Comment 2 for bug 1824299

So, it appears that the race happens in case relationship is defined before the class it relates to. For example in bgpvpn case:

  class BGPVPNPortAssociationRoute:
    ...
    bgpvpn = orm.relationship("BGPVPN",...)

  class BGPVPN(...)
    ...

in order to fix the race should be changed to:

  class BGPVPN(...)
    ...

  class BGPVPNPortAssociationRoute:
    ...
    bgpvpn = orm.relationship("BGPVPN",...)

and alike.