SQLAlchemy – what is declarative_base
Question:
I am learning sqlalchemy
.
Here is my initial code :
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
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()
When I run main.py, it won’t create tables automatically and gives me an exception on session.commit()
.
Also, when I move line Base = declarative_base()
to any different module and use the same Base
variable in main.py and in user.py – it creates the table.
My question: "What is declarative_base
" ?
Answers:
declarative_base()
is a factory function that constructs a base class for declarative class definitions (which is assigned to the Base
variable in your example). The one you created in user.py is associated with the User
model, 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 Base
from user.py
from user import User, Base
instead of creating a new Base
class in main.py.
I am learning sqlalchemy
.
Here is my initial code :
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
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()
When I run main.py, it won’t create tables automatically and gives me an exception on session.commit()
.
Also, when I move line Base = declarative_base()
to any different module and use the same Base
variable in main.py and in user.py – it creates the table.
My question: "What is declarative_base
" ?
declarative_base()
is a factory function that constructs a base class for declarative class definitions (which is assigned to the Base
variable in your example). The one you created in user.py is associated with the User
model, 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 Base
from user.py
from user import User, Base
instead of creating a new Base
class in main.py.