XSD Tutorial - Part 3 of 5 - Extending Existing Types

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XSD Tutorial Parts

1. Elements and Attributes
2. Conventions and Recommendations
3. Extending Existing Types
4. Namespaces
5. Other Useful bits...

Introduction

It is often useful to be able to take the definition for an existing entity, and extend it to add more specific information. In most development languages, we would call this inheritance or sub classing. The same concepts also exist in the XSD standard. This allows us to take an existing type definition and extend it. We can also restrict an existing type (although this behavior has no real parallel in most development languages).

• Extending a ComplexType
• Restricting an Existing ComplexType
• Use of Extended/Restricted Types
• Extending Simple Types (Union, List, Restriction)

Extending an Existing ComplexType

It is possible to take an existing <xs:complexType> and extend it. Let's see how this may be useful with an example.

Looking at the AddressType that we defined earlier (in part 1), let's assume our company has now gone international and we need to capture country specific addresses. In this case, we need specific information for UK addresses (County and Postcode), and for US addresses (State and ZipCode).

So we can take our existing definition of address and extend it as follows:

<xs:complexType name="UKAddressType">
    <xs:complexContent>
        <xs:extension base="AddressType"> 
            <xs:sequence>
                <xs:element name="County" type="xs:string"/>
                <xs:element name="Postcode" type="xs:string"/>
            </xs:sequence>
        </xs:extension>
    </xs:complexContent>
</xs:complexType>

<xs:complexType name="USAddressType">
    <xs:complexContent>
        <xs:extension base="AddressType"> 
            <xs:sequence>
                <xs:element name="State" type="xs:string"/>
                <xs:element name="Zipcode" type="xs:string"/>
            </xs:sequence>
        </xs:extension>
    </xs:complexContent>
</xs:complexType> 

This is clearer when viewed graphically. But basically, it is saying - we are defining a new <xs:complexType> called "USAddressType", this extends the existing type "AddressType", and adds to it a sequence containing the elements "State", and "Zipcode".

There are 2 new things here the <xs:extension> element and the <xs:complexContent> element; we'll get to these shortly.

We can now use these new types as follows:

<xs:element name="UKAddress" type="UKAddressType"/>
<xs:element name="USAddress" type="USAddressType"/>

Some sample XML for these elements may look like this:

<UKAddress>
    <Line1>34 thingy street</Line1>
    <Line2>someplace</Line2>
    <County>somerset</County>

We are defining a new type "InternalAddressType". The <xs:restriction> element says we are restricting the existing type "AddressType" , and we are only allowing the existing child element "Line1" to be used in this new definition.

Note: Because we are restricting an existing type, the only definitions that can appear in the <xs:restriction> are a subset of the ones defined in the base type "AddressType". They must also be enclosed in the same compositor (in this case, a sequence) and appear in the same order.

We can now use this new type as follows:

<xs:element name="InternalAddress" type="InternalAddressType"/>

Some sample XML for this element may look like this:

<InternalAddressType>
    <Line1>Desk 4, Second Floor/<Line1>
</InternalAddressType>

Note: The <xs:complexContent> element is just a container for the extension or restriction - we can largely ignore it for now.

Use of Extended/Restricted Types

We have just shown how we can create new types based on existing one. This in itself is pretty useful, and will potentially reduce the amount of complexity in your schemas, making them easier to maintain and understand. However, there is an aspect to this that has not yet been covered. In the above examples, we created 3 new types (UKAddressType, USAddressType and InternalAddressType), all based on AddressType.

So, if we have an element that specifies it is of type UKAddressType, then that is what must appear in the XML document. But if an element specifies it is of type "AddressType", then any of the 4 types can appear in the XML document (UKAddressType, USAddressType, InternalAddressType or AddressType).

The thing to consider now is, how will the XML parser know which type you meant to use, surely it needs to know otherwise it cannot do proper validation?

Well, it knows because if you want to use a type other than the one explicitly specified in the schema (in this case AddressType) then you have to let the parser know which type you are using. This is done in the XML document using the xsi:type attribute.

Let's look at an example.

<xs:element name="Person">
  <xs:complexType>
    <xs:sequence>
      <xs:element name="Name" type="xs:string" />
      <xs:element name="HomeAddress" type="AddressType" />
    </xs:sequence>
  </xs:complexType>
</xs:element>

This sample XML is the kind of thing you would expect to see.

