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

官方文档:http://docs.sqlalchemy.org/en/rel_1_0/orm/basic_relationships.html#relationship-patterns

最近在学习到Flask中的Sqlalchemy, 不过在看到数据库关系db.relations()时对lazy这个参数一直很模糊。主要是看到Flask Web开发这本书中对关注与被关注的关系建模中，被lazy的使用绕晕了。

看官方文档，也得不到多少信息，于是就自己实践，从lazy参数的不同值所执行的sql语句出发，结合one-to-many和many-to-many的关系，分析lazy参数取不同值(dynamic, joined, select)在不同场景下的选择，因为涉及到数据库性能问题，选择不同差别很大，尤其在数据量比较大时。
以下的实例均是基于如下的模型和表:主要侧重对relationship中的backref的lazy属性做修改。

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

基本介绍

首先看官网的关于lazy的说明:

lazy 决定了 SQLAlchemy 什么时候从数据库中加载数据:，有如下四个值:(其实还有个noload不常用)
select: (which is the default) means that SQLAlchemy will load the data as necessary in one go using a standard select statement.
joined: tells SQLAlchemy to load the relationship in the same query as the parent using a JOIN statement.
subquery: works like ‘joined’ but instead SQLAlchemy will use a subquery.
dynamic : is special and useful if you have many items. Instead of loading the items SQLAlchemy will return another query object which
you can further refine before loading the items. This is usually what you want if you expect more than a handful of items for this relationship

通俗了说，select就是访问到属性的时候，就会全部加载该属性的数据。joined则是在对关联的两个表进行join操作，从而获取到所有相关的对象。dynamic则不一样，在访问属性的时候，并没有在内存中加载数据，而是返回一个query对象, 需要执行相应方法才可以获取对象，比如.all().下面结合实例解释这几个的使用场景。

实例

首先是最开始一对多关系中，改动如下:将一的lazy改为select:

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

这样的话， class.students会直接返回结果列表:

>>> from app.models import Student as S, Class as C
>>> c1=C.query.first()
>>> c1.students
[<Student: u'test'>, <Student: u'test2'>, <Student: u'test3'>]

这种情况下，在数据量较大或者想做进一步操作时候，不太方便，因此这个时候, dynamic就用上了:

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

同样看看结果:

>>> from app.models import Student as S, Class as C
>>> s1=S.query.first()
>>> c1=C.query.first()
>>> c1.students
<sqlalchemy.orm.dynamic.AppenderBaseQuery object at 0x7f007d2e8ed0>
>>> print c1.students
SELECT students.id AS students_id, students.name AS students_name
FROM students, registrations
WHERE :param_1 = registrations.class_id AND students.id = registrations.student_id
>>> c1.students.all()
[<Student: u'test'>, <Student: u'test2'>, <Student: u'test3'>]

可以看到, 执行c1.student返回的是是一个 query对象，并且该对象的sql语句也可以看到，就是简单查询了Student。而如果lazy=select 或者 joined均是直接返回结果。　需要注意的是， lazy="dynamic"只可以用在一对多和多对对关系中，不可以用在一对一和多对一中，如果返回结果只有一个的话，也就无需要延迟加载数据了。
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'　即可。这个在多对多关系需要进行考量。
先看一个最基本的多对多关系:

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')) 这里需要注释，不需要外键了
    def __repr__(self):
        return '<Student: %r>' %self.name
class Class(db.Model):
    __tablename__ = 'classes'
    id = db.Column(db.Integer, primary_key=True)
    students = db.relationship('Student', secondary=registrations, backref='_class', lazy="dynamic")　#这里指定关联表
    name = db.Column(db.String(64))
    def __repr__(self):
        return '<Class: %r>' %self.name

同样执行结果可以看到:

>>> s1=S.query.first()
>>> c1=C.query.first()
>>> s1._class
[<Class: u'class1'>, <Class: u'class2'>]
>>> c1.students
<sqlalchemy.orm.dynamic.AppenderBaseQuery object at 0x7ff8691a8610>
>>> c1.students.all()
[<Student: u'test'>, <Student: u'test2'>, <Student: u'test3'>]
>>> print c1.students
SELECT students.id AS students_id, students.name AS students_name
FROM students, registrations
WHERE :param_1 = registrations.class_id AND students.id = registrations.student_id

可以看到这个跟一对多关系中的很类似，只不过s1._class成为了集合形式, 因为backref="_class"默认仍然是select，所以直接返回结果，而c1.students的sql语句也仅仅是查询了students。但是如果修改反向引用的lazy为joined:

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

然后看看结果:

....
>>> print c1.students
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 JOIN classes AS classes_1 ON classes_1.id = registrations_1.class_id) ON students.id = registrations_1.student_id
WHERE :param_1 = registrations.class_id AND students.id = registrations.student_id
>>> c1.students.all()
[<Student: u'test'>, <Student: u'test2'>, <Student: u'test3'>]
>>> s1._class
[<Class: u'class1'>, <Class: u'class2'>]

