1. select: 就是访问到属性的时候，就会全部加载该属性的数据
2. joined: 对关联的两个表使用联接
3. subquery: 与joined类似，但使用子子查询
4. dynamic: 不加载记录，但提供加载记录的查询，也就是生成query对象

 

students = db.relationship('Student', backref='_class', lazy="dynamic")

同樣來看結果：

lazy爲dynamic值的students 得到一個查詢對象，而不是一個列表，可以在上面添加過濾器進行操作，如filter() all()

并且该对象的sql语句也可以看到，就是简单查询了Student。

lazy="dynamic"只可以用在一对多和多对多关系中，不可以用在一对一和多对一中

即 lazy屬性的對向關系模型只能是多的一側，在這裏，Class的對向Student爲多的一側

前面说的都是给当前属性加lazy属性，backref的lazy默认都是select,如果给反向引用backref加lazy属性呢? 直接使用backref=db.backref('students', lazy='dynamic'）　？？根據上面原則，那麼反向引用對向側即Class模型必須爲多的一側，而此時模型關系變爲多對多，多對多關系比較復雜，需要關聯表作爲中間表

先看一個基本多對多關系，即一個學生可選擇多個課程 一個課程有多個學生

registrations = db.Table('registrations',db.Column('student_id', db.Integer, db.ForeignKey('students.id')),db.Column('class_id', db.Integer, db.ForeignKey('classes.id')))class Student(db.Model):__tablename__ = 'students'id = db.Column(db.Integer, primary_key=True)name = db.Column(db.String(64))#class_id = db.Column(db.Integer, db.ForeignKey('classes.id'))def __repr__(self):return '<Student: %r>' %self.nameclass Class(db.Model):__tablename__ = 'classes'id = db.Column(db.Integer, primary_key=True)name = db.Column(db.String(64))students = db.relationship('Student', secondary=registrations,backref='_class', lazy="dynamic")def __repr__(self):return '<Class: %r>' %self.name

手動插入多對多數據：

# insert student and class many-to-many relationshipc1 = Class(id=1, name='class1')
c2 = Class(id=2, name='class2')
s1 = Student(id=1, name='susan')
s2 = Student(id=2, name='bob')
s3 = Student(id=3, name='may')
c1.students = [s1, s3]
c2.students = [s1, s2]#update db
db.session.add_all([s1, s2, s3, c1, c2])
db.session.commit()

students = db.relationship('Student', secondary=registrations,backref=db.backref('_class', lazy='joined'), lazy="dynamic")

同樣執行結果爲：

SELECT students.id AS students_id, students.name AS students_name, classes_1.id AS classes_1_id, classes_1.name AS classes_1_name
FROM registrations, students LEFT OUTER JOIN (registrations AS registrations_1 INNER JOIN classes AS classes_1 ON classes_1.id = registrations_1.class_id) ON students.id = registrations_1.student_id
WHERE %(param_1)s = registrations.class_id AND students.id = registrations.student_id

class Registration(db.Model):__tablename__ = 'registrations'student_id = db.Column(db.Integer, db.ForeignKey('students.id'), primary_key=True)class_id = db.Column(db.Integer, db.ForeignKey('classes.id'), primary_key=True)create_at = db.Column(db.DateTime, default=datetime.utcnow)class Student(db.Model):__tablename__ = 'students'id = db.Column(db.Integer, primary_key=True)name = db.Column(db.String(64))_classes = db.relationship('Registration', foreign_keys=[Registration.student_id],backref = db.backref('_student', lazy='joined'),lazy = 'dynamic')def __repr__(self):return '<Student: %r>' %self.nameclass Class(db.Model):__tablename__ = 'classes'id = db.Column(db.Integer, primary_key=True)name = db.Column(db.String(64))_students = db.relationship('Registration', foreign_keys=[Registration.class_id],backref=db.backref('_class', lazy='joined'),lazy='dynamic')def __repr__(self):return '<Class: %r>' %self.name

手動插入數據：

#insert student and classes many to many relationship
c1 = Class(id=1, name='class1')
c2 = Class(id=2, name='class2')
s1 = Student(id=1, name='susan')
s2 = Student(id=2, name='bob')
s3 = Student(id=3, name='may')
r1 = Registration(student_id=3, class_id=1)
r2 = Registration(student_id=2, class_id=2)
r3 = Registration(student_id=1, class_id=1)
r4 = Registration(student_id=1, class_id=2)#update db
db.session.add_all([s1, s2, s3, c1, c2, r1, r2, r3, r4])
db.session.commit()

查看表結構：

    _students = db.relationship('Registration', foreign_keys=[Registration.class_id],backref=db.backref('_class', lazy='select'),lazy='dynamic')

    _classes = db.relationship('Registration', foreign_keys=[Registration.student_id],backref = db.backref('_student', lazy='select'),lazy = 'dynamic')

flask-sqlalchemy中 backref lazy的参数实例解释和选择

SQLalchemy relationship之lazy属性 学习笔记

sqlalchemy中多对多的关系 - CSDN博客