Currently to relate two collections or objects of two collections a dedicated type is necessary. This datatype usually has a float weight as single datamember and then two OneToOneRelations to connect two objects.

E.g. from EDM4hep

  edm4hep::MCRecoParticleAssociation:
    Description: "Used to keep track of the correspondence between MC and reconstructed particles"
    Author: "C. Bernet, B. Hegner"
    Members:
      - float weight                        // weight of this association
    OneToOneRelations :
     - edm4hep::ReconstructedParticle  rec  // reference to the reconstructed particle
     - edm4hep::MCParticle sim              // reference to the Monte-Carlo particle

LCIO allows for arbitrary relations between two objects, via the LCRelation. In order to handle all of the relations that are used in the wild many new Association types have to be introduced to EDM4hep, e.g.

• Extend data model for clicRecontruction key4hep/EDM4hep#122
• Add association between ReconstructedParticle and Vertex key4hep/EDM4hep#133

It might be simpler to implement a generic (templated) association or relation type in podio that then allows to use (and persist) arbitrary relations between objects.