Yes, every time, the first file is imported, the tables are created again.
To prevent this, use the following statements:
if __name__ == '__main__':
Base.metadata.create_all(engine)
Pregunta
I took a look at the tutorial here: http://www.pythoncentral.io/introductory-tutorial-python-sqlalchemy/
The decisive parts for my question are first:
import os
import sys
from sqlalchemy import Column, ForeignKey, Integer, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship
from sqlalchemy import create_engine
Base = declarative_base()
class Person(Base):
__tablename__ = 'person'
# Here we define columns for the table person
# Notice that each column is also a normal Python instance attribute.
id = Column(Integer, primary_key=True)
name = Column(String(250), nullable=False)
class Address(Base):
__tablename__ = 'address'
# Here we define columns for the table address.
# Notice that each column is also a normal Python instance attribute.
id = Column(Integer, primary_key=True)
street_name = Column(String(250))
street_number = Column(String(250))
post_code = Column(String(250), nullable=False)
person_id = Column(Integer, ForeignKey('person.id'))
person = relationship(Person)
# Create an engine that stores data in the local directory's
# sqlalchemy_example.db file.
engine = create_engine('sqlite:///sqlalchemy_example.db')
# Create all tables in the engine. This is equivalent to "Create Table"
# statements in raw SQL.
Base.metadata.create_all(engine)
and second:
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy_declarative import Address, Base, Person
engine = create_engine('sqlite:///sqlalchemy_example.db')
# Bind the engine to the metadata of the Base class so that the
# declaratives can be accessed through a DBSession instance
Base.metadata.bind = engine
DBSession = sessionmaker(bind=engine)
# A DBSession() instance establishes all conversations with the database
# and represents a "staging zone" for all the objects loaded into the
# database session object. Any change made against the objects in the
# session won't be persisted into the database until you call
# session.commit(). If you're not happy about the changes, you can
# revert all of them back to the last commit by calling
# session.rollback()
session = DBSession()
# Insert a Person in the person table
new_person = Person(name='new person')
session.add(new_person)
session.commit()
# Insert an Address in the address table
new_address = Address(post_code='00000', person=new_person)
session.add(new_address)
session.commit()
If the second part is run (e.g. "python second_part"), won't the import statement
from sqlalchemy_declarative import Address, Base, Person
cause all executable code in the first part to run? (Which would trigger the code which creates tables in a db each time the first part is run)
Best regards
Solución
Yes, every time, the first file is imported, the tables are created again.
To prevent this, use the following statements:
if __name__ == '__main__':
Base.metadata.create_all(engine)