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First, we will ammend the schema to support the property "optional" to datastream declarations. The JIRA issue for this is http://fedora-commons.org/jira/browse/FCREPO-531
The semantic meaning of optional is:
1. The datastream can exist in the substribing objects but does not have to
2. If the datastream exist, it must adhere to the specification in the content model
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<dsCompositeModel
xmlns="info:fedora/fedora-system:def/dsCompositeModel#">
<dsTypeModel ID="DC">
<form MIME="text/xml"/>
<extension name="SCHEMA">
</extensions>extension>
</dsTypeModel>
<dsTypeModel ID="ORIGIN">
<form MIME="text/xml"/>
<extension name="SCHEMA">
</extensions>extension>
</dsTypeModel>
</dsCompositeModel>
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<dsCompositeModel
xmlns="info:fedora/fedora-system:def/dsCompositeModel#"
xmlns:schema="http://ecm.sourceforge.net/types/dscompositeschema/0/1/#">
<!-- The DC datastream is declared. It's mime type must be text/xml. It must adhere to the xml schema residing in on the specified URL.
<dsTypeModel ID="DC">
<form MIME="text/xml"/>
<extension name="SCHEMA">
<reference type="url" value="http://www.openarchives.org/OAI/2.0/oai_dc.xsd"/>
</extensions>extension>
</dsTypeModel>
<!-- The PBCORE datastream is declared. It's mime type must be text/xml. It must adhere to the xml schema residing in the PDCORE_SCHEMA datastream in this content model-->
<dsTypeModel ID="PBCORE">
<form MIME="text/xml"/>
<extension name="SCHEMA">
<reference type="datastream" value="PDCORE_SCHEMA"/>
</extensions>extension>
</dsTypeModel>
<!-- The ORIGIN datastream is declared. It's mime type must be text/xml. It must adhere to the xml schema inlined here
<dsTypeModel ID="ORIGIN">
<form MIME="text/xml"/>
<extension name="SCHEMA">
<schema targetNamespace="originNamespace" xmlns="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" attributeFormDefault="unqualified">
<element name="origin" type="string"/>
</schema>
</extensions>extension>
</dsTypeModel>
</dsCompositeModel>
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Notice that in the import tag, the schema location is given as "$THIS$/SIMPLEDC-SCHEMA". This means that the schema location is the residing in the SIMPLEDC-SCHEMA datastream in this content model. This functionality only works in schemas, and only when validating with the built-in validate method. It works both for schemas inlined in DS-COMPOSITE-MODEL, and stored in separate datastreams. You can have multiple schemas imported, in a hirachical structure, but the $THIS$ will always refer to the object containing the DS-COMPOSITE-MODEL in question.
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