How to use both recusion and inheritance in XML Schema? - xsd

Question

I have a use case to define XSD schema to describe a structure that is similar to Files and Directories.
File is a complex type with multiple attributes.
Directory is a File, so it should be an extension of a File or in other words, inherited from a File.
Directory can contain more Files and sub Directories, so it should be defined recursively.
File could be an element.
Directory could be an root element.
Can anyone provide an example schema file?
For example, the following C++ code compiles/runs fine:
#include <iostream>
#include <vector>
using namespace std;
class FileClass {
public:
string name;
int type;
string path;
long long size;
virtual int open(){
};
virtual int close (){
};
virtual ~FileClass(){
};
};
class DirectoryClass: public FileClass {
public:
vector<FileClass*> dir_contents;
};
int main() {
cout<<"started"<<endl;
DirectoryClass root;
root.open();
FileClass* newFile = new FileClass();
root.dir_contents.push_back(newFile);
DirectoryClass* subDir = new DirectoryClass();
root.dir_contents.push_back(subDir);
root.close();
cout<<"finished"<<endl;
}
As you can see, the DirectoryClass contains only one vector of FileClass, and it's capable of storing sub dirs since DirectoryClass is derived from FileClass.

Answer 1

I'd agree with Tony, why derive Directory from File?
But if thats what your trying to model...
<?xml version="1.0" encoding="utf-8" ?>
<!--Created with Liquid XML Studio 2013 Designer Edition 11.0.0.0 (http://www.liquid-technologies.com)-->
<xs:schema elementFormDefault="qualified"
xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:complexType name="FileType">
<xs:attribute name="name"
type="xs:string"
use="required" />
<xs:attribute name="creationDate"
type="xs:dateTime"
use="required" />
</xs:complexType>
<xs:element name="File"
type="FileType" />
<xs:element name="Directory">
<xs:complexType>
<xs:complexContent>
<xs:extension base="FileType">
<xs:sequence>
<xs:element ref="File"
minOccurs="0"
maxOccurs="unbounded" />
<xs:element ref="Directory"
minOccurs="0"
maxOccurs="unbounded" />
</xs:sequence>
</xs:extension>
</xs:complexContent>
</xs:complexType>
</xs:element>
</xs:schema>
Then if you are then trying to read/write the structured XML data from C#, C++, Java etc you could look at XML Data Binding. This generates a set of strongly typed objects from your XSD.

Answer 2

That changes things a bit and compilcates the XML. There are 2 approaches ComplexType Inheritance or Substitution Groups. Complex Types are a better fit to the breif.
<?xml version="1.0" encoding="utf-8" ?>
<!--Created with Liquid XML Studio 2013 Designer Edition (Trial) 11.0.0.0 (http://www.liquid-technologies.com)-->
<xs:schema elementFormDefault="qualified"
xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:complexType name="FileType">
<xs:attribute name="name"
type="xs:string"
use="required" />
<xs:attribute name="creationDate"
type="xs:dateTime"
use="required" />
<xs:attribute name="type"
type="xs:int" />
<xs:attribute name="size"
type="xs:long" />
</xs:complexType>
<xs:complexType name="DirectoryType">
<xs:complexContent>
<xs:extension base="FileType">
<xs:sequence>
<xs:element name="Content"
type="FileType"
minOccurs="0"
maxOccurs="unbounded" />
</xs:sequence>
</xs:extension>
</xs:complexContent>
</xs:complexType>
<xs:element name="Directory"
type="DirectoryType" />
</xs:schema>
Sample XML
<?xml version="1.0" encoding="utf-8"?>
<!-- Created with Liquid XML Studio 2013 Designer Edition (Trial) 11.0.0.0 (http://www.liquid-technologies.com) -->
<Directory xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="FoldersCT.xsd"
name="string"
creationDate="1975-12-12T08:25:00.94"
type="393"
size="-5191">
<Content name="FileA1"
creationDate="1993-02-23T21:13:52.11"
type="4931"
size="-1984" />
<Content name="FileA2"
creationDate="1991-12-26T15:57:44.30"
type="-6155"
size="18" />
<Content name="FileA3"
creationDate="1979-01-05T21:36:13.80"
type="8362"
size="5579" />
<Content xsi:type="DirectoryType"
name="DirA4"
creationDate="1979-01-05T21:36:13.80"
type="8362"
size="5579">
<Content name="FileB1"
creationDate="1979-01-05T21:36:13.80"
type="8362"
size="5579" />
<Content name="FileB2"
creationDate="1979-01-05T21:36:13.80"
type="8362"
size="5579" />
</Content>
</Directory>
Note the use of the xsi:type="DirectoryType" attribute in the directories. This tells the XML Parser that the content is defined by DirectoryType. Without this its a file. This is what your spec says, but its pretty messy I would say.
Another solution is to use substitution groups, which personally I don't like as they are difficult to read in a schema. This effectivley this is the same as the orginal solution.
<?xml version="1.0" encoding="utf-8" ?>
<!--Created with Liquid XML Studio 2013 Designer Edition (Trial) 11.0.0.0 (http://www.liquid-technologies.com)-->
<xs:schema elementFormDefault="qualified"
xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:complexType name="FileType">
<xs:attribute name="name"
type="xs:string"
use="required" />
<xs:attribute name="creationDate"
type="xs:dateTime"
use="required" />
<xs:attribute name="type"
type="xs:int" />
<xs:attribute name="size"
type="xs:long" />
</xs:complexType>
<xs:element name="File"
type="FileType"
substitutionGroup="File" />
<xs:element name="Directory"
substitutionGroup="File">
<xs:complexType>
<xs:complexContent>
<xs:extension base="FileType">
<xs:sequence>
<xs:element ref="File"
minOccurs="0"
maxOccurs="unbounded" />
</xs:sequence>
</xs:extension>
</xs:complexContent>
</xs:complexType>
</xs:element>
</xs:schema>
Sample XML
<?xml version="1.0" encoding="utf-8"?>
<!-- Created with Liquid XML Studio 2013 Designer Edition (Trial) 11.0.0.0 (http://www.liquid-technologies.com) -->
<Directory xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="FoldersSG.xsd"
name="string"
creationDate="1992-01-13T17:58:08.38"
type="-2296"
size="-8752">
<File name="string"
creationDate="1982-11-03T17:57:40.96"
type="6813"
size="5278" />
<File name="string"
creationDate="1986-12-08T12:38:03.46"
type="4479"
size="8453" />
<Directory name="string"
creationDate="1992-01-13T17:58:08.38"
type="-2296"
size="8752">
<File name="string"
creationDate="1982-11-03T17:57:40.96"
type="6813"
size="5278" />
<File name="string"
creationDate="1986-12-08T12:38:03.46"
type="4479"
size="8453" />
</Directory>
</Directory>