Flask学习记录之Flask-SQLAlchemy

Flask-SQLAlchemy库让flask更方便的使用SQLALchemy,是一个强大的关系形数据库框架,既可以使用orm方式操作数据库,也可以使用原始的SQL命令.

Flask-Migrate 是一个数据迁移框架,需要通过Flask-script库来操作.

一.配置Flask-SQLAlchemy

程序使用的数据库地址需要配置在SQLALCHEMY_DATABASE_URI中,SQLALchemy支持多种数据库,配置格式如下:

　　Postgres:

　　postgresql://scott:tiger@localhost/mydatabase

　　MySQL:

　　mysql://scott:tiger@localhost/mydatabase

　　Oracle:

　　oracle://scott:tiger@127.0.0.1:1521/sidname

　　SQLite:

　　sqlite:////absolute/path/to/foo.db

db是SQLALchemy类的实例,表示程序使用的数据库,为用户提供Flask-SQLALchemy的所有功能

from flask import Flask
from flask.ext.sqlalchemy import SQLAlchemy

app = Flask(__name__)
#配置数据库地址
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:////tmp/test.db'
#该配置为True,则每次请求结束都会自动commit数据库的变动
app.config['SQLALCHEMY_COMMIT_ON_TEARDOWN'] = True
db = SQLAlchemy(app)
#也可以db = SQLAlchemy()        db.init_app(app)

二.定义模型

Flask-SQLALchemy使用继承至db.Model的类来定义模型,如:

class User(db.Model, UserMixin):#UserMixin是Flask-Login库中所需要的
    __tablename__ = 'users'
    #每个属性定义一个字段
    id = db.Column(db.Integer,primary_key=True)
    username = db.Column(db.String(64),unique=True)
    password = db.Column(db.String(64))
    def __repr__(self):
        return '<User %r>' % self.username

定义完需要在Python Shell中导入db,调用db.create_all()来创建数据库

(1)常用字段选项:

　　primary_key 设置主键

　　unique 是否唯一

　　index 是否创建索引

　　nullable 是否允许为空

　　default 设置默认值,可以传入函数的引用 如传入 datetime.datetime.utcnow 则每次创建时时间都是最新时间

三.增删查改

(1) 插入数据:

from app.models import User
from app import db

#创建一个新用户
u = User()
u.username = 'abc'
u.password = 'abc'
#将用户添加到数据库会话中
db.session.add(u)
#将数据库会话中的变动提交到数据库中,如果不Commit,数据库中是没有改动的
db.commit()

(2)查找数据:

#返回所有用户保存到list中
user_list = User.query.all()

#查找username为abc的第一个用户,返回用户实例
u = User.query.filter_by(username='abc').first()

#模糊查找用户名以c结尾的所有用户
user_list  = User.query.filter(username.endswith('c')).all()

#查找用户名不是abc的用户
u = User.query.filter(username != 'abc').first()

(3)删除数据:

user = User.query.first()
db.session.delete(user)
db.session.commit()

(4)修改数据:

u = User.query.first()
u.username = 'sb'
db.session.commit()

四.一对多关系

我的理解是:在多的一边定义外键,而relathonship()函数是用来建立关系的,可以只在一边定义,也可以两边都使用(只在一边使用时加上了backref选项等同于两边都使用)

class Person(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))
    #backref将在Address表中创建个名为persons的Person引用,之后可以使用address.persons访问这个地址的所有人
    addresses = db.relationship('Address', backref='persons',
                                lazy='dynamic')

class Address(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    email = db.Column(db.String(50))
    #在多的一边使用db.ForeignKey声明外键
    person_id = db.Column(db.Integer, db.ForeignKey('person.id'))

五.多对多关系

多对多关系可以分解为原表和关联表之间两个多对一关系,如下代码建立了学生与所选课程之间的关系:

#创建关联表,两个字段的外键是另两个表,一个学生对应多个关联表,一个关联表对应多个课程
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)
    classes = db.relationship('Class',
                              secondary = registrations, #关联表,只需要在一个表建立关系,sqlalchemy会负责处理好另一个表
                              backref = db.backref('students',lazy='dynamic'),
                              lazy = 'dynamic')

class Class(db.Model):
    __tablename__ = 'classes'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String)

多对多的使用:

#学生1增加一门选课
student1.classes.append(class1)
#学生1退选class1
student1.classes.remove(class1)
#学生1所选课程,由于指定了lazy='dynamic'所以没有直接返回列表,而需要使用.all()
student1.classes.all()

