo,)

    def bar(self, *args):
        return sum(captain.age for captain in self.captains)

class FooView(EntityView):
    __regid__ = 'mycube.fooview'
    __select__ = implements('IFoo')

    def cell_call(self, row, col):
        entity = self.cw_rset.get_entity(row, col)
        self.w('bar: %s' % entity.bar())

Converting to:

class IFooAdapter(EntityAdapter):
    __regid__ = 'IFoo'
    __select__ = is_instance('MyEntity')

    def bar(self, *args):
        return sum(captain.age for captain in self.entity.captains)

class FooView(EntityView):
     __regid__ = 'mycube.fooview'
     __select__ = adaptable('IFoo')

     def cell_call(self, row, col):
         entity = self.cw_rset.get_entity(row, col)
         self.w('bar: %s' % entity.cw_adapt_to('IFoo').bar())

Note

When migrating an entity method to an adapter, the code can be moved as is except for the self of the entity class, which in the adapter must become self.entity.

4.6.3. Adapters defined in the library

More are defined in web/views.