前言

一般来说, 解决 sqlalchemy 连接多个库的最简单的方式是新建两个或多个 db.session 相互没有关联,modle 配置不同的 db.session 来连接, 这样的话,relationship 正常配置就行, 不用特殊配置. 如果这样解决的话, 也就不用看下面的配置了

# -*- coding:utf-8 -*-

import flask
from flask_sqlalchemy import SQLAlchemy  # Flask-SQLAlchemy 2.3.2
from datetime import datetime
from sqlalchemy.orm import backref, foreign  # SQLAlchemy 1.3.1

app = flask.Flask(__name__)
app.config['DEBUG'] = True
app.config['SQLALCHEMY_BINDS'] = {
    'read_db': 'mysql://reader:test@127.0.0.1:3306/test?charset=utf8',
    'write_db': 'mysql://writer:test@127.0.0.2:3306/test?charset=utf8'
}

app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False
app.config['SQLALCHEMY_ECHO'] = False
db = SQLAlchemy(app)


class RDriver(db.Model):
    __bind_key__ = 'read_db'
    __tablename__ = 'driver'
    # __table_args__ = {'schema': 'test'} # 不可以加上

    id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    fk_user_id = db.Column(db.Integer, db.ForeignKey("user.id"))
    driver_name = db.Column(db.String(7))
    create_time = db.Column(db.TIMESTAMP, default=datetime.now)


class RUser(db.Model):
    __bind_key__ = 'read_db'
    __tablename__ = 'user'
    # __table_args__ = {'schema': 'test'}

    id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    user_name = db.Column(db.String(32), index=True, unique=True)
    user_password = db.Column(db.String(32))
    create_time = db.Column(db.TIMESTAMP, default=datetime.now)
    update_time = db.Column(db.TIMESTAMP, default=datetime.now)
    # 如下的五种方式都是可以的
    # driver_fk = db.relationship("RDriver", foreign_keys='RDriver.fk_user_id')
    # driver_fk = db.relationship("RDriver", primaryjoin=lambda: RDriver.fk_user_id == RUser.id, viewonly=True)
    # driver_fk = db.relationship("RDriver", primaryjoin=RDriver.fk_user_id == id)
    fk_driver = db.relationship("RDriver", primaryjoin='RDriver.fk_user_id == RUser.id')
    # driver_fk = db.relationship("RDriver", backref=db.backref('user', lazy=True),
    #                             primaryjoin=lambda: RDriver.fk_user_id == RUser.id, viewonly=True)


class WDriver(db.Model):
    __bind_key__ = 'write_db'
    __tablename__ = 'driver'
    __table_args__ = {'schema': 'test', 'extend_existing': True} # 这个配置很关键

    id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    fk_user_id = db.Column(db.Integer, db.ForeignKey("test.user.id"))   # test.user.id 很关键
    plate = db.Column(db.String(7))
    create_at = db.Column(db.TIMESTAMP, default=datetime.now)


class WUser(db.Model):
    __bind_key__ = 'write_db'
    __tablename__ = 'user'
    __table_args__ = {'schema': 'test', 'extend_existing': True}    # 这个配置很关键

    id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    hash = db.Column(db.String(256), nullable=False)
    user_no = db.Column(db.String(32), index=True, unique=True)  # 用户工号
    create_time = db.Column(db.TIMESTAMP, default=datetime.now)
    update_time = db.Column(db.TIMESTAMP, default=datetime.now)
    # 以下五种方式都是可以的
    # fk_driver = db.relationship("WDriver", foreign_keys='WDriver.fk_user_id', uselist=False)
    # fk_driver = db.relationship("WDriver", primaryjoin=lambda: WDriver.fk_user_id == WUser.id)
    fk_driver = db.relationship("WDriver", primaryjoin=WDriver.fk_user_id == id)
    # fk_driver = db.relationship("WDriver", primaryjoin='WDriver.fk_user_id == WUser.id')
    # fk_driver = db.relationship("WDriver", backref=db.backref('test.user', lazy=True),
    #                             primaryjoin=lambda: WDriver.fk_user_id == WUser.id)



r_user_obj = RUser.query.filter_by().first()
print("r_user_obj:", r_user_obj)
print("r_user_obj.driver_fk:", r_user_obj.fk_driver)
w_user_obj = WUser.query.filter_by(id=2188).first()
print("w_user_obj:", w_user_obj)
print("w_user_obj.driver_fk:", w_user_obj.fk_driver)

参考文档:

* https://docs.sqlalchemy.org/en/13/orm/relationship_api.html # 值得细看
* https://www.osgeo.cn/sqlalchemy/orm/relationship_api.html # 同上, 中文
* https://www.cnblogs.com/srd945/p/9851227.html
* extend_existing: (False) 当表已经存在于元数据中时，如果元数据中存在与 column_list 中的列同名的列，column_list 中同名的列会替换掉元数据中已经有的列
* useexisting 已被废弃, 新版本使用 extend_existing

总结

关系配置参数真的很多, 如下, 很容易就会出错, 需要多读读官方文档, 还有就是建立 modle 时候尽量简洁, 风格统一, 不要在数据库层建立外键.

sqlalchemy.orm.relationship(argument, secondary=None, primaryjoin=None, secondaryjoin=None, foreign_keys=None,
uselist=None, order_by=False, backref=None, back_populates=None, post_update=False, cascade=False, extension=None,
viewonly=False, lazy='select', collection_class=None, passive_deletes=False, passive_updates=True, remote_side=None,
enable_typechecks=True, join_depth=None, comparator_factory=None, single_parent=False, innerjoin=False,
distinct_target_key=None, doc=None, active_history=False, cascade_backrefs=True, load_on_pending=False,
bake_queries=True, _local_remote_pairs=None, query_class=None, info=None, omit_join=None)