六.分页导航

Flask-SQLALchemy的Pagination对象可以方便的进行分页,

对一个查询对象调用pagenate(page, per_page=20, error_out=True)函数可以得到pagination对象,第一个参数表示当前页,第二个参数代表每页显示的数量,error_out=True的情况下如果指定页没有内容将出现404错误,否则返回空的列表

#从get方法中取得页码
page = request.args.get('page', 1, type = int)
#获取pagination对象
    pagination = Post.query.order_by(Post.timestamp.desc()).paginate(page, per_page=10, error_out = False)

#pagination对象的items方法返回当前页的内容列表
    posts = pagination.items

pagination对象常用方法:

has_next :是否还有下一页

has_prev :是否还有上一页

items : 返回当前页的所有内容

next(error_out=False) : 返回下一页的Pagination对象

prev(error_out=False) : 返回上一页的Pagination对象

page : 当前页的页码(从1开始)

pages : 总页数

per_page : 每页显示的数量

prev_num : 上一页页码数

next_num :下一页页码数

query :返回 创建这个Pagination对象的查询对象

total :查询返回的记录总数

iter_pages(left_edge=2, left_current=2, right_current=5, right_edge=2)

在模版中使用

{% macro render_pagination(pagination, endpoint) %}
  <div class=pagination>
  {%- for page in pagination.iter_pages() %}
    {% if page %}
      {% if page != pagination.page %}
        <a href="{{ url_for(endpoint, page=page) }}">{{ page }}</a>
      {% else %}
        <strong>{{ page }}</strong>
      {% endif %}
    {% else %}
      <span class=ellipsis>…</span>
    {% endif %}
  {%- endfor %}
  </div>
{% endmacro %}

七.事件监听

Flask-SQLALchemy不但提供了方便的数据库操作,还提供了事件的监听,如下

from sqlalchemy import event

def my_append_listener(target, value, initiator):
    print "received append event for target: %s" % target

event.listen(MyClass.collection, 'append', my_append_listener)

Listeners have the option to return a possibly modified version of the value, when the retval=Trueflag is passed to listen():

def validate_phone(target, value, oldvalue, initiator):
    "Strip non-numeric characters from a phone number"

    return re.sub(r'(?![0-9])', '', value)

# setup listener on UserContact.phone attribute, instructing
# it to use the return value
listen(UserContact.phone, 'set', validate_phone, retval=True)

A validation function like the above can also raise an exception such as ValueError to halt the operation.

Several modifiers are available to the listen() function.

Parameters:

active_history=False – When True, indicates that the “set” event would like to receive the “old” value being replaced unconditionally, even if this requires firing off database loads. Note that active_history can also be set directly viacolumn_property() and relationship(). propagate=False – When True, the listener function will be established not just for the class attribute given, but for attributes of the same name on all current subclasses of that class, as well as all future subclasses of that class, using an additional listener that listens for instrumentation events. raw=False – When True, the “target” argument to the event will be theInstanceState management object, rather than the mapped instance itself. retval=False – when True, the user-defined event listening must return the “value” argument from the function. This gives the listening function the opportunity to change the value that is ultimately used for a “set” or “append” event.

append(target, value, initiator)

Receive a collection append event.

Parameters:	target – the object instance receiving the event. If the listener is registered with raw=True, this will be the InstanceState object. value – the value being appended. If this listener is registered withretval=True, the listener function must return this value, or a new value which replaces it. initiator – the attribute implementation object which initiated this event.
Returns:	if the event was registered with retval=True, the given value, or a new effective value, should be returned.

remove(target, value, initiator)

Receive a collection remove event.

Parameters:	target – the object instance receiving the event. If the listener is registered with raw=True, this will be the InstanceState object. value – the value being removed. initiator – the attribute implementation object which initiated this event.
Returns:	No return value is defined for this event.

set(target, value, oldvalue, initiator)

Receive a scalar set event.

Parameters:

target – the object instance receiving the event. If the listener is registered with raw=True, this will be the InstanceState object. value – the value being set. If this listener is registered with retval=True, the listener function must return this value, or a new value which replaces it. oldvalue – the previous value being replaced. This may also be the symbol NEVER_SET or NO_VALUE. If the listener is registered withactive_history=True, the previous value of the attribute will be loaded from the database if the existing value is currently unloaded or expired. initiator – the attribute implementation object which initiated this event.

Returns:

if the event was registered with retval=True, the given value, or a new effective value, should be returned.