postgresql – 设置关系表的server_default值

assignments = db.Table('assignments',db.Column('role_id',db.Integer,db.ForeignKey('roles.id')),db.Column('administrator_id',db.ForeignKey('administrators.id')),db.Column('created_at',db.DateTime,server_default=db.func.now()),db.Column('updated_at',server_default=db.func.now(),onupdate=db.func.now()),db.ForeignKeyConstraint(['administrator_id'],['administrators.id']),db.ForeignKeyConstraint(['role_id'],['roles.id'])
)

class Administrator(db.Model,UserMixin):
    __tablename__ = 'administrators'

    id = Column(Integer,primary_key=True,server_default=text("nextval('administrators_id_seq'::regclass)"))
    email = Column(String(255),nullable=False,unique=True,server_default=text("''::character varying"))
    name = Column(String(255))
    surname = Column(String(255))

    roles = db.relationship('Role',secondary=assignments,backref=db.backref('users',lazy='dynamic'))


class Role(db.Model):
    __tablename__ = 'roles'

    id = Column(Integer,server_default=text("nextval('roles_id_seq'::regclass)"))
    name = Column(String(255))

但是当我将一个角色分配给管理员时

admin.roles = [role1]
db.session.add(admin)
db.session.commit()

它打破了以下错误：

IntegrityError: (psycopg2.IntegrityError) null value in column "created_at" violates not-null constraint
DETAIL:  Failing row contains (1265,19,3,null,null).
[SQL: 'INSERT INTO assignments (role_id,administrator_id) VALUES (%(role_id)s,%(administrator_id)s)'] [parameters: {'administrator_id': 19,'role_id': 3}]

有没有办法在赋值表中为created_at和updated_at字段设置默认值？