Using the Codeathon at Wikimania 2009, we have now implemented support for "inverse properties". This email gives an overview of the features (usr perspective) and internals (developer perspectives). Some SMW extensions will need to be adjusted to work with the new feature.
== Introduction ==
The inverse of a property simply is the property that points into the other
direction. For example, if "developer of" connects a person to a software
tool, then the inverse of "developer of" connects software tools to persons.
So far, it was very hard to use properties in other directions. In our
example, you could, e.g., have an ask query showing the all persons who
develop some open source tool, but you could not have a query that shows all
software tools that are developed by people living in Argentina. To do the
latter, you would have had to use a property "is developed by" that explicitly
connects software tools to their developers, and you would have had to add
data for that again. If you need both directions, you would have to maintain
both properties individually, which is not very convenient.
== Feature Details ==
SMW (SVN) now allows you to directly refer to the inverse of any property by
simply putting a "-" in front of its name. For example, "-developer of" gives
you access to what I called "is developed by" above. This can be used in any
place where property names occur, including browsing special pages, ask
queries, and output directives of queries. So the projects developed by people
from Argentina can be obtained with a query
{{#ask: [[-developer of:: <q>[[lives in::Argentina]]</q> ]] }}
or simply
{{#ask: [[-developer of.lives in::Argentina]] }}
We are aware that the "-" prefix is not the most readable way of solving the
problem. However, the solution we have now avoids a number of complications
that other solution would introduce (for example, you cannot have chains of
inverses, like Property:A inverse_of Property:B inverse_of Property:C
inverse_of ...; also, there is no relevant interplay of "subproperty of" and
inverses).
If applicable, inverse properties generally are linked to the page of the
corresponding property (so "-developer of" links to "Property:developer of").
It is strongly suggested not to create property pages that are called like
inverse properties (it won't destroy anything, but it might create unnecessary
confusion). Also, you cannot use inverse properties to enter semantic data
into the wiki: all annotations must be on the page that is the subject of the
non-inverted property.
Inverses in queries are currently only supported if they are of Type:Page.
== Development Information ==
If your code uses SMW properties that are generated from user inputs, then you
need to be aware of inverse properties to make sure everything works
correctly. Internally, an inverse property, like any property, is represented
by an SMWPropertyValue object. It will look like the property value object for
the non-inverse version of the property. To check if it actually is the
inverse, you can use the new method isInverse(). You can pass inverse
properties to the SMW store, and it will take care of everything.
The main changes that I expect to be required in SMW extensions is in places
where you deal with property subjects and silently expect them to be of
Type:Page. If users can enter inverse properties, then a query for a subject
can also return datavalues of other datatypes, so you need to use the generic
datavalue API, or check the type of the datavalue first.
Another place where changes might be needed is when passing on parameters as
strings. If you use getWikiValue() on an inverse property, then you will
obtain the property name with "-" in front, as expected. But if you use
"getShortWiikiText()" and other functions, you will get the name of the
property without any "-".
I hope that SMW already works properly in all cases where inverses can occur.
Feedback is welcome.
Cheers,
Markus
--
Markus Krötzsch <mar...@semantic-mediawiki.org>
* Personal page: http://korrekt.org
* Semantic MediaWiki: http://semantic-mediawiki.org
* Semantic Web textbook: http://semantic-web-book.org
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