Metadata-Version: 1.0
Name: z3c.relationfield
Version: 0.6.2
Summary: A relation field framework for Zope 3.
Home-page: UNKNOWN
Author: Martijn Faassen
Author-email: faassen@startifact.com
License: ZPL 2.1
Description: z3c.relationfield
        *****************
        
        Introduction
        ============
        
        This package implements a new schema field Relation, and the
        RelationValue objects that store actual relations. It can index these
        relations using the ``zc.relation`` infractructure, and using these
        indexes can efficiently answer questions about the relations.
        
        The package `z3c.relationfieldui`_ in addition provides a widget to
        edit and display Relation fields.
        
        .. _`z3c.relationfieldui`: http://pypi.python.org/pypi/z3c.relationfieldui
        
        Setup
        =====
        
        ``z3c.relationfield.Relation`` is a schema field that can be used to
        express relations. Let's define a schema IItem that uses a relation
        field::
        
          >>> from z3c.relationfield import Relation
          >>> from zope.interface import Interface
          >>> class IItem(Interface):
          ...   rel = Relation(title=u"Relation")
        
        We also define a class ``Item`` that implements both ``IItem``
        and the special ``z3c.relationfield.interfaces.IHasRelations``
        interface::
        
          >>> from z3c.relationfield.interfaces import IHasRelations
          >>> from persistent import Persistent
          >>> from zope.interface import implements
          >>> class Item(Persistent):
          ...   implements(IItem, IHasRelations)
          ...   def __init__(self):
          ...     self.rel = None
        
        The ``IHasRelations`` marker interface is needed to let the relations
        on ``Item`` be cataloged (when they are put in a container and removed
        from it, for instance). It is in fact a combination of
        ``IHasIncomingRelations`` and ``IHasOutgoingRelations``, which is fine
        as we want items to support both.
        
        Finally we need a test application::
        
          >>> from zope.app.component.site import SiteManagerContainer
          >>> from zope.app.container.btree import BTreeContainer
          >>> class TestApp(SiteManagerContainer, BTreeContainer):
          ...   pass
        
        We set up the test application::
        
          >>> root = getRootFolder()['root'] = TestApp()
        
        We make sure that this is the current site, so we can look up local
        utilities in it and so on. Normally this is done automatically by
        Zope's traversal mechanism::
        
          >>> from zope.app.component.site import LocalSiteManager
          >>> root.setSiteManager(LocalSiteManager(root))
          >>> from zope.app.component.hooks import setSite
          >>> setSite(root)
        
        For this site to work with ``z3c.relationship``, we need to set up two
        utilities. Firstly, an ``IIntIds`` that tracks unique ids for objects
        in the ZODB::
        
          >>> from zope.app.intid import IntIds
          >>> from zope.app.intid.interfaces import IIntIds
          >>> root['intids'] = intids = IntIds()
          >>> sm = root.getSiteManager()
          >>> sm.registerUtility(intids, provided=IIntIds)
        
        And secondly a relation catalog that actually indexes the relations::
        
          >>> from z3c.relationfield import RelationCatalog
          >>> from zc.relation.interfaces import ICatalog
          >>> root['catalog'] = catalog = RelationCatalog()
          >>> sm.registerUtility(catalog, provided=ICatalog)
        
        Using the relation field
        ========================
        
        We'll add an item ``a`` to our application::
        
          >>> root['a'] = Item()
        
        All items, including the one we just created, should have unique int
        ids as this is required to link to them::
        
          >>> from zope import component
          >>> from zope.app.intid.interfaces import IIntIds
          >>> intids = component.getUtility(IIntIds)
          >>> a_id = intids.getId(root['a'])
          >>> a_id >= 0
          True
        
        The relation is currently ``None``::
        
          >>> root['a'].rel is None
          True
        
        Now we can create an item ``b`` that links to item ``a`` (through its
        int id)::
        
          >>> from z3c.relationfield import RelationValue
          >>> b = Item()
          >>> b.rel = RelationValue(a_id)
        
        We now store the ``b`` object in a container, which will also set up
        its relation (as an ``IObjectAddedEvent`` will be fired)::
        
          >>> root['b'] = b
        
        Let's examine the relation. First we'll check which attribute of the
        pointing object ('b') this relation is pointing from::
        
          >>> root['b'].rel.from_attribute
          'rel'
        
