I am trying to create an application using SQLAlchemy. It worked fine as long as I only had one file with one Class. Now I want to have multiple classes/tables in different files. I stumbled upon this question, and tried to do it like it was suggested there: I now have three files
base.py
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
blind.py
from sqlalchemy import Column, String
from .base import Base
class Blind(Base):
__tablename__ = 'blinds'
blind = Column(String)
data_processor_uuid = Column(String, primary_key=True)
data_source_uuid = Column(String)
timestamp = Column(String, primary_key=True)
and data.py
from sqlalchemy import Column, Integer, String, Float
from .base import Base
class Datum(Base):
__tablename__ = 'data'
data_source_uuid = Column(Integer, primary_key=True)
sensor_type = Column(String)
timestamp = Column(String, primary_key=True)
value = Column(Float)
I now want to initialize the database in db_setup.py using
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from .base import Base
engine = create_engine('sqlite:///test.db', echo=True)
Base.metadata.bind = engine
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)
session = Session()
def get_db_session():
return session
This works, however, it does not create the tables in the database as expected. When I try to insert something into the table, I get an error saying "table does not exist". Can someone tell me what I am doing wrong here?
The problem was that I wasn't importing the class definitions for Blinds and Datum anywhere, so they weren't evaluated! Before I split them up into different files, I had imported them to get to Base. Thanks to #IljaEverilä for this answer!
Related
I have an existing model that I can't change, written in Flask-SQLAlchemy.
I'm writing another app that uses the same model, but without the need for Flask, therefore I'm working with the regular SQLAlchemy module.
Unfortunately, I'm getting a lot of:
'AttributeError: module 'DB' has no attribute 'Model'
for all kind of attributes - such as Column, Integer, etc
Is there a way to use Flask-SQLAlchemy with a regular SQLAlchemy app?
There is an example of one of my Model Class:
class Table_name(Model):
__tablename__ = 'table_name'
id = db.Column(db.INTEGER, primary_key=True)
field1 = db.Column(db.INTEGER, db.ForeignKey('table1.id'), nullable=False)
field2 = db.Column(db.TEXT, nullable=False)
field3 = db.Column(db.INTEGER, db.ForeignKey('table2.id'), nullable=False)
time = db.Column(db.TIMESTAMP, nullable=False)
Unfortunately I can't change them
I've had the same dilemma. If it's a small or one-off, you can hack in a Flask app object without really using it, like so:
from flask_sqlalchemy import SQLAlchemy
throw_away_app = Flask(__name__)
db = SQLAlchemy(throw_away_app)
with self.throw_away_app.app_context():
(perform your db operation)
That works relatively well for simple things like scripts. But if you're sharing a model across multiple projects and you simply cannot alter the existing Flask project, it's unfortunately probably not a good solution.
If that's the case, and you simply cannot alter the existing codebase at all, it probably makes sense to create a new model class and connect to the existing database using vanilla SQLAlchemy.
BTW, for any future programmers wondering how to get some of the benefits of Flask-SQLAlchemy without Flask, consider: sqlalchemy-mixins.
from sqlalchemy import create_engine
from sqlalchemy import Column, Integer, String, Text, TIMESTAMP, ForeignKey
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker
Base = declarative_base()
# connect in memory sqlite database or you can connect your own database
engine = create_engine('sqlite:///:memory:', echo=True)
# create session and bind engine
Session = sessionmaker(bind=engine)
class Table_name(Base):
__tablename__ = 'table_name'
id = Column(Integer, primary_key=True)
field1 = Column(Integer, ForeignKey('table1.id'), nullable=False)
field2 = Column(Text, nullable=False)
field3 = Column(Integer, ForeignKey('table2.id'), nullable=False)
time = Column(TIMESTAMP, nullable=False)
table = Table_name(field1=1, fi....)
session.add(table)
Now you can use your ORM as usual like flask-sqlalchemy .
Docs: https://docs.sqlalchemy.org/en/13/orm/tutorial.html
I'm using SQLAlchemy ORM for a Flask project where I want to join across an eagerly loaded model but this leads to two joins to the same intermediary model. If you run the code below you'll see in the generated SQL that there are two joins between the Author model and the Book model. If the lazy=joined bit is removed the sql generated is perfect.
I don't know if I'm doing something wrong or this is by design. How do I get SQLAlchemy to emit the right SQL while maintaining the joinedload in this case?
