Flask-SQLAlchemy多对多

来自Django，我很难弄清多对多关系的工作原理。

以下models.py无法正常工作，出现错误：InvalidRequestError：一个或多个映射器无法初始化-无法继续进行其他映射器的初始化。最初的例外是：在关系'Sector.companies'上创建backref'sectors'时出错：该名称的属性存在于映射器'Mapper | Company | companies'

代码如下：

`    
company_contacts = db.Table('company_contacts',
                       db.Column('company_id', db.Integer, db.ForeignKey('companies.id')),
                       db.Column('contact_id', db.Integer, db.ForeignKey('contacts.id'))
                       )


company_sectors = db.Table('company_sectors',
                   db.Column('company_id', db.Integer, db.ForeignKey('companies.id')),
                   db.Column('sector_id', db.Integer, db.ForeignKey('sectors.id'))
                   )

company_worklists = db.Table('company_worklists',
                   db.Column('company_id', db.Integer, db.ForeignKey('companies.id')),
                   db.Column('worklist_id', db.Integer, db.ForeignKey('worklists.id'))
                   )


class Sector(db.Model):

    __tablename__ = 'sectors'
    id = db.Column(db.Integer, primary_key = True)
    name_srb= db.Column(db.String(64), unique = True)
    name_ita= db.Column(db.String(64), unique = True)

    companies = db.relationship('Company',
                           secondary = company_sectors,
                           backref = db.backref('sectors', lazy = 'dynamic'),
                           lazy = 'dynamic')



class Contact(db.Model):

    __tablename__ = 'contacts'
    id = db.Column(db.Integer, primary_key = True)
    name = db.Column(db.String(64), unique = False)
    last_name = db.Column(db.String(64), unique = False)
    position_srb = db.Column(db.String(64), unique = False)
    position_ita = db.Column(db.String(64), unique = False)
    email = db.Column(db.String(120), unique = True)
    phone = db.Column(db.String(100))
    created = db.Column(db.DateTime(), default=datetime.now)
    updated = db.Column(db.DateTime(), default=datetime.now,onupdate=datetime.now)

    #companies = db.relationship('Company',
                           #secondary = company_contacts,
                           #backref = db.backref('companies', lazy = 'dynamic'),
                           #lazy = 'dynamic')



class WorkList(db.Model):

    __tablename__ = 'worklists'
    id = db.Column(db.Integer, primary_key = True)
    title = db.Column(db.String(64), unique = True)
    created = db.Column(db.DateTime(), default = datetime.now)
    updated = db.Column(db.DateTime(), default=datetime.now,onupdate=datetime.now)
    slug =  db.Column(db.String(100), unique = True)


class Company(db.Model):

    __tablename__ = 'companies'
    id = db.Column(db.Integer, primary_key = True)
    name = db.Column(db.String(64), unique = True)
    address = db.Column(db.String(120), unique = False)
    website = db.Column(db.String(100), unique = False)
    city = db.Column(db.String(20), unique = False)
    desc = db.Column(db.Text())
    email = db.Column(db.String(120), unique = True)
    created = db.Column(db.DateTime(), default=datetime.now)
    updated = db.Column(db.DateTime(), default=datetime.now,onupdate=datetime.now)
    slug =  db.Column(db.String(100), unique = True)
    phone = db.Column(db.String(100))
    fax = db.Column(db.String(100), unique = False)



    contacts = db.relationship('Contact',
                           secondary = company_contacts,
                           backref = db.backref('contacts', lazy = 'dynamic'),
                           lazy = 'dynamic')

    sectors = db.relationship('Sector',
                           secondary = company_sectors,
                           backref = db.backref('sectors', lazy = 'dynamic'),
                           lazy = 'dynamic')


    wlists= db.relationship('WorkList',
                           secondary = company_worklists,
                           backref = db.backref('wlists', lazy = 'dynamic'),
                           lazy = 'dynamic')

我希望能够访问诸如Company.contacts和Contact.companies ...之类的东西。