sqlalchemy.exc.NoForeignKeysError: Could not determine join condition between 
parent/child tables on relationship CurriculumVersion.enrollments - there are 
no foreign keys linking these tables.  Ensure that referencing columns are 
associated with a ForeignKey or ForeignKeyConstraint,or specify
a 'primaryjoin' expression.

在我的车型上：

class User(db.Model,AuthUser):
    id = db.Column(db.Integer,primary_key=True)
    tf_login = db.Column(db.String(255),unique=True,nullable=False) # can assume is an email
    password = db.Column(db.String(120),nullable=False)
    salt = db.Column(db.String(80))
    role = db.Column(db.String(80)) # for later when have different permission types
    zoho_contactid = db.Column(db.String(20),nullable=False)
    created_asof = db.Column(db.DateTime,default=datetime.datetime.utcnow)
    enrollments = db.relationship('Enrollment',backref='enrollment',lazy='dynamic')
    firstname = db.Column(db.String(80))
    lastname = db.Column(db.String(80))
 
 
    def __repr__(self):
        return '#%d tf_login: %s,First Name: %s Last Name: %s created_asof %s' % (self.id,self.tf_login,self.firstname,self.lastname,self.created_asof)
 
    def __getstate__(self):
        return {
            'id': self.id,'tf_login': self.tf_login,'firstname': self.firstname,'lastname': self.lastname,'role': self.role,'created_asof': self.created_asof,}
 
    @classmethod
    def load_current_user(cls,apply_timeout=True):
        data = get_current_user_data(apply_timeout)
        if not data:
            return None
        return cls.query.filter(cls.email==data['email']).one()
        return '#%d Course Name: %s,Course Version: %s,Closing Date: %s' %(self.id,self.course_name,self.course_version,self.closing_date)
 
class Enrollment(db.Model,primary_key=True)
    user_id = db.Column(db.Integer,db.ForeignKey('user.id'))
    version_id = db.Column(db.Integer,db.ForeignKey('curriculumversion.id'))
    cohort_id = db.Column(db.Integer,db.ForeignKey('cohort.id'))
 
    def __repr__(self):
        return '#%d User ID: %d Version ID: %d,Cohort ID: %d' %(self.id,self.user_id,self.version_id,self.cohort_id)
 
class Cohort(db.Model,primary_key=True)
    days_to_completion = db.Column(db.String(20))
    course_id = db.Column(db.Integer,db.ForeignKey('course.id'))
    enrollments = db.relationship('Enrollment',lazy='dynamic')
 
    def __repr__(self):
        return '#%d Days To Completion: %s' %(self.id,self.days_to_completion)
 
 
class CurriculumVersion(db.Model,primary_key=True)
    version_number = db.Column(db.String(6))
    date_implemented = db.Column(db.DateTime)
    course_id = db.Column(db.Integer,lazy='dynamic')
 
    def __repr__(self):
        return '#%d Version Number: %s,Date Implemented: %s' %(self.id,self.version_number,self.date_implemented)
 
class Course(db.Model,primary_key=True)
    course_code = db.Column(db.String(20))
    course_name = db.Column(db.String(50))
    versions = db.relationship('CurriculumVersion',backref='version',lazy='dynamic')
    cohorts = db.relationship('Cohort',backref='cohort',lazy='dynamic')
 
 
    def __repr__(self):
        return '#%d Course Code: %s,Course Name: %s' %(self.id,self.course_code,self.course_name)

任何帮助将不胜感激！