Question

I have an XML Schema that mostly makes sense, but I encountered a bit that I don’t understand. Below is a snapshot of the schema:

<xs:complexType name="DATA-FIELD">
    <xs:sequence>
        <xs:group ref="FIELD-CONTENT" />
        <xs:element name="Field" type="FIELD" minOccurs="0" />
    </xs:sequence>
</xs:complexType>

<xs:complexType name="FIELD">
    <xs:group ref="FIELD-CONTENT" />
    <xs:attribute name="att" type="xs:string" use="required" />
</xs:complexType>

<xs:group name="FIELD-CONTENT">
    <xs:sequence>
        <xs:element name="Elem" type="ELEM" minOccurs="0" />
    </xs:sequence>
</xs:group>

Mostly what I don’t understand is how the group element is being used, and what a sample XML file that adheres to this schema would look like? Would it look like this:

<DataField>
    <Field att="test">
        <Elem>element</Elem>
    </Field>
</DataField>

OR

<DataField>
    <Elem>element</Elem>
</DataField>

Answer 1

The group element represents a fragment of a content model, and is largely used to represent common parts of multiple elements. In your example, it’s used to encapsulate the option Elem field in both the DATA-FIELD and FIELD types. In most cases, you can substitute the contents of the group element for the group references to get an idea of what the schema is accomplishing:

<xs:complexType name="DATA-FIELD">
    <xs:sequence>
        <xs:element name="Elem" type="xs:string" minOccurs="0" />
        <xs:element name="Field" type="FIELD" minOccurs="0" />
    </xs:sequence>
</xs:complexType>

<xs:complexType name="FIELD">
    <xs:element name="Elem" type="xs:string" minOccurs="0" />
    <xs:attribute name="att" type="xs:string" use="required" />
</xs:complexType>

In terms of validation, both the instance documents you specified are valid given the schema, since both elements are optional. In the first instance document the Elem field from the group is omitted, while in the second case the Field field is omitted. The following would also be legal documents given this schema:

<DataField>
    <Elem>foo</Elem>
    <Field att="test">
        <Elem>element</Elem>
    </Field>
</DataField>

And:

<DataField/>