<?xml version="1.0"?>
<Person>
    <Name>Fred</Name>
    <HomeAddress>
        <Line1>22 whatever place, someplace</Line1>
        <Line2>sometown, ss1 6gy </Line2>
    </HomeAddress>
</Person>

But the following is also valid.

<?xml version="1.0"?>
<Person xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <Name>Fred</Name>
    <HomeAddress xsi:type="USAddressType">
       <Line1>234 Lancaseter Av</Line1>
       <Line2>SmallsVille</Line2>
       <State>Florida</State>
       <Zipcode>34543</Zipcode>
    </HomeAddress>
</Person>

Let's look at that in more detail.

• We have added the attribute xsi:type="USAddressType" to the "HomeAddress" element. This tells the XML parser that the element actually contains data described by "USAddressType".
• The xmlns:xsi attribute in the root element (Person) tells the XML parser that the alias xsi maps to the namespace "http://www.w3.org/2001/XMLSchema-instance".
• The xsi: part of the xsi:type attribute is a namespace qualifier. It basically says the attribute "type" is from the namespace aliased by "xsi" which was defined earlier to mean "http://www.w3.org/2001/XMLSchema-instance".
• The "type" attribute in this namespace is an instruction to the XML Parser to tell it which definition to use to validate the element.

But more about namespaces in the next section.

Extending Simple Types

There are 3 ways in which a simpleType can be extended; Restriction, List or Union. The most common is Restriction, but we will cover the other 2 as well.

Restriction

Restriction is a way to constrain an existing type definition. We can apply a restriction to the built in data types xs:string, xs:integer, xs:date, etc. or ones we create ourselves.

Here, we are defining a restriction the existing type "string", and applying a regular expression to it to limit the values it can take.

<xs:simpleType name="LetterType">
    <xs:restriction base="xs:string">
        <xs:pattern value="[a-zA-Z]"/>
    </xs:restriction>
</xs:simpleType>

Shown graphically in Liquid XML Studio as follows:

Let's go through this line by line:

1. A <simpleType> tag is used to define our new type, we must give the type a unique name - in this case, "LetterType"
2. We are restricting an existing type - so the tag is <restriction> (you can also extend an existing type - but more about this later). We are basing our new type on a string so type="xs:string"
3. We are applying a restriction in the form of a Regular expression, this is specified using the <pattern> element. The regular expression means the data must contain a single lower or upper case letter a through to z.
4. Closing tag for the restriction
5. Closing tag for the simple type

Restrictions may also be referred to as "Facets". For a complete list, see the XSD Standard, but to give you an idea, here are a few to get you started.

<xs:enumeration value="Hippo"/><br />
			<xs:enumeration value="Zebra"/><br />
			<xs:enumeration value="Lion"/>

It is important to note that not all facets are valid for all data types - for example, maxInclusive has no meaning when applied to a string. For the combinations of facets that are valid for a given data type, refer to the XSD standard.

Union

A union is a mechanism for combining 2 or more different data types into one.

The following defines 2 simple types "SizeByNumberType" all the positive integers up to 21 (e.g. 10, 12, 14), and "SizeByStringNameType" the values small, medium and large.

<xs:simpleType name="SizeByNumberType"> 
    <xs:restriction base="xs:positiveInteger"> 
        <xs:maxInclusive value="21"/> 
    </xs:restriction> 
</xs:simpleType> 

<xs:simpleType name="SizeByStringNameType"> 
    <xs:restriction base="xs:string"> 
        <xs:enumeration value="small"/> 
        <xs:enumeration value="medium"/> 
        <xs:enumeration value="large"/> 
    </xs:restriction> 
</xs:simpleType>

We can then define a new type called "USClothingSizeType", we define this as a union of the types "SizeByNumberType" and "SizeByStringNameType" (although we can add any number of types, including the built in types - separated by whitespace).

<xs:simpleType name="USClothingSizeType">
    <xs:union memberTypes="SizeByNumberType SizeByStringNameType" />
</xs:simpleType>

This means the type can contain any of the values that the 2 members can take (e.g. 1, 2, 3, ...., 20, 21, small, medium, large). This new type can then be used in the same way as any other <xs:simpleType>

List

A list allows the value (in the XML document) to contain a number of valid values separated by whitespace.

A List is constructed in a similar way to a Union. The difference being that we can only specify a single type. This new type can contain a list of values that are defined by the itemType property. The values must be whitespace separated. So, a valid value for this type would be "5 9 21".

<xs:simpleType name="SizesinStockType">
    <xs:list itemType="SizeByNumberType" />
</xs:simpleType>