public class FooFinder {
	private List<Foo> _result;
	private ResourceSet _resourceSet;

	public Result find(IProject project) throws CoreException, IOException {
		_result = new ArrayList<>();
		
		getResources(project);
		fetchFoo();
		
		return _result;
	}

	private void getResources(IProject project) throws CoreException, IOException {
		if (project == null) {
			return;
		}

		getResourceSet(project);
		processContainer(project);
	}
	
	private void getResourceSet(IProject project) {
		var inj = BarActivator.getInstance().getInjector(BarActivator.LANGUAGE_ID);
		var prov = inj.getInstance(IResourceSetProvider.class);
		_resourceSet = prov.get(project);
	}
	
	private void processContainer(IContainer container) throws CoreException, IOException {
		for (var member : container.members()) {
			if (member instanceof IContainer c) {
				processContainer(c);
			} else if (member instanceof IFile f) {
				processFile(f);
			}
		}
	}
	
	private void processFile(IFile file) throws IOException {
		if (!isBarFile(file)) {
			return;
		}
		
		var uri = URI.createPlatformResourceURI(file.getFullPath().toString(), true);
		_resourceSet.getResource(uri, true);
	}
	
	private boolean isBarFile(IFile file) {
		return file.getFileExtension().equals("bar");  // there is probably a better way of checking this
	}
	
	private void fetchFoo() {
		for (var res : _resourceSet.getResources()) {			
			_result.addAll(EcoreUtil2.typeSelect(EcoreUtil2.eAllContentsAsList(res), Foo.class));
		}
	}
}

• it seems unidiomatic, and liable to be manually performing tasks that something in Xtext or Sirius should be taking care of;
  
• I've been advised that it is probably performing redundant loading and validation activities;
  
• this is especially true given that the Bar language has a Sirius view that will almost certainly be open when this query is run, and will likely already have some or all of the resources available;
  
• if I do this, how do I write back any changes I want to make to the models programmatically, given I've mostly abandoned any of the Xtext infrastructure on the way to getting these models?
  

• the call to perform the query is coming from a Command from within the eclipse3 API stack (eg a button or menu item in some view), and isn't initiated by any action on the model;
  
• the database is exposed as an Eclipse service, which might make it visible at the Xtext side;
  
• I have access to the IProject, but not directly to the EObjects, Resources, or ResourceSets;
  
• there is a graphical view of part of the model available through Sirius, which I could try call into to try get to the EMF model, but not all of the things that I want to do will involve Sirius so that feels strange.
  

This is not necessarily true. You have access to the current selection which via a derived ISelection will reference directly or indirectly an EObject from which you can navigate up to the ResourceSet and down to all the Foo's.

you might also consider querying the Xtext index depending your usecase

• the API I could see for filtering to visible containers seemed to be expecting a resource description to serve as the reference point... unless I should just be directly using the index and not doing that filtering? Would I need to filter the resources to make sure they're in the project, the correct language, etc.?
  
• I wasn't sure how to go from a resource description to a resource, unless I should just simply be taking the URI and loading it into a resource set (which seems like it'd be similar to scraping the project directories for xtext source files and reading them in).