如何在子表修改时更新父表的时间戳？

Question

如何修改子表时修改父时间戳？

我想使用父表时间戳检查我的休息客户端是否应该更新它的这些表的本地版本。

class Parent(db.Model): 
    id = db.Column(db.Integer, primary_key=True) 
    version = db.Column(db.Integer) 
    timestamp = db.Column(db.DateTime, 
          default=datetime.utcnow, 
          onupdate=datetime.utcnow) 
    childs = db.relationship('Children', 
          backref='parent', 
          lazy='dynamic', 
          cascade="all, delete-orphan") 

class Children(db.Model): 
    id = db.Column(db.Integer, primary_key=True) 
    version = db.Column(db.Integer) 
    timestamp = db.Column(db.DateTime, 
          default=datetime.utcnow, 
          onupdate=datetime.utcnow) 
    parent_id = db.Column(db.Integer, db.ForeignKey('parent.id'), nullable=False) 

和测试这样的：

db.create_all() 
    parent = Parent(version=1) 
    child = Children(version=1, parent=parent) 
    db.session.add_all([parent, child]) 
    db.session.commit() 
    print "parent timestamp: %s, child timestamp %s" % (parent.timestamp, child.timestamp) 
    previous_timestamp = parent.timestamp 
    parent.version = 2 
    db.session.add(parent) 
    db.session.commit() 
    assert parent.timestamp != previous_timestamp # this works 
    print "parent timestamp: %s, child timestamp %s" % (parent.timestamp, child.timestamp) 
    previous_timestamp = parent.timestamp 
    child.version = 2 
    db.session.add(child) 
    db.session.commit() 
    # this fails. Parent timestamp is not updated when child is modified 
    assert parent.timestamp != previous_timestamp 
    print "parent timestamp: %s, child timestamp %s" % (parent.timestamp, child.timestamp) 

Answer 1

使用SQLAlchemy events本question作为回答。

见下面一个独立的瓶例如使用一个内存中的SQLite数据库和你的数据模型（请注意我已经改变了你的Children类Child和关系childs到children。

浏览到三条路线/ insert_child /，/ delete_child /和/ update_child /查看父时间戳更改。

from datetime import datetime 
from flask import Flask 
from flask.ext.sqlalchemy import SQLAlchemy, event 

app = Flask(__name__) 
app.config['DEBUG'] = True 
app.config['SECRET_KEY'] = 'super-secret' 
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///:memory:' 
app.config['SQLALCHEMY_ECHO'] = True 
db = SQLAlchemy(app) 


class Parent(db.Model): 
    id = db.Column(db.Integer, primary_key=True) 
    version = db.Column(db.Integer) 
    timestamp = db.Column(db.DateTime, 
          default=datetime.utcnow, 
          onupdate=datetime.utcnow) 
    children = db.relationship('Child', 
          backref='parent', 
          lazy='dynamic', 
          cascade="all, delete-orphan") 


class Child(db.Model): 
    id = db.Column(db.Integer, primary_key=True) 
    version = db.Column(db.Integer) 
    timestamp = db.Column(db.DateTime, 
          default=datetime.utcnow, 
          onupdate=datetime.utcnow) 
    parent_id = db.Column(db.Integer, db.ForeignKey('parent.id'), nullable=False) 


@event.listens_for(Parent.children, 'append') 
@event.listens_for(Parent.children, 'remove') 
def receive_append_or_remove(target, value, initiator): 
    # Update when a child is added or removed 
    target.timestamp = datetime.utcnow() 


@event.listens_for(Child.version, 'set') 
def receive_attribute_change(target, value, oldvalue, initiator): 
    # Update when a child's "version" attribute is set 
    if target.parent: 
     target.parent.timestamp = datetime.utcnow() 


def render_html(): 
    _html = "" 
    parents = Parent.query.all() 
    for parent in parents: 
     _html += "<h4>Parent : {version}; Timestamp: {timestamp}</h4>".format(version=parent.version, timestamp=parent.timestamp) 
     _html += "<ul>" 
     for child in parent.children: 
      _html += "<li>Child : {version}; Timestamp: {timestamp}</li>".format(version=child.version, timestamp=child.timestamp) 
     _html += "</ul>" 
    return _html 


@app.route('/') 
def index(): 
    return render_html() 


@app.route('/insert_child/') 
def insert_child(): 
    parent = Parent.query.first() 
    child_version = parent.children.count() + 1 
    child = Child(version=child_version, parent=parent) 
    db.session.add(child) 
    db.session.commit() 
    return render_html() 


@app.route('/delete_child/') 
def delete_child(): 
    parent = Parent.query.first() 
    if parent.children.count() > 0: 
     last_child = parent.children[-1] 
     parent.children.remove(last_child) 
     db.session.commit() 
    return render_html() 


@app.route('/update_child/') 
def update_child(): 
    parent = Parent.query.first() 
    first_child = parent.children[0] 
    first_child.version += 1 
    db.session.commit() 
    return render_html() 


@app.before_first_request 
def before_first_request(): 
    db.drop_all() 
    db.create_all() 

    parent = Parent(version=1) 
    child = Child(version=1, parent=parent) 
    db.session.add_all([parent, child]) 
    db.session.commit() 

if __name__ == '__main__': 
    app.run(debug=True)