XML schema may include constraints on the elements in the document to ensure that they have unique keys. In particular:

• Key Uniqueness : That a given identifier is unique within the document

• Referential integrity : that a given reference to another element is satisfied, that is, the referenced element is declared elsewhere in the document.

Referential integrity constraints are added in the root document (For NeTEx the file netexPublication.xsd.

Each entity in a NeTEx Xml document is a versioned instance, that is, the combination of its id and version attributes must be unique within the document .
A key constraint is specified such that:

• Most First class NeTEx entities are unique within their XML element class and version (e.g. LINE, SCHEDULED STOP POINT,
• Certain first class NeTEx Bentities are unique within their XML element superclass, (e.g. ADDRESS / POSTAL ADDRESS / ROAD ADDRESS ? TO DO ADD LIST]
• NeTEx Child entities are unique within their XML element class and order number.

By convention "any" can be used as a version id when no specifc version is intended,

Unique ids for versions of first class entities LINEs and SCHEDULED STOP POINTs.

  <Line id="X43A" version="any">  :::: </Line>
  <Line id="X22" version="any">  :::: </Line>

  <ScheduledStopStopPoint id="A" version="any">  :::: </ScheduledStopStopPoint>
  <ScheduledStopStopPoint id="B" version="any">  :::: </ScheduledStopStopPoint>
  <ScheduledStopStopPoint id="C" version="any">  :::: </ScheduledStopStopPoint>

Unique ids for versions of a SERVICE PATTERN and three child entity instances STOP POINTs IN PATTERN.

  <ServicePattern id="foo" version="any">
     <StopPointInPattern id="foo" version="any" order="1">   <ScheduledStopStopPointRef  ref="A" version="any"/>  </StopPointInPattern>
     <StopPointInPattern id="foo" version="any" order="2">  <ScheduledStopStopPointRef  ref="B" version="any"/>  </StopPointInPattern>
     <StopPointInPattern id="foo" version="any" order="3>   <ScheduledStopStopPointRef  ref="C" version="any"/>  </StopPointInPattern>
  <ServicePattern>

Unique ids for versions of a COUNTRY and a CODESPACE.

     <Country id="uk" province-UK-WLS">  :::: </Country>
     <Codespace id="epip_cs"  ">  :::: </Codespace>

NeTEx uses a uniform VersionRef mechanism to reference entities uniquely within a document. (Each reference can be considered as the serialisation of an association relationship from the representational model). In general, there is a referential integrity constraint for each different element class.

• Each reference has two keyattributes, a ref and a version.
• Each reference to a versioned child has key attributes, a ref and a version and an order.

There are two special cases

• Each reference to a CODESPACE has a single key attributes ref.
• Each reference to a COUNTRY has two key attributes ref and a province.

In consequence, referential integrity constraints are only validated if teh key is fully specified

• If the version attribute is** present** the reference will be validated
• If the version attribute is missing, the reference will note be validated

This makes it possible to decide on an individual basis whether references are internal (within the same NeTEx XML document) or external (in a separate document ),

The following example example shows a SERVICE LINK that references two different stop points of a specifed version.

   <scheduledStopPoints>
		<ScheduledStopPoint version="001"  id="mybus:SSP0001">
			<Name>Museum</Name> 
		</ScheduledStopPoint>

		<ScheduledStopPoint version="001"  id="mybus:SSP0002">
			<Name>Bahnhof</Name> 
		</ScheduledStopPoint>
   </scheduledStopPoints>
   <serviceLinks>
		<ServiceLink version="002"  id="mybus:SSP0001+SSP0002">
			<Distance>1.12</Distance>
			<FromPointRef version="001" ref="mybus:SSP0001"/>
			<ToPointRef version="001" ref="mybus:SSP000B"/>
		</ServiceLink>
   </serviceLinks>

The following example shows the same link using unvalidated references,

  <serviceLinks>
   	<ServiceLink version="002"  id="mybus:SSP0001+SSP0002">
   		<Distance>1.12</Distance>
   		<FromPointRef ref="mybus:SSP0001"/>
   		<ToPointRef   ref="mybus:SSP0002"/>
   	</ServiceLink>
  </serviceLinks>

Sometimes it is useful to indicate a version on an external reference. This can be done using the auxilliary ** versionRef** attribute. The following eample example shows the same link using unvalidated external references,

   <serviceLinks>
		<ServiceLink version="002"  id="mybus:SSP0001+SSP0002">
			<Distance>1.12</Distance>
			<FromPointRef  versionRef="001"  ref="mybus:SSP0001"/>
			<ToPointRef   versionRef="001" ref="mybus:SSP0002"/>
		</ServiceLink>
</serviceLinks>

It is possible to have two seperate versions of the same entity in the same NeTEx Xml document. For example, the following is allowed

  <ScheduledStopStopPoint id="A" version="001">  <:::: </ScheduledStopStopPoint>
  <ScheduledStopStopPoint id="A" version="002">  :::: </ScheduledStopStopPoint>