bfaivre / alembic-sqlalchemy-demo

Alembic & SQLAlchemy Demo

A simple demo of using Alembic and SQLAlchemy.

Prerequisites

This demo assumes you have access to a MariaDB or MySQL database. Create an empty database named "Demo" on your server.

Steps

1. Clone the repository:

$ git clone https://github.com/bfaivre/alembic-sqlalchemy-demo

2. Change directory into it:

$ cd alembic-sqlalchemy-demo

3. Create a virtual environment:

$ python -m venv venv

4. Activate the newly created virtual environment (Windows):

$ .\venv\Scripts\activate

5. Install pip packages:

(venv) $ pip install -r requirements.txt

6. Edit .env and set DATABASE, PASSWORD, SERVER and USER variables for your MariaDB/MySQL server. For example:

DATABASE=Demo
PASSWORD=root
SERVER=localhost
USER=root

7. Initialize Alembic:

(venv) $ alembic init alembic

8. Edit alembic.ini and set sqlalchemy.url for your environment. For example mysql://root:root@localhost/Demo for MariaDB/MySQL, username "root", password "root, server "localhost" and database "Demo".

9. Edit env.py in the alembic folder to read our models.py file for generating scripts. Comment out "target_metadata = None" and add the following below:

import os
import sys
sys.path.append(os.getcwd())
import models
target_metadata = models.Base.metadata

10. Take a look at the models.py file. We have defined a simple table named "Fv". Create the initial database migration script based off of the models.py file:

(venv) $ alembic revision --autogenerate -m "Initial migration."

10. Run the the initial database migration script to create the "Fv" table:

(venv) $ alembic upgrade head

11. At this point the database "Demo" should now contain a table for "Fv" and "alembic_version". You'll also notice that a script has been created in folder alembic/versions if you want to inspect/modify the upgrade and/or downgrade commands.

12. Edit models.py and uncomment the following lines to add a new table named "Gravity":

    # Gravities = relationship("Gravity", backref = "FV", lazy = "dynamic")

# class Gravity(Base):
#     __tablename__ = "Gravity"
#     GravityId = Column(Integer, primary_key = True)
#     FvId = Column(Integer, ForeignKey("Fv.FvId"), nullable = False) 
#     Gravity = Column(Float, nullable = False)
#     Timestamp = Column(DATETIME(fsp = 6), nullable = False)

13. Let's create our next database migration script to add the changes to our database from the modification of the models.py file in the step above:

(venv) $ alembic revision --autogenerate -m "Added Gravity table."

14. Let's run the alembic upgrade command to update our database with the Gravity table:

(venv) $ alembic upgrade head

15. Refresh your database and confirm that the Gravity table was added.

16. To downgrade to the previous version of the database (without the Gravity table), run the alembic downgrade command:

(venv) $ alembic downgrade -1

17. Confirm that the Gravity table was removed.

18. Run the alembic upgrade command one more time to prepare for using the SQLAlchemy demo scripts:

(venv) $ alembic upgrade head

19. At this point the database "Demo" should now contain a table for "Fv", "Gravity" and "alembic_version". Now you can modify and run demo.py to show some simple examples and demoReflection.py to demonstrate the use of SQLAlchemy if you don't have a models file that you've been maintaining for a database that you'd like to manipulate using SQLAlchemy and Python:

(venv) $ python demo.py
(venv) $ python demoReflection.py