Python SQLAlchemy - 什么是 declarative_base
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SQLAlchemy - what is declarative_base
提问by Aniruddha
I am learning sqlalchemy. Here is my initial code :
我正在学习 sqlalchemy。这是我的初始代码：
File : user.py
文件：user.py
from sqlalchemy import Column,Integer,Sequence, String
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class User(Base):
__tablename__ = 'users'
id = Column(Integer,Sequence('user_seq'),primary_key=True)
username = Column(String(50),unique=True)
fullname = Column(String(150))
password = Column(String(50))
def __init__(self,name,fullname,password):
self.name = name
self.fullname = fullname
self.password = password
File main.py
文件main.py
from sqlalchemy.orm import sessionmaker
from sqlalchemy import create_engine
from user import User
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
if __name__ == '__main__':
engine = create_engine('mysql://root:[email protected]:3306/test',echo=True)
Base.metadata.create_all(engine, checkfirst=True)
Session = sessionmaker(bind=engine)
session = Session()
ed_user = User('ed', 'Ed Jones', 'edspassword')
session.add(ed_user)
session.commit()
Now when I run main.py. It wont create table automatically & give me exception on 'session.commit()'. Now when I move line 'Base = declarative_base()' to different module & use same 'Base' variable in main.py & user.py. It create table.
现在当我运行 main.py 时。它不会自动创建表并在“session.commit()”上给我例外。现在，当我将“Base = declarative_base()”行移动到不同的模块并在 main.py 和 user.py 中使用相同的“Base”变量时。它创建表。
My question is what is 'declarative_base' ?
我的问题是什么是 'declarative_base' ？
采纳答案by Audrius Ka?ukauskas
declarative_base()is a factory function that constructs a base class for declarative class definitions (which is assigned to the Basevariable in your example). The one you created in user.py is associated with the Usermodel, while the other one (in main.py) is a different class and doesn't know anything about your models, that's why the Base.metadata.create_all()call didn't create the table. You need to import Basefrom user.py
declarative_base()是一个工厂函数，它为声明性类定义（Base在您的示例中分配给变量）构造基类。您在 user.py 中创建的一个与User模型相关联，而另一个（在 main.py 中）是一个不同的类并且对您的模型一无所知，这就是Base.metadata.create_all()调用没有创建表的原因。您需要Base从 user.py导入
from user import User, Base
instead of creating a new Baseclass in main.py.
而不是Base在 main.py中创建一个新类。
