[Zope-CMF] Extending FTI.isConstructionAllowed

[yuppie]

Hi Wichert!


Wichert Akkerman wrote:
> I have a use case where I need to put additional restrictions on object 
> creation, in particular I need to restrict the maximum depth of items 
> inside of a container of a specific type. The ideal place to put such a 
> restriction seems to be the isConstructionAllowed method on the FTI. 
> Currently this method is not very extensible, which leads to complicated 
> code in various FTI types.
> 
> I am considering to add an extension point here, something like this:
> 
> class ITypeConstructionFilter(Interface):
>      def __init__(fti, container):
>          """Adapt on the FTI of the object being created and the target 
> container"""
> 
>      def allowed():
>          """Check if construction is allowed."""
> 
> 
> current checks such as the workflow check that was added in CMF 2.2, or 
> the type constraint logic Plone has in ATContentTypes could be moved to 
> such an adapter. The standard isConstructionAllowed method could then 
> query all registered adapters to check if construction should be possible.
> 
> Does this sound sensible?

After (re)reading all the comments and having a closer look at the code 
I came to these conclusions:

1.) CMF 2.1 checks two different restrictions: allowType() and 
isConstructionAllowed(). PortalFolderBase._verifyObjectPaste just checks 
allowType() because in CMF 2.1 isConstructionAllowed() does basically 
the same permission check as CopyContainer._verifyObjectPaste. Changing 
isConstructionAllowed() without changing 
PortalFolderBase._verifyObjectPaste creates inconsistent behavior. The 
_checkWorkflowAllowed change and your branch are both broken.

2.) The distinction between allowType() and isConstructionAllowed() was 
clear in CMF 2.1: allowType() checked a cheap, not permission related 
CMF specific restriction. isConstructionAllowed() checked generic 
permission related restictions. The new restrictions 
_checkWorkflowAllowed and ITypeConstructionFilter don't fit in one of 
these two categories.

3.) I was wrong about comparing isConstructionAllowed with checkFactory 
and checkObject. These are used for checking general container 
constraints, not for checking user specific permissions. checkFactory 
doesn't work for CMF because it doesn't take the portal type as argument.


My conclusion:

allowType() and isConstructionAllowed() are both the wrong place for 
checking additional restrictions. But allowType() could become part of a 
more general precondition that could be checked by checkObject and a new 
checkPortalType (=CMF specific checkFactory) function.

Plone could use its own precondition that checks registered 
ITypeConstructionFilter adapters.


Cheers,

	Yuppie