        We can ask for the object it is pointing at::
        
          >>> to_object = root['b'].rel.to_object
          >>> to_object.__name__
          u'a'
        
        We can also get the object that is doing the pointing; since we
        supplied the ``IHasRelations`` interface, the event system took care
        of setting this::
        
          >>> from_object = root['b'].rel.from_object
          >>> from_object.__name__
          u'b'
        
        This object is also known as the ``__parent__``; again the event
        sytem took care of setting this::
        
          >>> parent_object = root['b'].rel.__parent__
          >>> parent_object is from_object
          True
        
        The relation also knows about the interfaces of both the pointing object
        and the object that is being pointed at::
        
          >>> from pprint import pprint
          >>> pprint(sorted(root['b'].rel.from_interfaces))
          [<InterfaceClass zope.location.interfaces.IContained>,
           <InterfaceClass z3c.relationfield.interfaces.IHasRelations>,
           <InterfaceClass __builtin__.IItem>,
           <InterfaceClass persistent.interfaces.IPersistent>]
        
          >>> pprint(sorted(root['b'].rel.to_interfaces))
          [<InterfaceClass zope.location.interfaces.IContained>,
           <InterfaceClass z3c.relationfield.interfaces.IHasRelations>,
           <InterfaceClass __builtin__.IItem>,
           <InterfaceClass persistent.interfaces.IPersistent>]
        
        We can also get the interfaces in flattened form::
        
          >>> pprint(sorted(root['b'].rel.from_interfaces_flattened))
          [<InterfaceClass zope.location.interfaces.IContained>,
           <InterfaceClass z3c.relationfield.interfaces.IHasIncomingRelations>,
           <InterfaceClass z3c.relationfield.interfaces.IHasOutgoingRelations>,
           <InterfaceClass z3c.relationfield.interfaces.IHasRelations>,
           <InterfaceClass __builtin__.IItem>,
           <InterfaceClass zope.location.interfaces.ILocation>,
           <InterfaceClass persistent.interfaces.IPersistent>,
           <InterfaceClass zope.interface.Interface>]
        
          >>> pprint(sorted(root['b'].rel.to_interfaces_flattened))
          [<InterfaceClass zope.location.interfaces.IContained>,
           <InterfaceClass z3c.relationfield.interfaces.IHasIncomingRelations>,
           <InterfaceClass z3c.relationfield.interfaces.IHasOutgoingRelations>,
           <InterfaceClass z3c.relationfield.interfaces.IHasRelations>,
           <InterfaceClass __builtin__.IItem>,
           <InterfaceClass zope.location.interfaces.ILocation>,
           <InterfaceClass persistent.interfaces.IPersistent>,
           <InterfaceClass zope.interface.Interface>]
        
        Paths
        =====
        
        We can also obtain the path of the relation (both from where it is
        pointing as well as to where it is pointing). The path should be a
        human-readable reference to the object we are pointing at, suitable
        for serialization. In order to work with paths, we first need to set
        up an ``IObjectPath`` utility.
        
        Since in this example we only place objects into a single flat root
        container, the paths in this demonstration can be extremely simple:
        just the name of the object we point to. In more sophisticated
        applications a path would typically be a slash separated path, like
        ``/foo/bar``::
        
          >>> import grokcore.component as grok
          >>> from z3c.objpath.interfaces import IObjectPath
          >>> class ObjectPath(grok.GlobalUtility):
          ...   grok.provides(IObjectPath)
          ...   def path(self, obj):
          ...       return obj.__name__
          ...   def resolve(self, path):
          ...       try:
          ...           return root[path]
          ...       except KeyError:
          ...           raise ValueError("Cannot resolve path %s" % path)
        
          >>> grok.testing.grok_component('ObjectPath', ObjectPath)
          True
        
        After this, we can get the path of the object the relation points to::
        
          >>> root['b'].rel.to_path
          u'a'
        
        We can also get the path of the object that is doing the pointing::
        
          >>> root['b'].rel.from_path
          u'b'
        
        Comparing and sorting relations
        ===============================
        
        Let's create a bunch of ``RelationValue`` objects and compare them::
        
          >>> rel_to_a = RelationValue(a_id)
          >>> b_id = intids.getId(root['b'])
          >>> rel_to_b = RelationValue(b_id)
          >>> rel_to_a == rel_to_b
          False
        
        Relations of course are equal to themselves::
        
