XSD: Restricting a substitutionGroup - xsd

Question

It seems like it should be possible to restrict a substitution group to a single element in that group. Due to the handling of substitution groups and elements during validation, however, the intuition only applies to certain cardinalities.
For example, the following schema is not valid:
<xs:element name="representedOrganization" type="Organization"/>
<xs:element name="scopingOrganization" type="Organization" substitutionGroup="representedOrganization"/>
<xs:complexType name="Test.Parent">
<xs:sequence>
<xs:element ref="representedOrganization" minOccurs="0" maxOccurs="1"/>
</xs:sequence>
</xs:complexType>
<xs:complexType name="Test.Child">
<xs:complexContent>
<xs:restriction base="Test.Parent">
<xs:sequence>
<xs:element name="scopingOrganization" type="Organization" minOccurs="0" maxOccurs="1"/>
</xs:sequence>
</xs:restriction>
</xs:complexContent>
</xs:complexType>
However, a small change to the cardinality (i.e. minOccurs) of the restriction is valid:
<xs:element name="representedOrganization" type="Organization"/>
<xs:element name="scopingOrganization" type="Organization" substitutionGroup="representedOrganization"/>
<xs:complexType name="Test.Parent">
<xs:sequence>
<xs:element ref="representedOrganization" minOccurs="0" maxOccurs="1"/>
</xs:sequence>
</xs:complexType>
<xs:complexType name="Test.Child">
<xs:complexContent>
<xs:restriction base="Test.Parent">
<xs:sequence>
<xs:element name="scopingOrganization" type="Organization" minOccurs="1" maxOccurs="1"/>
</xs:sequence>
</xs:restriction>
</xs:complexContent>
</xs:complexType>
This does not behave intuitively... but why?

Answer 1

This is a vicious interaction between the handling in two locations.
Under Schema Component Constraint: Particle Valid (Restriction), the specification clarifies that:
2.1 Any top-level element declaration particle (in R or B) which is the {substitution group affiliation} of one or more other element declarations and whose ·substitution group· contains at least one element declaration other than itself is treated as if it were a choice group whose {min occurs} and {max occurs} are those of the particle, and whose {particles} consists of one particle with {min occurs} and {max occurs} of 1 for each of the declarations in its ·substitution group·.
Under Particle Derivation OK (Elt:All/Choice/Sequence -- RecurseAsIfGroup), the specification clarifies that:
For an element declaration particle to be a ·valid restriction· of a group particle (all, choice or sequence) a group particle of the variety corresponding to B's, with {min occurs} and {max occurs} of 1 and with {particles} consisting of a single particle the same as the element declaration must be a ·valid restriction· of the group as defined by Particle Derivation OK (All:All,Sequence:Sequence -- Recurse) (§3.9.6), Particle Derivation OK (Choice:Choice -- RecurseLax) (§3.9.6) or Particle Derivation OK (All:All,Sequence:Sequence -- Recurse) (§3.9.6), depending on whether the group is all, choice or sequence.
This creates the perverse situation where the invalid schema (above) is converted into the following:
<xs:complexType name="Test.Parent">
<xs:sequence>
<xs:choice minOccurs="0" maxOccurs="1">
<xs:element name="representedOrganization" type="Organization.Organization" minOccurs="1" maxOccurs="1"/>
<xs:element name="scopingOrganization" type="Organization.Organization" minOccurs="1" maxOccurs="1"/>
</xs:choice>
</xs:sequence>
</xs:complexType>
<xs:complexType name="Test.Child">
<xs:complexContent>
<xs:restriction base="Test.Parent">
<xs:sequence>
<xs:choice minOccurs="1" maxOccurs="1">
<xs:element name="scopingOrganization" type="Organization.Organization" minOccurs="0" maxOccurs="1"/>
</xs:choice>
</xs:sequence>
</xs:restriction>
</xs:complexContent>
</xs:complexType>
This schema passes the choice-level cardinality check in the Particle Derivation OK (Choice:Choice -- RecurseLax). However, it fails the test for the (nested) particles because the cardinality of the restricted item is larger than the cardinality of the matching particle in the base type.
NOTE: Parsers may report this as an issue in Particle Derivation OK (Elt:All/Choice/Sequence -- RecurseAsIfGroup). This is the section that contains instructions to treat the element as a choice and run the nested choice test. For example, XMLSpy reports:
rcase-RecurseAsIfGroup: <element name="scopingOrganization" minOccurs="0"> is not a valid restriction of model group particle <element ref="representedOrganization" minOccurs="0">.
If you have control over the deriving schema, you could overcome this issue by wrapping the element in a choice and moving the cardinality to this level.