首先不变的还是s1._class还是直接返回数据。有变化的是c1.students的sql语句，　不仅仅是查询Student对象, 而且还通过与关联表做join操作，把相关联的Class也查询了。相关联的意思是什么呢？看下直接执行sql语句的结果就知道了:

mysql> 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 JOIN classes AS classes_1 ON classes_1.id = registrations_1.class_id) ON students.id = registrations_1.student_id  WHERE 1 = registrations.class_id AND students.id = registrations.student_id;
+-------------+---------------+--------------+----------------+
| students_id | students_name | classes_1_id | classes_1_name |
+-------------+---------------+--------------+----------------+
|           1 | test          |            1 | class1         |
|           1 | test          |            2 | class2         |
|           2 | test2         |            1 | class1         |
|           3 | test3         |            1 | class1         |
+-------------+---------------+--------------+----------------+
4 rows in set (0.00 sec)

也就是说把查询得到的students的对应的class实体也都查询出来了。　但是貌似在这个例子中没有意义，因为这种多对多的关系比较简单，关联表甚至都不是模型，只有两个外键的id, 上述代码中的registrations是直接被sqlalchemy接管的，程序无法直接访问的。
在下面的多对多例子中，我们可以看到上述的lazy方式的优势，我们把关联表改为实体model，并且额外增加一个时间信息。模型代码如下:

class Registration(db.Model):
    '''关联表'''
    __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))
    _class = db.relationship('Registration', foreign_keys=[Registration.student_id],
                             backref=db.backref('student', lazy="joined"), lazy="dynamic")
    def __repr__(self):
        return '<Student: %r>' %self.name
class Class(db.Model):
    __tablename__ = 'classes'
    id = db.Column(db.Integer, primary_key=True)
    students = db.relationship('Registration', foreign_keys=[Registration.class_id],
                               backref=db.backref('_class', lazy="joined"), lazy="dynamic")
    name = db.Column(db.String(64))
    def __repr__(self):
        return '<Class: %r>' %self.name

提前准备数据:

mysql> select * from classes;
+----+--------+
| id | name   |
+----+--------+
|  1 | class1 |
|  2 | class2 |
+----+--------+
2 rows in set (0.00 sec)
mysql> select * from students;
+----+-------+
| id | name  |
+----+-------+
|  1 | test  |
|  2 | test2 |
|  3 | test3 |
+----+-------+
3 rows in set (0.00 sec)
mysql> select * from registrations;
+------------+----------+-----------+
| student_id | class_id | create_at |
+------------+----------+-----------+
|          1 |        1 | NULL      |
|          2 |        1 | NULL      |
|          3 |        1 | NULL      |
|          1 |        2 | NULL      |
+------------+----------+-----------+
4 rows in set (0.00 sec)

之后看看结果:

>>> s1._class.all()
[<app.models.Registration object at 0x7f0348018ed0>, <app.models.Registration object at 0x7f0348018f50>]
>>> c1.students.all()
[<app.models.Registration object at 0x7f0348018ed0>, <app.models.Registration object at 0x7f03480412d0>, <app.models.Registration object at 0x7f034c32f250>]

可以看到返回值是Registration两个对象，　不再直接返回Student和Class对象了。如果想要获取的话，可以使用给Registration加的反向引用:

>>> map(lambda x: x._class, s1._class.all())
[<Class: u'class1'>, <Class: u'class2'>]
>>> map(lambda x: x.student, c1.students.all())
[<Student: u'test'>, <Student: u'test2'>, <Student: u'test3'>]

那么问题就来了，这里在调用Registration的_class和student时候，　还需不需要再查询一遍数据库呢？

下面通过查看执行的sql语句来看看:

>>> print s1._class
SELECT registrations.student_id AS registrations_student_id, registrations.class_id AS registrations_class_id, registrations.create_at AS registrations_create_at, classes_1.id AS classes_1_id, classes_1.name AS classes_1_name, students_1.id AS students_1_id, students_1.name AS students_1_name
FROM registrations LEFT OUTER JOIN classes AS classes_1 ON classes_1.id = registrations.class_id LEFT OUTER JOIN students AS students_1 ON students_1.id = registrations.student_id
WHERE :param_1 = registrations.student_id

我们可以发现: 跟上一个例子一样，s1._class不仅查询了对应的class信息，而且通过join操作，获取到了相应的Student和Class对象，换句话说，把Registration的student和_class两个回引属性均指向了对应的对象，　也就是说，s1._class这一条查询语句就可以把上述操作都完成。这个就是backref=db.backref('_class', lazy='joined')的作用。　
下面再看看把lazy改为select的情况:

###
_class = db.relationship('Registration', foreign_keys=[Registration.student_id],
                         backref=db.backref('student', lazy="select"), lazy="dynamic")
###
students = db.relationship('Registration', foreign_keys=[Registration.class_id],
                           backref=db.backref('_class', lazy="select"), lazy="dynamic")

这样看看查询语句:

>>> s1=S.query.first()
>>> print s1._class
SELECT registrations.student_id AS registrations_student_id, registrations.class_id AS registrations_class_id, registrations.create_at AS registrations_create_at
FROM registrations
WHERE :param_1 = registrations.student_id
>>> map(lambda x : x._class , s1._class)
[<Class: u'class1'>, <Class: u'class2'>]

十分简单的sql语句，仅仅查询返回了　Registration对象，　虽然结果一样，但是每一个Registration对象访问_class属性时，又各自都查询了一遍数据库！　这是很重的！　比如一个class有100个student, 那么获取class.students需要额外查询100次数据库！　每一次数据库的查询代价很大，因此这就是joined的作用了。