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RE: group or abstract type?

From: Patrick Lisser <patrick.lisser@canoo.com>
Date: Wed, 9 Jan 2002 04:52:10 -0500 (EST)
To: "Jeni Tennison" <jeni@jenitennison.com>
Cc: <xmlschema-dev@w3.org>
Message-ID: <MLEOJBDKOPLHJHPOOEIHCEHOCOAA.patrick.lisser@canoo.com>
Wow - great answer! Many aspects I never would have thought of!

At the moment I'm intending to use a model group, however I have
a question about an additional aspect: unique/key constraints.

My menu entries have additionally an attribute "menuId" of the
type xs:ID (which is referenced by other element's attributes
of the type xs:IDREF). Because the ID value must be unique in
the whole XML document and this is an unnecessarily strong
constraint (the menu id only needs to be unique within the
<MenuStructure>...</MenuStructure> subtree) I intend to change
the types to xs:string and to add unique or key constraints.
For example:

<xs:element MenuStructure ...>
  ...
  <xs:unique name="menuIdConstraint">
    <xs:selector xpath="AbstractMenuEntry"/> <!-- substitution group -->
    <xs:field xpath="@menuId"/>
  </xs:unique>
</xs:element>

<xs:element MenuStructure ...>
  ...
  <xs:unique name="menuIdConstraint">
    <xs:selector xpath="SomeMenu"/> <!-- model group -->
    <xs:field xpath="@menuId"/>
  </xs:unique>
</xs:element>

Now I wonder whether such constraints work better/easier/not-at-all
with model group and substitution group (I didn't try hard yet, 
but until now nothing worked. Have first to read more about these
unique/key constraints). I.e. should the constraints above work
even if there are some menus (e.g. <Spacer/>) which do not have
a menuId (I'd guess that it works with the unique constraint but
not with the key costraint)? Or would I have to specify a constraint
covering the concrete menu entries, which would be MenuEntry and
DynamicMenuEntry, directly?

thanks again for the valuable support!

pat



> -----Original Message-----
> From: Jeni Tennison [mailto:jeni@jenitennison.com]
> Sent: Tuesday, January 08, 2002 6:24 PM
> To: Patrick Lisser
> Cc: xmlschema-dev@w3.org
> Subject: Re: group or abstract type?
> 
> 
> Hi Patrick,
> 
> > So the <xs:choice .... </xs:choice> part is dublicated. To simplify
> > (and to express that it's really 2 times the same) I'd like to
> > consolidate it and I see two possibilities to do so:
> >
> > 1. I extract the part, add and use it as a group:
> [snip]
> > 2. Specify a abstract type and substitute it in the concrete menu
> > entries:
> [snip]
> > What are the advantages and disadvantages of both solutions? Are
> > there recommendations on what should be preferred under certain
> > circumstances? I could imagine that if part of the schema (i.e. the
> > group/abstract type) would be external and not modifiable for those
> > who wanted to add new concrete menu entries, the choice should be
> > the abstract type, however in my situation this is not the case.
> 
> Interesting question. Differences that leap out at me:
> 
> Model group control:
> When you use a substitution group, you get a very basic choice, with
> the min/max occurrence of the head element determining how many
> occurrences there can be of the other elements. On the other hand,
> with a xs:choice in a xs:group you can force different elements to be
> repeated different numbers of times or, if you need to later on,
> refine the model group to give tighter control over the content.
> 
> Namespaces:
> When you use a substitution group, all the elements have to be
> declared at the top level of the schema, as global elements. This
> means that they must all be in the target namespace for the schema. On
> the other hand, if you declare the elements within the xs:group, they
> are local elements and could be in no namespace (unqualified) instead.
> 
> Naming conflicts:
> In general, declaring global elements leads to the possibility of
> conflicting element declarations - if you had another element of the
> same name but with a different type, and this element also had to be
> declared at the top level of the schema (because it was the document
> element, or because it was a member of a substitution group too) then
> you would run into difficulties.
> 
> Multiple groups:
> An element can only belong to one substitution group, but (if it's
> declared as a global element) it can be in multiple xs:groups. So if
> you have the same issue with a group involving the same element
> elsewhere in the schema, it probably makes more sense to use xs:group
> throughout.
> 
> Extending into different namespaces:
> If someone were to take your schema and want to extend the choice
> group to include elements in their own namespace, the amount of effort
> they would have to go to is different in the two cases. To extend the
> xs:group, they'd have to create an 'adapter' schema in your namespace
> that redefined the xs:group, adding references to the elements in
> their namespace. On the other hand, with a substitution group they
> just need to declare their elements (at the top level) and point to
> the abstract substitution group head element, effectively extended the
> choice with their own elements. The same issues are involved with
> extensions in the same namespace, but the effort is slightly less
> marked.
> 
> Control over element types:
> Using a substitution group gives you some control over the content of
> the extra elements that other people might add, because the types of
> the elements that are members of a substitution group must be derived
> from the type of the head of the substitution group (your abstract
> element). On the other hand, people could put all sorts of elements in
> the xs:group if they redefined it.
> 
> 
> I think my conclusion is that if you're fairly happy that you're not
> going to need to change your basic schema (and assuming that it
> doesn't contain any conflicts), want to make it easy for people to
> extend the allowed elements, and especially if you want to control
> what kinds of elements they can extend with, go for a substitution
> group. Otherwise, use a model group.
> 
> Cheers,
> 
> Jeni
> 
> ---
> Jeni Tennison
> http://www.jenitennison.com/
> 
> 
Received on Wednesday, 9 January 2002 09:41:09 GMT

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