          >>> rel_to_a == rel_to_a
          True
        
        A relation that is stored is equal to a relation that isn't stored yet::
        
          >>> root['b'].rel == rel_to_a
          True
        
        We can also sort relations::
        
          >>> [(rel.from_path, rel.to_path) for rel in
          ...  sorted([root['b'].rel, rel_to_a, rel_to_b])]
          [('', u'a'), ('', u'b'), (u'b', u'a')]
        
        
        
        Relation queries
        ================
        
        Now that we have set up and indexed a relationship between ``a`` and
        ``b``, we can issue queries using the relation catalog. Let's first
        get the catalog::
        
          >>> from zc.relation.interfaces import ICatalog
          >>> catalog = component.getUtility(ICatalog)
        
        Let's ask the catalog about the relation from ``b`` to ``a``::
        
          >>> l = sorted(catalog.findRelations({'to_id': intids.getId(root['a'])}))
          >>> l
          [<z3c.relationfield.relation.RelationValue object at ...>]
        
        We look at this relation object again. We indeed go the right one::
        
          >>> rel = l[0]
          >>> rel.from_object.__name__
          u'b'
          >>> rel.to_object.__name__
          u'a'
          >>> rel.from_path
          u'b'
          >>> rel.to_path
          u'a'
        
        Asking for relations to ``b`` will result in an empty list, as no such
        relations have been set up::
        
          >>> sorted(catalog.findRelations({'to_id': intids.getId(root['b'])}))
          []
        
        We can also issue more specific queries, restricting it on the
        attribute used for the relation field and the interfaces provided by
        the related objects. Here we look for all relations between ``b`` and
        ``a`` that are stored in object attribute ``rel`` and are pointing
        from an object with interface ``IItem`` to another object with the
        interface ``IItem``::
        
          >>> sorted(catalog.findRelations({
          ...   'to_id': intids.getId(root['a']),
          ...   'from_attribute': 'rel',
          ...   'from_interfaces_flattened': IItem,
          ...   'to_interfaces_flattened': IItem}))
          [<z3c.relationfield.relation.RelationValue object at ...>]
        
        There are no relations stored for another attribute::
        
          >>> sorted(catalog.findRelations({
          ...   'to_id': intids.getId(root['a']),
          ...   'from_attribute': 'foo'}))
          []
        
        There are also no relations stored for a new interface we'll introduce
        here::
        
          >>> class IFoo(IItem):
          ...   pass
        
          >>> sorted(catalog.findRelations({
          ...   'to_id': intids.getId(root['a']),
          ...   'from_interfaces_flattened': IItem,
          ...   'to_interfaces_flattened': IFoo}))
          []
        
        Changing the relation
        =====================
        
        Let's create a new object ``c``::
        
          >>> root['c'] = Item()
          >>> c_id = intids.getId(root['c'])
        
        Nothing points to ``c`` yet::
        
          >>> sorted(catalog.findRelations({'to_id': c_id}))
          []
        
        We currently have a relation from ``b`` to ``a``::
        
          >>> sorted(catalog.findRelations({'to_id': intids.getId(root['a'])}))
          [<z3c.relationfield.relation.RelationValue object at ...>]
        
        We can change the relation to point at a new object ``c``::
        
          >>> root['b'].rel = RelationValue(c_id)
        
        We need to send an ``IObjectModifiedEvent`` to let the catalog know we
        have changed the relations::
        
          >>> from zope.event import notify
          >>> from zope.lifecycleevent import ObjectModifiedEvent
          >>> notify(ObjectModifiedEvent(root['b']))
        
        We should find now a single relation from ``b`` to ``c``::
        
          >>> sorted(catalog.findRelations({'to_id': c_id}))
          [<z3c.relationfield.relation.RelationValue object at ...>]
        
        The relation to ``a`` should now be gone::
        
          >>> sorted(catalog.findRelations({'to_id': intids.getId(root['a'])}))
          []
        
        Removing the relation
        =====================
        
        We have a relation from ``b`` to ``c`` right now::
        
          >>> sorted(catalog.findRelations({'to_id': c_id}))
          [<z3c.relationfield.relation.RelationValue object at ...>]
        
        We can clean up an existing relation from ``b`` to ``c`` by setting it
        to ``None``::
        
          >>> root['b'].rel = None
        
        We need to send an ``IObjectModifiedEvent`` to let the catalog know we
        have changed the relations::
        
