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[news.eclipse.tools.emf] Re: Complex EMF Query for a "Simple Query" 
Hello Christian,

Thanks a lot.

I managed to reduce some of my condition objects based on your input.
The code now looks like this (and as you said - simple)

EObjectCondition authorCondition = new EObjectCondition() {
	@Override
	public boolean isSatisfied(EObject object) {
		//Removed reflection for simplicity
		if (object instanceof Writer)
			if (((Writer) object).getBooks().size() > 10)
				return true;

		return false;
	}
};

SELECT select = new SELECT(new FROM(selectedEObjects), new WHERE(
		authorCondition));

I didn't extend EObjectReferenceValueCondition as my check was not on an EReference of the context, but a computation on my contextual object itself (size of Elist containing books).

I have different question now about EMF model query solutions available. I will post it as a separate thread.

Thanks
Nirmal


Christian W. Damus schrieb: 
Hi, Nirmal,

You're right, it shouldn't have to be so complicated, and I don't think it actually is.

I think you can do this query with two one objects. Create a subclass of the EObjectStructuralFeatureCondition. By default, this condition matches objects that have the feature (meaning not that they have values for the feature, but that the feature is available/defined for their EClasses). In your subclass, extend the isSatisfied(EObject) method to check the size of the feature's value list (by extend, I mean do your additional check if super returns true).

That should do it.

Your scenario raises a good point: the API currently is missing a framework for aggregate functions on multi-valued features. It would have been nice to be able to create an EObjectReferenceValueCondition with a COUNT aggregate function object. The current EObjectReferenceValueCondition (like EObjectAttributeValueCondition) only knows how to evaluate a condition on the individual collection elements with a "forAll" or "exists" quantifier. COUNT would mix nicely with the NumberCondition class ...

That would make a great contribution  :-D

Cheers,

Christian


On Thu, 2009-05-07 at 07:43 +0000, Nirmal Sasidharan wrote: 
Hello,

I am playing around with EMF Query currently. I wanted to write a simple query based on the Extended Library Model. 

SELECT Authors
FROM Library
WHERE COUNT(Authors.Books) > 10

I ended up writing a query like this. Would like to know if there is a simpler way of doing this.

EObjectCondition adaptedCondition = new EObjectCondition(){
@Override
public boolean isSatisfied(EObject object) {
if (object.eClass().equals(EXTLibraryPackage.eINSTANCE.getWriter())) {
if (((EList)object.eGet(EXTLibraryPackage.eINSTANCE.getWriter_Books())).size() > 10)
return true;
}
return false;
}
};

EObjectCondition contextCondition = new EObjectTypeRelationCondition(
		EXTLibraryPackage.eINSTANCE.getWriter());

EObjectCondition valueCondition = new EObjectReferenceValueCondition(
		EXTLibraryPackage.eINSTANCE.getBook_Author(),
		adaptedCondition);

EObjectCondition masterCondition = new EObjectReferenceValueCondition(
contextCondition,
EXTLibraryPackage.eINSTANCE.getWriter_Books(), valueCondition);

What I find funny is the fact that, I started with Writer as the context (because this is what I want to return). Then I get the Books of the Writer, then the Writer of the Books, then the Books of the writer again. Arrrrg.... Has to be a better way.

Thanks in advance,

Nirmal