Moriarty DataTables: Active Record for RDF
DataTables are a new addition to the Moriarty PHP library. They are an implementation of the ActiveRecord pattern for use with RDF data in Talis Platform stores. It draws inspiration from the active record implementation in CodeIgniter.
The intention is to allow querying of RDF data in a natural way for most PHP coders. For example:
$dt->select('firstname')->from('person')->where('surname','Evans');
$dt->get();
In a relational database that kind of code would select the firstname column for every record in the person table that has a surname column with a value of Evans. With RDF we have two problems:
- there are no columns or tables, instead we have properties and classes.
- URIs are used to name things and URIs are long, ugly and easy to get wrong.
Moriarty’s DataTable class attempts to solve these two problems. It solves the first by treating properties as columns and classes as tables. The second problem it solves by allowing the user to specify short names for URIs. So we can write:
$dt->map('http://xmlns.com/foaf/0.1/firstName', 'firstname');
$dt->map('http://xmlns.com/foaf/0.1/surname', 'surname');
$dt->map('http://xmlns.com/foaf/0.1/Person', 'person');
$dt->select('firstname')->from('person')->where('surname','Evans');
$dt->get();
We can read that as selecting all values of the foaf:firstName property for resources of type foaf:Person that also have a foaf:surname property with a value of Evans. The DataTable class converts that into a SPARQL select query behind the scenes.
This means you can very simply query and use RDF data from a Talis Platform store. To get the first 10 names and nicknames from a store:
$dt = new DataTable('http://api.talis.com/stores/mystore');
$dt->map('http://xmlns.com/foaf/0.1/name', 'name');
$dt->map('http://xmlns.com/foaf/0.1/nick', 'nick');
$dt->select('name,nick')->limit(10);
$res = $dt->get();
foreach ($res->result() as $row) {
echo $row->name;
echo $row->nick;
}
I’ve written up a collection of example queries based on the education data held in the data.gov.uk service.
When I was thinking about how to map the ideas from active record into RDF I was stumped at how to implement table joins. This bothered me because if there is one thing RDF excels at it’s links between resources. Here’s an example of how CodeIgniter implements the join syntax:
$this->db->select('firstname, blog.title');
$this->db->from('person');
$this->db->join('blog', 'person.id = blog.id');
It turned out that the answer was incredibly simple and elegant: you don’t need them! The whole concept of the join method in most active record implementations is to compensate for the fact that relational databases don’t name their relationships (some do but it is very rarely used in practice and not commonly supported in SQL). If you think about the RDF equivalent of that query it becomes clearer: select the name of each resource of type person and for each of their blogs select its title. That join is just the property relating the person resource to the blog resource, probably foaf:weblog.
When you use a DataTable you specify a join simply by including a dotted property path in the select method, e.g. blog.title where blog and title both map to properties. That lets us write our query like this:
$dt->map('http://xmlns.com/foaf/0.1/firstName', 'firstname');
$dt->map('http://xmlns.com/foaf/0.1/weblog', 'blog');
$dt->map('http://purl.org/dc/elements/title', 'title');
$this->db->select('firstname, blog.title');
$this->db->from('person');
Ignoring the mappings this is much simpler than the relational database equivalent! Here’s a good example of using these joinless queries.
DataTables aren’t just for querying. They also support insert and update. To insert a new description of a resource:
$dt = new DataTable('http://api.talis.com/stores/mystore');
$dt->map('http://xmlns.com/foaf/0.1/name', 'name');
$dt->map('http://xmlns.com/foaf/0.1/Person', 'person');
$dt->set('name', 'scooby');
$response = $dt->insert('person');
This translates to submitting a description of a blank node, with a foaf:name property having a value of scooby and an rdf:type of foaf:Person. If you want to submit a description about a resource with a known URI, then you need to set the special _uri field like this:
$dt->set('_uri', 'http://example.com/people/1');
$dt->set('name', 'scooby');
$response = $dt->insert('person');
Behind the scenes the insert method generates the RDF and POSTs it into the store’s metabox. Updates work in a similar way:
$dt->set('_uri', 'http://example.com/people/1');
$dt->set('name', 'scooby');
$dt->where('nick', 'scoob');
$response = $dt->update();
Here the update method queries the store for the current value of the name property for the specified resource and generates a changeset which it then submits to the store’s metabox. This also works for multiple resources, so to update the resource description for anything with a name of shaggy to have a name of scooby:
$dt->set('_uri', 'http://example.com/people/1');
$dt->set('name', 'scooby');
$dt->where('name', 'shaggy');
$response = $dt->update();
Full documentation can be found here: DataTable and DataTableResult
About Moriarty… Moriarty is a simple PHP library for accessing the Talis Platform. It follows the Platform API very closely and wraps up many common tasks into convenient classes while remaining very lightweight. It also provides some simple RDF classes that are based on the excellent ARC2 class library. Moriarty is being developed by small community of developers and is in continual beta, subject to a slow stream of updates. To find out more visit its Google Code project