          >>> notify(ObjectModifiedEvent(root['b']))
        
        Setting the relation on ``b`` to ``None`` should remove that relation
        from the relation catalog, so we shouldn't be able to find it anymore::
        
          >>> sorted(catalog.findRelations({'to_id': intids.getId(root['c'])}))
          []
        
        Let's reestablish the removed relation::
        
          >>> root['b'].rel = RelationValue(c_id)
          >>> notify(ObjectModifiedEvent(root['b']))
        
          >>> sorted(catalog.findRelations({'to_id': c_id}))
          [<z3c.relationfield.relation.RelationValue object at ...>]
        
        Copying an object with relations
        ================================
        
        Let's copy an object with relations::
        
          >>> from zope.copypastemove.interfaces import IObjectCopier
          >>> IObjectCopier(root['b']).copyTo(root)
          u'b-2'
          >>> u'b-2' in root
          True
        
        Two relations to ``c`` can now be found, one from the original, and
        the other from the copy::
        
          >>> l = sorted(catalog.findRelations({'to_id': c_id}))
          >>> len(l)
          2
          >>> l[0].from_path
          u'b'
          >>> l[1].from_path
          u'b-2'
        
        Relations are sortable
        ======================
        
        Relations are sorted by default on a combination of the relation name,
        the path of the object the relation is one and the path of the object
        the relation is pointing to.
        
        Let's query all relations availble right now and sort them::
        
          >>> l = sorted(catalog.findRelations())
          >>> len(l)
          2
          >>> l[0].from_attribute
          'rel'
          >>> l[1].from_attribute
          'rel'
          >>> l[0].from_path
          u'b'
          >>> l[1].from_path
          u'b-2'
        
        Removing an object with relations
        =================================
        
        We will remove ``b-2`` again. Its relation should automatically be removed
        from the catalog::
        
          >>> del root['b-2']
          >>> l = sorted(catalog.findRelations({'to_id': c_id}))
          >>> len(l)
          1
          >>> l[0].from_path
          u'b'
        
        Breaking a relation
        ===================
        
        We have a relation from ``b`` to ``c`` right now::
        
          >>> sorted(catalog.findRelations({'to_id': c_id}))
          [<z3c.relationfield.relation.RelationValue object at ...>]
        
        We have no broken relations::
        
          >>> sorted(catalog.findRelations({'to_id': None}))
          []
        
        The relation isn't broken::
        
          >>> b.rel.isBroken()
          False
        
        We are now going to break this relation by removing ``c``::
        
          >>> del root['c']
        
        The relation is broken now::
        
          >>> b.rel.isBroken()
          True
        
        The original relation still has a ``to_path``::
        
          >>> b.rel.to_path
          u'c'
        
        It's broken however as there is no ``to_object``::
        
          >>> b.rel.to_object is None
          True
        
        The ``to_id`` is also gone::
        
          >>> b.rel.to_id is None
          True
        
        We cannot find the broken relation in the catalog this way as it's not
        pointing to ``c_id`` anymore::
        
          >>> sorted(catalog.findRelations({'to_id': c_id}))
          []
        
        We can however find it by searching for relations that have a
        ``to_id`` of ``None``::
        
          >>> sorted(catalog.findRelations({'to_id': None}))
          [<z3c.relationfield.relation.RelationValue object at ...>]
        
        A broken relation isn't equal to ``None`` (this was a bug)::
        
          >>> b.rel == None
          False
        
        RelationChoice
        ==============
        
        A ``RelationChoice`` field is much like an ordinary ``Relation`` field
        but can be used to render a special widget that allows a choice of
        selections.
        
        We will first demonstrate a ``RelationChoice`` field has the same effect
        as a ``Relation`` field itself::
        
          >>> from z3c.relationfield import RelationChoice
          >>> class IChoiceItem(Interface):
          ...   rel = RelationChoice(title=u"Relation", values=[])
          >>> class ChoiceItem(Persistent):
          ...   implements(IChoiceItem, IHasRelations)
          ...   def __init__(self):
          ...     self.rel = None
        
        Let's create an object to point the relation to::
        
          >>> root['some_object'] = Item()
          >>> some_object_id = intids.getId(root['some_object'])
        
        And let's establish the relation::
        
