修改子表时如何更新父表时间戳? [英] How to update parent table timestamp when child table is modified?

问题描述

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该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)

并对此进行测试:

    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)

解决方案

使用SQLAlchemy 事件，由该问题回答.

>

请参见下面的一个独立的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Children类更改为Child，并将关系childs更改为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)

How can I update parent timestamp when child table is modified?

I would like to use parent table timestamp for checking if my rest client should update it's local version of these tables.

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)

And test this:

    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)

解决方案

Use SQLAlchemy events as answered by this question.

See below a self-contained Flask example using an in-memory SQLite database and your data models (note I've changed your Children class to Child and the relationship childs to children.

Browse to the three routes /insert_child/, /delete_child/ and /update_child/ to see the parent timestamp change.

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)

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