class Site(db.Model):

    organization = db.relationship(
        "Organization",
        secondary="join(Operation, Region).join(Area)",
        primaryjoin="Site.area_id == Area.id",
        secondaryjoin="Organization.id == Operation.organization_id",
        uselist=False,
        innerjoin=True)

class Region(db.Model):

    organization = db.relationship(
        "Organization",
        secondary="operation",
        uselist=False,
        innerjoin=True)