          >>> choice_item = ChoiceItem()
          >>> choice_item.rel = RelationValue(some_object_id)
          >>> root['choice_item'] = choice_item
        
        We can query for this relation now::
        
          >>> l = sorted(catalog.findRelations({'to_id': some_object_id}))
          >>> l
          [<z3c.relationfield.relation.RelationValue object at ...>]
        
        RelationList
        ============
        
        Let's now experiment with the ``RelationList`` field which can be used
        to maintain a list of relations::
        
          >>> from z3c.relationfield import RelationList
          >>> class IMultiItem(Interface):
          ...   rel = RelationList(title=u"Relation")
        
        We also define a class ``MultiItem`` that implements both
        ``IMultiItem`` and the special
        ``z3c.relationfield.interfaces.IHasRelations`` interface::
        
          >>> class MultiItem(Persistent):
          ...   implements(IMultiItem, IHasRelations)
          ...   def __init__(self):
          ...     self.rel = None
        
        We set up a few object we can then create relations between::
        
          >>> root['multi1'] = MultiItem()
          >>> root['multi2'] = MultiItem()
          >>> root['multi3'] = MultiItem()
        
        Let's create a relation from ``multi1`` to both ``multi2`` and
        ``multi3``::
        
          >>> multi1_id = intids.getId(root['multi1'])
          >>> multi2_id = intids.getId(root['multi2'])
          >>> multi3_id = intids.getId(root['multi3'])
        
          >>> root['multi1'].rel = [RelationValue(multi2_id),
          ...                       RelationValue(multi3_id)]
        
        We need to notify that we modified the object
        
          >>> notify(ObjectModifiedEvent(root['multi1']))
        
        Now that this is set up, let's verify whether we can find the
        proper relations in in the catalog::
        
          >>> len(list(catalog.findRelations({'to_id': multi2_id})))
          1
          >>> len(list(catalog.findRelations({'to_id': multi3_id})))
          1
          >>> len(list(catalog.findRelations({'from_id': multi1_id})))
          2
        
        Temporary relations
        ===================
        
        If we have an import procedure where we import relations from some
        external source such as an XML file, it may be that we read a relation
        that points to an object that does not yet exist as it is yet to be
        imported. We provide a special ``TemporaryRelationValue`` for this
        case.  A ``TemporaryRelationValue`` just contains the path of what it
        is pointing to, but does not resolve it yet. Let's use
        ``TemporaryRelationValue`` in a new object, creating a relation to
        ``a``::
        
          >>> from z3c.relationfield import TemporaryRelationValue
          >>> root['d'] = Item()
          >>> root['d'].rel = TemporaryRelationValue('a')
        
        A modification event does not actually get this relation cataloged::
        
          >>> before = sorted(catalog.findRelations({'to_id': a_id}))
          >>> notify(ObjectModifiedEvent(root['d']))
          >>> after = sorted(catalog.findRelations({'to_id': a_id}))
          >>> len(before) == len(after)
          True
        
        We will now convert all temporary relations on ``d`` to real ones::
        
          >>> from z3c.relationfield import realize_relations
          >>> realize_relations(root['d'])
          >>> notify(ObjectModifiedEvent(root['d']))
        
        We can see the real relation object now::
        
          >>> root['d'].rel
          <z3c.relationfield.relation.RelationValue object at ...>
        
        The relation will also now show up in the catalog::
        
          >>> after2 = sorted(catalog.findRelations({'to_id': a_id}))
          >>> len(after2) > len(before)
          True
        
        Temporary relation values also work with ``RelationList`` objects::
        
          >>> root['multi_temp'] = MultiItem()
          >>> root['multi_temp'].rel = [TemporaryRelationValue('a')]
        
        Let's convert this to a real relation::
        
          >>> realize_relations(root['multi_temp'])
          >>> notify(ObjectModifiedEvent(root['multi_temp']))
        
        Again we can see the real relation object when we look at it::
        
          >>> root['multi_temp'].rel
          [<z3c.relationfield.relation.RelationValue object at ...>]
        
        And we will now see this new relation appear in the catalog::
        
          >>> after3 = sorted(catalog.findRelations({'to_id': a_id}))
          >>> len(after3) > len(after2)
          True
        
        Broken temporary relations
        ==========================
        
        Let's create another temporary relation, this time a broken one that
        cannot be resolved::
        
          >>> root['e'] = Item()
          >>> root['e'].rel = TemporaryRelationValue('nonexistent')
        
        Let's try realizing this relation::
        
          >>> realize_relations(root['e'])
        
        We end up with a broken relation::
        
          >>> root['e'].rel.isBroken()
          True
        
        It's pointing to the nonexistent path::
        
          >>> root['e'].rel.to_path
          'nonexistent'
        
        CHANGES
        *******
        
        0.6.2 (2012-12-06)
        ==================
        
        * Updated test setup and test to run with current versions of dependent
          packages, thus running with Python 2.6, too.
        