Note: I have tried this with MySQL and SQLite and it happens with both those dbs.
from sqlalchemy import create_engine, Integer, String, Column
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import ForeignKey
from sqlalchemy.orm import relationship
engine = create_engine('sqlite:///:memory:', echo=True)
Session = sessionmaker(bind=engine)
Base = declarative_base()
from sqlalchemy import create_engine, Integer, String, Column
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import ForeignKey
from sqlalchemy.orm import relationship
engine = create_engine('sqlite:///:memory:', echo=True)
Session = sessionmaker(bind=engine)
Base = declarative_base()
class Author(Base):
__tablename__ = 'authors'
id = Column(Integer, primary_key=True)
name = Column(String)
pseudo = Column(String)
books = relationship("Book", lazy='joined')
def __repr__(self):
return "<User(name='%s', fullname='%s', password='%s')>" % (
self.name, self.fullname, self.password)
class Book(Base):
__tablename__ = 'books'
id = Column(Integer, primary_key=True)
author_id = Column(Integer, ForeignKey('authors.id'))
name = Column(String)
user = relationship("Author", back_populates="books")
pages = relationship("Page")
class Page(Base):
__tablename__ = 'pages'
id = Column(Integer, primary_key=True)
book_id = Column(Integer, ForeignKey('books.id'))
text = Column(String)
book = relationship("Book", back_populates="pages")
Base.metadata.create_all(engine)
session = Session()
print(str(session.query(Author).outerjoin(Author.books, Page)))
It is by design – read The Zen of Joined Eager Loading:
It is critical to understand the distinction that while Query.join() is used to alter the results of a query, joinedload() goes through great lengths to not alter the results of the query, and instead hide the effects of the rendered join to only allow for related objects to be present.
There are multiple somewhat similar questions in sqlalchemy, though couldn't find one that'd fit the bill exactly.
If you manually add a join, and want to use it to eager load a relationship as well, you need contains_eager():
session.query(Author).\
outerjoin(Author.books, Book.pages).\
options(contains_eager(Author.books).contains_eager(Book.pages))
Note that the relationship definitions Author.books and Book.pages would seem to be missing the back_populates= argument.
i'm using sqlalchemy + alembic + Flask and i can't map circular classes.
apps/users/models.py:
class User(Base):
__tablename__ = 'users'
id = Column(Integer, primary_key=True)
email = Column(String)
password = Column(String)
session = relationship("Session", back_populates='user', cascade='all,delete', lazy='dynamic')
notes = relationship('Note2User', back_populates='user', cascade='all,delete', lazy='dynamic')
apps/notes/models.py:
class Note2User(Base):
__tablename__ = 'notes_users_m2m'
id = Column(Integer, primary_key=True)
user_id = Column(Integer, ForeignKey('users.id', ondelete='CASCADE'), nullable=False)
user = relationship('User', back_populates='notes')
note_id = Column(Integer, ForeignKey('notes.id', ondelete='CASCADE'), nullable=False)
note = relationship('Note', back_populates='users')
Table Note2User made for m2m relationship User <-> Notes, but when i start app and done some request, gets error:
InvalidRequestError: When initializing mapper Mapper|User|users,
expression 'Note2User' failed to locate a name ("name 'Note2User' is
not defined"). If this is a class name, consider adding this
relationship() to the class after
both dependent classes have been defined.
Initializing db in db/init.py: (dunder name)
from sqlalchemy import create_engine, MetaData
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import Session
engine = create_engine('postgresql+psycopg2://server:12345#localhost:5432/test')
Base = declarative_base()
meta = MetaData()
meta.reflect(bind=engine)
db_session = Session(bind=engine)
Add an import for Note2User class in apps/users/models.py file so this model gets defined first before initializing that relatioship in User class which refrences it.
like this
# file: apps/users/models.py
from ..notes.models import Note2User
You need to import the user.models module into the notes.model module and vice versa. It would look something like this:
# file app/users/models.py
import app.notes.models as notes
# use it like this
notes.Notes2User()
# file app/notes/models.py
import app.users.models as users
users.User()
The advantage to this is that you will avoid circular dependency problems as your program inevitably grows. I had so many problems with circular dependencies when I was creating an app with your same stack. The only solution was to ditch the
from . import Foo
and only use
import bar.foo as foo
It is considered best practice to use the import syntax for this reason.
Reference.
When I use sqlAlchemy to read data from mysql DB, it work when my class and the search code are in the same py file just like this:
class odd_basis(Base):
__tablename__ = 'odd_basis'
NO = Column(Integer, primary_key=True)
LEAGUE = Column(String(20))
TIME = Column(String(20))
engine = create_engine("......")
session = sessionmaker(bind=engine)
DBsession = session()
basis_data = DBsession.query(odd_basis).filter(odd_basis.NO=='16039').all()
when I put the class definition in another py file, then use the sys.path.append to import, then it will have this error:
sqlalchemy.exc.ArgumentError: SQL expression object or string expected, got object of type instead
my main code:
import sys
sys.path.append("~/analysis/table_class.py")
engine = create_engine("......")
session = sessionmaker(bind=engine)
DBsession = session()
basis_data = DBsession.query(odd_basis).filter(odd_basis.NO=='16039').all()
table_class.py:
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, Numeric, ForeignKey, UniqueConstraint, Index
Base = declarative_base()
class odd_basis(Base):
__tablename__ = 'odd_basis'
NO = Column(Integer, primary_key=True)
LEAGUE = Column(String(20))
TIME = Column(String(20))
Sys.path.append does not import anything. It just adds to your search path. You need to import your class from there:
from table_class import odd_basis
and so forth.
Hannu
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
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)