Extra Constraints for XML Schemas
This Schematron schema is an example of using path expressions to express
the various constraints on W3C XML Schemas that cannot be expressed using
XML Schemas (i.e. in the "schema for schemas").
This schema is only a fraction of the constraints. "Representation
Constraints" can be fairly simply expressed: the number of terms in the
tests are only complex because the constraints themselves are complex.
"Component Constraints" are more difficult to express, because they may
rely on datatype or components in included or imported documents. In those
cases, the assertion (text) is given but no test is given.
Some constraints are further divided into subgroups. The diagnostics
element has been used to note these, however they could be separate patterns.
Some constraints are not testable, and so do not require assertions.
Schema Representation Constraint: Schema Document Location Strategy OK
Schema Representation Constraint: Annotation Definition Representation OK
Schema Representation Constraint: Attribute Declaration Representation OK
In addition to the conditions imposed on <attribute> element information items by the schema for schemas, all of the following must be true:
default and fixed must not both be present.
If default and use are both present, use must have the ·actual value· optional.
If the item's parent is not <schema>, then
one of ref or name must be present, but not both.
If the item's parent is not <schema>, then
if ref is present, then all of <simpleType>, form and type must be absent.
type and <simpleType> must not both be present.
Constraints on Attribute Declaration Schema Components
All attribute declarations must satisfy the following constraints.
The values of the properties of an attribute declaration must be as
described in the property tableau in The Attribute Declaration Schema
Component (§3.2.1), modulo the impact of Missing Sub-components (§5.3).
if there is a {value constraint}, the canonical
lexical representation of its value must be ·valid· with respect to
the {type definition} as defined in String Valid (§3.14.4).
If the {type definition} is or is derived from ID then there must not
be a {value constraint}.
The {name} of an attribute declaration must not match xmlns.
The {target namespace} of an attribute declaration, whether local
or top-level, must not match http://www.w3.org/2001/XMLSchema-instance
unless it is one of the four built-in declarations given in the next
section).
To Do
We want the constraint that only one attribute can be declared.
Schema Component Constraint: Attribute Declaration Properties Correct
Schema Component Constraint: xmlns Not Allowed
Schema Component Constraint: xsi: Not Allowed
Note: This reinforces the special status of these attributes,
so that they not only need not be declared to be allowed in instances,
but must not be declared. It also removes any temptation to experiment
with supplying global or fixed values for e.g. xsi:type or xsi:nil,
which would be seriously misleading, as they would have no effect.