View source for CDOQuery OCL

Based on CDO 4.0-SR2

For some basic references to OCL please find http://atlanmod.emn.fr/atldemo/oclturorial/ or http://cs.ulb.ac.be/public/_media/teaching/infoh302/oclnotes.pdf

=== Model ===
We have the following, deliberately trivial, model:

[[Image:trivialModel.png]]

We have the entities <tt>Patient</tt> and <tt>Konsultation</tt> (Consultation), which hold some generic attributes resp. features. The enumeration <tt>Geschlecht</tt> (sex) is self explaining. Each Konsultation <tt>belongsTo</tt> exactly one <tt>Patient</tt>, it is however not contained by the patient but exists as independent entity.

=== Queries ===
''This section documents some queries based on the aforementioned model. It is work in progress.''

Queries are carried out on the whole Repository, so it is not important to what Resource an element belongs to.

==== Finding object references ====
The following OCL query finds all <tt>Konsultation</tt> entries in the entire repository, that belong to a certain <tt>Patient p</tt>, "injected" by the context. The element provided by the context is refered as '''self''' within the query.

<source lang="java">
CDOSession session = ...

CDOTransaction cdoTransaction = session.openTransaction();

CDOQuery cqo = cdoTransaction.createQuery("ocl",
		"Konsultation.allInstances()->select(k:Konsultation|k.belongsTo=self)",
		((Patient)p).cdoID(), false);

List<Konsultation> lre = cqo.getResult(Konsultation.class);
</source>

The most important part in this query is the fact that we are comparing real objects, as <tt>k.belongsTo</tt> refers to an element of type <tt>Patient</tt>. For OCL CDO to evaluate this query correct, we need to pass the context objects <tt>CDOID</tt> object as otherwise we will receive an error. A direct comparison by simply passing <tt>(Patient)p</tt> as context is hence not possible.

==== OCL Query Examples ====
''These query examples are pure OCL only, the need to be embedded into an OCL query!''

Find all instances of Patient <pre>Patient.allInstances()</pre>

Get the number of instances of Patient <pre>Patient.allInstances()->size()</pre>

Note: The OCLQueryHandler uses lazy extents by default. That means that the OCL interpreter can already start using the extent before it's fully populated. But this means that the size of the extent may not be known at that time. If your query needs to know the size of the allInstances() set (which triggers the CDOExtentCreator) you need to tell the query handler not to use lazy extents:

<pre>query.setParameter("cdoLazyExtents", false);</pre>

Get all instances of Patient where <tt>nachname</tt> is exactly <tt>Descher</tt> <pre>Patient.allInstances()->select(p:Patient|p.nachname='Descher')</pre>

Get all instances of Patient where <tt>nachname</tt> starts with <tt>G</tt> <pre>Patient.allInstances()->select(p:Patient|p.nachname.substring(1,1)='G')</pre>

Get the dates of all Konsultations <pre>Konsultation.allInstances().datum</pre>

Get the attribute numericId of all instances of Patient <pre>Patient.allInstances()->collect(numericId)</pre>