Simple XML file and XSD

Question

I have following xml file:
<?xml version="1.0" encoding="UTF-8"?>
<data xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="C:\schema.xsd">
     <trans>
          <element>aaa</element>
          <element>bbb</element>
          <element>ccc</element>
     </trans>
</data>
and for this file I created schema (XSD) file:
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
     <xs:element name="trans">
          <xs:complexType>
               <xs:sequence>
                    <xs:element ref="element" maxOccurs="unbounded"/>
               </xs:sequence>
          </xs:complexType>
     </xs:element>
     <xs:element name="element">
          <xs:simpleType>
               <xs:restriction base="xs:string">
                    <xs:enumeration value="aaa"/>
                    <xs:enumeration value="bbb"/>
                    <xs:enumeration value="ccc"/>
               </xs:restriction>
          </xs:simpleType>
     </xs:element>
     <xs:element name="data">
          <xs:complexType>
               <xs:sequence>
                    <xs:element ref="trans"/>
               </xs:sequence>
          </xs:complexType>
     </xs:element>
</xs:schema>
My XML file is compatible with this schema file.
But when I remove element <data> from my XML file, like this:
<?xml version="1.0" encoding="UTF-8"?>
<trans xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="C:\schema.xsd">
     <element>aaa</element>
     <element>bbb</element>
     <element>ccc</element>
</trans>
then, that file is compatible with schema file as well and it is problem for me.
Element data is required in my XML file as like other elements.
How should look schema file with restriction which informs that element <data> is always required?
Is it possible to do?
regards,
Jarek

Answer 1

That is a good question and the answer can depend upon how complex your schema is. For the simple example you gave, it could be rewritten as
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
     <xs:element name="data" type="dataType"/>
     <xs:complexType name="dataType">
          <xs:sequence>
               <xs:element name="trans">
                    <xs:complexType>
                         <xs:sequence>
                              <xs:element name="element" maxOccurs="unbounded">
                                   <xs:simpleType>
                                        <xs:restriction base="xs:string">
                                             <xs:enumeration value="aaa"/>
                                             <xs:enumeration value="bbb"/>
                                             <xs:enumeration value="ccc"/>
                                        </xs:restriction>
                                   </xs:simpleType>
                              </xs:element>
                         </xs:sequence>
                    </xs:complexType>
               </xs:element>
          </xs:sequence>
     </xs:complexType>
</xs:schema>This has only one element in it that can be used as a root node in the corresponding XML. If the root node of the XML is anything but data, then validation will fail.
If your schema is too large or cumbersome to redo as that, then you could perform a simple check just to see what the root node is and create an error situation when it is not the data node. Another option could be to create a second schema that has all the nodes (or maybe just the root desired node of type xs:anyType) that is used for cardinality validation. I'm not a fan of using a second schema for validation purposes but it is an option.