xml: using conditioned attributes - xsd

Question

I'm looking for a way to realize a scheme, where an element does have an attribute "topic" only when its enumeration has a specific value like in the upcoming example:
<article>
<type>Blogpost</type>
</article>
<article>
<type topic='news'>Article</type>
</article>
<article>
<type>Comment</type>
</article>
The challenge is to make it that the attribute will only be used when the value "Article" or "Documentary" is chosen. Furthermore I want only enumerations from predefined list to be valid. In case the right enumeration is chosen I want the attribute to be required in all other cases I don't want to have the attribute at all.
So far I tried the following schemes in order to do so but it doesn't work at all:
<xs:element name="type" minOccurs="0">
<xs:alternative test="#name='type'" type="type1"/>
<xs:alternative test="#name='type'" type="type2"/>
<xs:alternative type="xs:error"/>
</xs:element>
(...)
<xs:simpleType name="type1">
<xs:restriction base="xs:string">
<xs:enumeration value="Blogpost"/>
<xs:enumeration value="Comment"/>
<xs:enumeration value="Sentence"/>
</xs:restriction>
</xs:simpleType>
<xs:complexType name="type2">
<xs:simpleContent>
<xs:extension base="type1">
<xs:attribute name="topic" type="topics" use='required'>
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="Article"/>
<xs:enumeration value="Documentary"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:extension>
</xs:simpleContent>
</xs:complexType-->
I always get an error that any chosen element is 'not a valid value of "error"'. The other example I tried worked to make use the attribute however it allowed any text and didn't care about the enumeration restrictions making it basicly useless:
<xs:element name="type" minOccurs="0">
<xs:alternative test="#ItemType='type'" type="type1"/>
<xs:alternative test="#ItemType='type'" type="type2"/>
</xs:element>
(...)
How do I do it that the same element does only have the attribute when it is actually the right enumeration and still the restrictions to enumerations are valid? Is my approach to work with xs:alternative the right one in order to have a same element one time with and one time without an attribute?
I was not able to figure out right test conditions in order to work with xs:assert.

Answer 1

I now have a useful answer - sorry for the non-useful comments!
This question is almost identical yours: XSD 1.1 alternative test the contents of text()
. On that basis, I suggest that you try something like this (not tested, because I don't have a XSD1.1 processor)
<xs:element name="type" type="type2" minOccurs="0"/>
<xs:complexType name="type2">
<xs:simpleContent>
<xs:extension base="type1">
<xs:attribute name="topic" type="topics" use='required'/>
<xs:assert test="not ( #topic or ( #topic and ($value eq 'Article' or $value eq 'Documentary'))"/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
<xs:simpleType name="type1">
<xs:restriction base="xs:string">
<xs:enumeration value="Article"/>
<xs:enumeration value="Documentary"/>
<xs:enumeration value="Blogpost"/>
<xs:enumeration value="Comment"/>
<xs:enumeration value="Sentence"/>
</xs:restriction>
</xs:simpleType>```

Answer 2

Thank you again for your approach #kimbert.
With your help I was able to figure out the assertion that is fullfilling the demand. I just had to adjust your idea slightly:
<xs:element name="type" type="type2" minOccurs="0"/>
<xs:complexType name="type2">
<xs:simpleContent>
<xs:extension base="type1">
<xs:attribute name="topic" type="topics"/>
<xs:assert test="#topic and ($value eq 'Article' or $value eq 'Documentary') or not ($value eq 'Article' or $value eq 'Documentary')"/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
<xs:simpleType name="type1">
<xs:restriction base="xs:string">
<xs:enumeration value="Article"/>
<xs:enumeration value="Documentary"/>
<xs:enumeration value="Blogpost"/>
<xs:enumeration value="Comment"/>
<xs:enumeration value="Sentence"/>
</xs:restriction>
</xs:simpleType>
Like this the compiler now always demands the attribute "topic" in case the value "Article" or "Documentary" is asked for. In all other cases the attribute is ignored as it should be.