Today we require that a <ReferentialConstraint> element contain the same number of <PropertyRef> elements, in the same order, as the <PropertyRef> elements of the <Key> of the related entity.

This is the only place where we rely on the ordering of the properties of the key.

We should instead add a TargetKey attribute to explicitly specify the key property of the related entity to which a <PropertyRef> within a <ReferentialConstraint> applies.