        * Added missing (test) dependencies.
        
        * Rename __neq__ method to __ne__ since __neq__ is not the right builtin
          name for != handlers.
        
        
        0.6.1 (2009-10-11)
        ==================
        
        * Fixes broken release.
        
        0.6 (2009-10-11)
        ================
        
        * Ensure that the value_type of a RelationList is not overwritten to be 'None'
          when the field is constructed.
        
        0.5 (2009-06-30)
        ================
        
        * Move lxml and schema2xml dependencies to an [xml] extra so that people can
          use this package without having to install lxml, which still causes issues
          on some platforms. If z3c.schema2xml and lxml are not importable, the
          relevant adapters will not be defined, but everything else will still work.
        
        * Subscribe to IIntIdAddedEvent instead of IObjectAddedEvent to prevent
          errors due to subscriber ordering.
        
        
        0.4.3 (2009-06-04)
        ==================
        
        * Add missing dependency for lxml.
        
        
        0.4.2 (2009-04-22)
        ==================
        
        * Prevent the event failures from failing when utilities are missing or when
          objects do not implement IContained.
        
        
        0.4.1 (2009-02-12)
        ==================
        
        * Don't handle ``IObjectModified`` events for objects that do not yet
          have a parent. There is no need to do so anyway, as these objects cannot
          have outgoing relations indexed.
        
        0.4 (2009-02-10)
        ================
        
        * Introduce a ``RelationChoice`` field that behaves like
          ``schema.Choice`` but tracks relations. In combination with a source
          (such as created by ``RelationSourceFactory`` provided by
          ``z3c.relationfieldui``) this can be used to create drop-down
          selections for relations.
        
        * Clarify the way comparing and sorting of ``RelationValue`` objects is
          done in order to better support choice support.
        
        0.3.2 (2009-01-21)
        ==================
        
        * When a relation is broken, properly re-catalog things.
        
        0.3.1 (2009-01-20)
        ==================
        
        * Introduce sensible sort order for relations, based on a
          ``(from_attribute, from_path, to_path)`` tuple.
        
        * Relations will now never compare to ``None``.
        
        0.3 (2009-01-19)
        ================
        
        * Introduce two new interfaces: ``IHasOutgoingRelations`` and
          ``IHasIncomingRelations``. ``IHasOutgoingRelations`` should be provided
          by objects that actually have relations set on them, so that
          they can be properly cataloged. ``IHasIncomingRelations`` should be
          set on objects that can be related to, so that broken relations
          can be properly tracked. ``IHasRelations`` now extends both,
          so if you provide those on your object you have an object that can
          have both outgoing as well as incoming relations.
        
        * Improve broken relations support. When you now break a relation (by
          removing the relation target), ``to_id`` and ``to_object`` become
          ``None``. ``to_path`` however will remain the path that the relation
          last pointed to. ``TemporaryRelation`` objects that when realized
          are broken relations can also be created.
        
          You can also for broken status by calling ``isBroken`` on a
          relation.
        
        * The signature of the top-level function ``create_relation``
          changed. It used to take the object to which the relation was to be
          created, but should now get the path (in ``IObjectPath`` terms).
          ``create_relation`` will now create a broken relation object if the
          path cannot be resolved.
        
        0.2 (2009-01-08)
        ================
        
        * Added support for ``RelationList`` fields. This allows one to
          maintain a list of ``RelationValue`` objects that will be cataloged
          like the regular ``Relation`` fields.
        
        * Get rid of ``IRelationInfo`` adapter requirement. Just define a
          ``create_relation`` function that does the same work.
        
        * When looking for relations on an object be more tolerant if those
          cannot be found (just skip them) - this can happen when a schema is
          changed.
        
        0.1 (2008-12-05)
        ================
        
        * Initial public release.
        
        Download
        ********
        
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