The CKAN software allows portal providers to include additional metadata fields in the metadata schema. When retrieving the metadata description of a dataset via the API, these keys are included in the resulting JSON under the key “extras”. However, it is not guaranteed that the DCAT conversion of the CKAN metadata contains these extra keys. Depending on the version and configuration of the DCAT export-extension there are three different cases:
1) Portal-specific mapping
The portal provider defines a mapping for certain CKAN fields to a specific RDF property.
For instance, data.gov.ie maps certain metadata keys to DCAT properties: the CKAN metadata description [json] for the dataset Modes of Travel in Dublin Region maps the keys theme-primary to the DCAT property dcat:theme (see the exported DCAT metadata [rdf]).
2) No mapping
Looking at the same dataset, we can see that there are other “extra” keys where no mapping to an RDF property exists, e.g., for the key collection-name. The metadata information will get lost if we only consider the exported DCAT.
3) Generic mapping by extension
Certain CKAN data portals map all available extra metadata keys by using the dct:relation (Dublin Core vocabulary) property. The key gets mapped to the rdfs:label property and the value to the rdf:value property, e.g., for the contact-email metadata key:
<http://example.com/example-dataset> dct:relation [ rdfs:label "contact-email" ; rdf:value "firstname.lastname@example.org" ] ;
This generic mapping can be found on various data portals, for instance at data.gov.uk: e.g., the DCAT export [rdf] for the Road Safety Data contains all extra keys by using the dct:relation property.
A more detailed report can be found here.