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Using FCC-EDM collections

This document describes some typical use-cases. A corresponding document for general usage of PODIO has some additional information.

Object Ownership

Every data object created and added to a collection is owned by the collection and garbage collected at the end of event processing.

Object creation

Each collection allows the creation of objects with the factory method create, using this ensures correct object ownership and is recommended. More details in the PODIO examples.

Data composition

In the design document we already mentioned that an influencing factor was to ease writing re-usable code. This lead to the introduction of the Core PODs you defined at the very top of the edm.yaml. For example both fcc::Particle and fcc::MCParticle have the member fcc::BareParticle:

void setKinematics(fcc::BareParticle& p, double pt, double eta, double phi) {
    p.P4.Px = TMath::Cos(phi)*pt;
    p.P4.Py = TMath::Sin(phi)*pt;
    p.P4.Pz = TMath::Sinh(eta)*pt;
}

// First get the collection (note that this will work differently in FCCSW, see below)
auto& myParticleColl = store.create<fcc::ParticleCollection>("recoParticles");
auto& myMCParticleCollection = store.create<fcc::MCParticleCollection>("genParticles");
auto particle = myParticleCollection.create();
auto mcParticle = myMCParticleCollection.create();

setKinematics(particle.Core, 20, 0.1, 0.5);
setKinematics(mcParticle.Core, 20, 0.1, 0.5);

Processing composed objects

In the design doc we describe why we chose to encode some objects through composition. E.g. Tags and Jets are saved as tag information and OneToOneRelation to the jet in the fcc::TaggedJet. We advise to directly iterate over those composed objects instead of iterating over the jet collection and then searching for the tag in the tag collection.

Processing associated objects

Some relations between objects are saved in association collections. One example is the relation between reconstructed and generated particles saved in the fcc::ParticleMCParticleAssociation. Again, we advise to directly iterate over this association collection if you need both the reconstructed and simulated particles.