The triples used so far have all had variables in both the subject and object. However, a string value may be used instead. Since the subject of an RDF arc is always a resource, the subject may be a resource URI. The object attribute may be a resource URI or a literal value. Using a literal as the object of a triple is a common way to filter out specific items. For instance, if items were identified with a type, the data could be filtered to include only items of a particular type.
Here is an example triple that we could use in the photos example:
<conditions> <content uri="?start"/> <member container="?start" child="?photo"/> <triple subject="?photo" predicate="http://purl.org/dc/elements/1.1/title" object="Canal"/> </conditions>
This new triple has a static value for the object attribute instead of a variable reference. You might be able to guess that it will cause a match for only one of the photos, the Canal photo. The others won't be able to match when evaluating the triple. To look closer, here is the data network after the member has been evaluated:
(?start = http://www.xulplanet.com/rdf/myphotos, ?photo = http://www.xulplanet.com/ndeakin/images/t/palace.jpg) (?start = http://www.xulplanet.com/rdf/myphotos, ?photo = http://www.xulplanet.com/ndeakin/images/t/canal.jpg) (?start = http://www.xulplanet.com/rdf/myphotos, ?photo = http://www.xulplanet.com/ndeakin/images/t/obselisk.jpg)
When evaluating the triple, the template builder will iterate over the three potential results found so far. For the first result, the value of the ?photo variable 'http://www.xulplanet.com/ndeakin/images/t/palace.jpg' will be used as the subject, the predicate will be 'http://purl.org/dc/elements/1.1/title', and the object will be 'Canal'. Since there are no variables to fill into the triple, the builder will just confirm that such an triple exists in the datasource. The triple doesn't exist for the first result since the title for the first photo is 'Palace from Above', so the potential result will be removed from the data. For the second result, such an arc does exist, so the result is kept. Like the first result, there is no arc for the third photo, so it is also removed.
The resulting data will look like the following:
(?start = http://www.xulplanet.com/rdf/myphotos, ?photo = http://www.xulplanet.com/ndeakin/images/t/canal.jpg)
Here is the example.
Naturally, we will need to add a binding for the ?title variable if we wish to display it, or we could just hardcode it since we know the value will be 'Canal'. Usually you won't be filtering on a title though, you would instead filter on a value that acts like a type.
There has to be at least one variable used in a triple in either the subject or the object attributes, or both. You can't have a triple with no variables. That's not an issue though, since it wouldn't be particularly useful. Also, you cannot use a variable in the predicate. Only the subject and object attributes may have variables. Another thing that you cannot have is a triple where both variables would be unknown as in the following:
<conditions> <content uri="?start"/> <member container="?start" child="?photo"/> <triple subject="?category" predicate="http://purl.org/dc/elements/1.1/title" object="?title"/> </conditions>
In this case when the template builder gets to the triple, neither the ?category nor the ?title variables can be filled in so the builder doesn't know what to generate. The builder won't generate any output in this situation.
The member tag must have variables for both the container and child attributes. It wouldn't be very common to filter for a specific child anyway.