z3c.relationfield 0.3.1

A relation field framework for Zope 3.

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.

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:

>>> sorted(root['b'].rel.from_interfaces)
[<InterfaceClass zope.app.container.interfaces.IContained>,
 <InterfaceClass z3c.relationfield.interfaces.IHasRelations>,
 <InterfaceClass __builtin__.IItem>,
 <InterfaceClass persistent.interfaces.IPersistent>]

>>> sorted(root['b'].rel.to_interfaces)
[<InterfaceClass zope.app.container.interfaces.IContained>,
 <InterfaceClass z3c.relationfield.interfaces.IHasRelations>,
 <InterfaceClass __builtin__.IItem>,
 <InterfaceClass persistent.interfaces.IPersistent>]

We can also get the interfaces in flattened form:

>>> sorted(root['b'].rel.from_interfaces_flattened)
[<InterfaceClass zope.app.container.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>]
>>> sorted(root['b'].rel.to_interfaces_flattened)
[<InterfaceClass zope.app.container.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'

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 ...>]

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

A broken relation isn’t equal to None (this was a bug):

>>> b.rel == None
False

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

0.3.1 (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.

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