修改子表后如何更新父表时间戳？

Question

修改子表后，如何更新父时间戳？

我想使用父表时间戳检查我的其余客户端是否应更新这些表的本地版本。

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)

并测试一下：

    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

使用此问题回答的SQLAlchemy 事件。

请参阅下面的一个独立的Flask示例，该示例使用内存中的SQLite数据库和您的数据模型（请注意，我已将您的Children类更改为Child，并将关系更改childs为children。

浏览到三个路由/ insert_child /，/ delete_child /和/ update_child /以查看父时间戳的更改。

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)