4.7. How to use entities objects and adapters

The previous chapters detailed the classes and methods available to the developer at the so-called ORM level. However they say little about the common patterns of usage of these objects.

Entities objects (and their adapters) are used in the repository and web sides of CubicWeb. On the repository side of things, one should manipulate them in Hooks and Operations.

Hooks and Operations provide support for the implementation of rules such as computed attributes, coherency invariants, etc (they play the same role as database triggers, but in a way that is independent of the actual data sources).

So a lot of an application’s business rules will be written in Hooks (or Operations).

On the web side, views also typically operate using entity objects. Obvious entity methods for use in views are the Dublin Core methods like dc_title. For separation of concerns reasons, one should ensure no ui logic pervades the entities level, and also no business logic should creep into the views.

In the duration of a transaction, entities objects can be instantiated many times, in views and hooks, even for the same database entity. For instance, in a classic CubicWeb deployment setup, the repository and the web front-end are separated process communicating over the wire. There is no way state can be shared between these processes (there is a specific API for that). Hence, it is not possible to use entity objects as messengers between these components of an application. It means that an attribute set as in obj.x = 42, whether or not x is actually an entity schema attribute, has a short life span, limited to the hook, operation or view within which the object was built.

Setting an attribute or relation value can be done in the context of a Hook/Operation, using the obj.cw_set(x=42) notation or a plain RQL SET expression.

In views, it would be preferable to encapsulate the necessary logic in a method of an adapter for the concerned entity class(es). But of course, this advice is also reasonable for Hooks/Operations, though the separation of concerns here is less stringent than in the case of views.

This leads to the practical role of objects adapters: it’s where an important part of the application logic lies (the other part being located in the Hook/Operations).

4.8. Anatomy of an entity class

We can look now at a real life example coming from the tracker cube. Let us begin to study the entities/project.py content.

from cubicweb.entities.adapters import ITreeAdapter

class ProjectAdapter(ITreeAdapter):
    __select__ = is_instance('Project')
    tree_relation = 'subproject_of'

class Project(AnyEntity):
    __regid__ = 'Project'
    fetch_attrs, cw_fetch_order = fetch_config(('name', 'description',
                                                'description_format', 'summary'))

    TICKET_DEFAULT_STATE_RESTR = 'S name IN ("created","identified","released","scheduled")'

    def dc_title(self):
        return self.name

The fact that the Project entity type implements an ITree interface is materialized by the ProjectAdapter class (inheriting the pre-defined ITreeAdapter whose __regid__ is of course ITree), which will be selected on Project entity types because of its selector. On this adapter, we redefine the tree_relation attribute of the ITreeAdapter class.

This is typically used in views concerned with the representation of tree-like structures (CubicWeb provides several such views).

It is important that the views themselves try not to implement this logic, not only because such views would be hardly applyable to other tree-like relations, but also because it is perfectly fine and useful to use such an interface in Hooks.

In fact, Tree nature is a property of the data model that cannot be fully and portably expressed at the level of database entities (think about the transitive closure of the child relation). This is a further argument to implement it at entity class level.

fetch_attrs configures which attributes should be pre-fetched when using ORM methods retrieving entity of this type. In a same manner, the cw_fetch_order is a class method allowing to control sort order. More on this in Loaded attributes and default sorting management.

We can observe the big TICKET_DEFAULT_STATE_RESTR is a pure application domain piece of data. There is, of course, no limitation to the amount of class attributes of this kind.

The dc_title method provides a (unicode string) value likely to be consumed by views, but note that here we do not care about output encodings. We care about providing data in the most universal format possible, because the data could be used by a web view (which would be responsible of ensuring XHTML compliance), or a console or file oriented output (which would have the necessary context about the needed byte stream encoding).

Note

The Dublin Core dc_xxx methods are not moved to an adapter as they are extremely prevalent in CubicWeb and assorted cubes and should be available for all entity types.

Let us now dig into more substantial pieces of code, continuing the Project class.

def latest_version(self, states=('published',), reverse=None):
    """returns the latest version(s) for the project in one of the given
    states.

    when no states specified, returns the latest published version.
    """
    order = 'DESC'
    if reverse is not None:
        warn('reverse argument is deprecated',
             DeprecationWarning, stacklevel=1)
        if reverse:
            order = 'ASC'
    rset = self.versions_in_state(states, order, True)
    if rset:
        return rset.get_entity(0, 0)
    return None

def versions_in_state(self, states, order='ASC', limit=False):
    """returns version(s) for the project in one of the given states, sorted
    by version number.

    If limit is true, limit result to one version.
    If reverse, versions are returned from the smallest to the greatest.
    """
    if limit:
        order += ' LIMIT 1'
    rql = 'Any V,N ORDERBY version_sort_value(N) %s ' \
          'WHERE V num N, V in_state S, S name IN (%s), ' \
          'V version_of P, P eid %%(p)s' % (order, ','.join(repr(s) for s in states))
    return self._cw.execute(rql, {'p': self.eid})

These few lines exhibit the important properties we want to outline:

  • entity code is concerned with the application domain
  • it is NOT concerned with database consistency (this is the realm of Hooks/Operations); in other words, it assumes a consistent world
  • it is NOT (directly) concerned with end-user interfaces
  • however it can be used in both contexts
  • it does not create or manipulate the internal object’s state
  • it plays freely with RQL expression as needed
  • it is not concerned with internationalization
  • it does not raise exceptions

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