Allowing/Restricting XML Attributes based other Attributes

Question

Is it possible to create an XML schema with the following behavior? I have an XML file that looks like the following. I'd like to either expose or restrict attributes of Object based on the value of Type. For example, if Type="Bike" I might want to expose attributes related only to Bike (i.e. Pedals, Frame, etc). If the Type="Car", I might want to expose attributes that are only relevant to Car (i.e. Make, Model, Miles, etc).

<Objects>
  <Object Type="Bike" Pedals="XXX" Frame="XXX" />
  <Object Type="Car" Make="XXX" Model="XXX" Miles="XXX" />
</Objects>

Thanks in advance for any help. Let me know if you have any questions.

Answer 1

My understanding is that this is beyond what you can do with xsd. Besides, it makes querying a nightmare. The preferred approach (if it is possible) is to use:

<vehicles>
  <bike pedals="XXX" frame="XXX" />
  <car make="XXX" model="XXX" miles="XXX" />
</vehicles>

Where car and bike presumably have a common root vehicle in the xsd, but their own specific properties (on top of the inherited ones).

Answer 2

I think you are cramming too many things into attributes. Things like Pedals and Frame should probably be elements.

Answer 3

I agree with Marc that what you're trying to do is outside of the capabilities of W3C XML-Schema. I'd also agree that you're data model is just a bit overloaded/ambiguous to be useful. However, if you have no choice in the matter then perhaps either Schematron or Relax-NG could provide a workable solution.

Answer 4

Yes, you just have to use xsi:type instead of Type for XSD polymorphism (and in the schema, derive the types from a common type):

<Objects>
  <Object xsi:type="Bike" Pedals="XXX" Frame="XXX" />
  <Object xsi:type="Car" Make="XXX" Model="XXX" Miles="XXX" />
</Objects>

For details, see the XML Schema Part 0: Primer. It has an example schema for this, in which USAddress and UKAddress types are derived from Address. Further down is an example instance using xsi:type.