archetypes.schemaextender 1.5

Introduction

This package allows you to modify an Archetypes schema, using simple adapters. This can be used to add new fields, reorder fields and fieldsets or make other changes.

The most common use of schema extension is to allow add-on products to enhance standard Plone content types, for example by adding an option that can be set to toggle special behaviour.

schemaextender hooks into the Archetypes framework by registering an ISchema adapter for BaseContent and BaseFolder, making it responsible for providing the schema for all types derived from those classes. This includes all standard Plone content types. Since only one ISchema adapter can be active schemaextender provides its own mechanism to modify schemas using named adapters. Named adapters are allowing to register more than one schemaextender per adapted interface.

There are three types of adapters available:

• ISchemaExtender: using this adapter you can add new fields to a schema.

• IOrderableSchemaExtender: this adapters makes it possible to both add new fields and reorder fields. This is more costly than just adding new fields.

• IBrowserLayerAwareExtender: this adapters are making use of plone.browserlayer, so that the extender is only available if a layer is registered.

• ISchemaModifier: this is a low-level hook that allows direct manipulation of the schema. This can be very dangerous and should never be used if one does not know exactly what she/he is doing!

The adapter types are documented in the ‘’interfaces.py’’ file in archetypes.schemaextender.

Simple example

As an example we will add a simple boolean field to the standard Plone document type. First we need to create a field class:

from Products.Archetypes.public import BooleanField
from archetypes.schemaextender.field import ExtensionField

class MyBooleanField(ExtensionField, BooleanField):
    """A trivial field."""

schemaextender can not use the standard Archetypes fields directly since those rely on the class generation logic generating accessors and mutator methods. By using the ExtensionField mix-in class we can still use them. Make sure the ExtensionField mix-in comes first, so it properly overwrites the standard methods.

Next we have to create an adapter that will add this field:

from zope.component import adapts
from zope.interface import implements
from archetypes.schemaextender.interfaces import ISchemaExtender
from Products.Archetypes.public import BooleanWidget
from Products.ATContentTypes.interface import IATDocument

class PageExtender(object):
    adapts(IATDocument)
    implements(ISchemaExtender)


    fields = [
        MyBooleanField("super_power",
        widget = BooleanWidget(
            label="This page has super powers")),
            ]

    def __init__(self, context):
        self.context = context

    def getFields(self):
        return self.fields

Try to store the fields on the class, that way they aren’t created each time the getFields method gets called. Generally you should make sure getFields does as few things as possible, because it’s called very often.

The final step is registering this adapter with the Zope component architecture. Since we already declared the interface we provide and which type of object we adapt this can be done very quickly in configure.zcml (assuming you put the code above in a file extender.py):

<configure xmlns="http://namespaces.zope.org/zope"
           xmlns:five="http://namespaces.zope.org/five">

    <include package="archetypes.schemaextender" />
    <adapter factory=".extender.PageExtender" />
</configure>

Custom fields

If you want you can make more complicated field types as well. The only requirement is that you need to have ExtensionField as the first parent class for your field type. As an example here is a field that toggles a marker interface on an object:

from zope.interface import Interface
from zope.interface import alsoProvides
from zope.interface import noLongerProvides
from Products.Archetypes.public import BooleanField
from archetypes.schemaextender.field import ExtensionField

def addMarkerInterface(obj, *ifaces):
    for iface in ifaces:
        if not iface.providedBy(obj):
            alsoProvides(obj, iface)


def removeMarkerInterface(obj, *ifaces):
    for iface in ifaces:
        if iface.providedBy(obj):
            noLongerProvides(obj, iface)


class ISuperPower(Interface):
    """Marker interface for classes that can do amazing things."""


class InterfaceMarkerField(ExtensionField, BooleanField):
    def get(self, instance, **kwargs):
        return ISuperPower.providedBy(instance)

    def getRaw(self, instance, **kwargs):
        return ISuperPower.providedBy(instance)

    def set(self, instance, value, **kwargs):
        if value:
            addMarkerInterface(instance, ISuperPower)
        else:
            removeMarkerInterface(instance, ISuperPower)

Caching

The package provides an alternative ISchema adapter, which implements a simple caching strategy to avoid redundant construction of the schema. To be on the safe side, the adapter for now only caches individual schemata on the current request object. In other words, the cache works per object per request. This should avoid most potential invalidation problems.

However, as the caching is still experimental and also affects most test setups, including those of other packages, it is not enabled by default. Please use the ‘’caching.zcml’’ instead of the regular ‘’configure.zcml’’ to activate it. This can be accomplished either via zcml, e.g. from another package like:

<include package="archetypes.schemaextender" file="caching.zcml" />

or else via buildout like:

[instance]
eggs = archetypes.schemaextender
zcml = archetypes.schemaextender:caching.zcml

Please also see below for more information about this.

Changelog