@Override
    public boolean equals(final Object o) {
        if (this == o) {
            return true;
        }
        if (o == null || getClass() != o.getClass()) {
            return false;
        }
        final EntityEvent that = (EntityEvent) o;
        return Objects.equals(docRef, that.docRef) && Objects.equals(oldDocRef,
                that.oldDocRef) && action == that.action;
    }
@return The {@link DocRef} of the {@link stroom.util.shared.Document} affected by this event,
as it is before the event happened. May be null.
stroom-util/src/main/java/stroom/util/entityevent/EntityEvent.java