Can someone explain how to organize the relationship between objects using Spring Boot and Firestore?

I am using spring-cloud-gcp-starter-data-firestore and I have two objects that must be located in different collections:

@Document(collectionName = "action_task")
public class ActionTask {

    @DocumentId
    private String id;
    ...........................
    
    private EventRule eventRule;
}

@Document(collectionName = "event_rule")
public class EventRulePair {

    @DocumentId
    private String id;

    ...................

    private String code;

    private String eventType;

    private Instant createdAt;
}

How organize relationships in the code and haw store in DB?