Thursday, September 28, 2023

Using SQLAlchemy 2.0 in Flask

Way back in January, the very popular Python ORM SQLAlchemy released version 2.0. This version makes SQLAlchemy code much more compatible with Python type checkers.

Typed model classes

Here's a SQLAlchemy 2.0 model with typed columns:

class BlogPost(Base):
    __tablename__ = "blog_post"
    id: Mapped[int] = mapped_column(primary_key=True)
    title: Mapped[str] = mapped_column(String(30))
    content: Mapped[str]

When you're using an IDE that understands type annotations (like VS Code with the Python extension), you can then get intellisense for those columns, like suggestions for functions that can be called on that data type.

Screenshot of intellisense suggestion for id column

You can also run a tool like mypy or pyright to find out if any of your code is using types incorrectly. For example, imagine I wrote a function to process the BlogPost model above:

def process_blog_posts(posts: list[BlogPost]):
    for post in posts:
        post.title = post.title.upper() =

Then running mypy would let me know if my code was using the typed columns incorrectly:

$ python3 -m mypy error: "int" has no attribute "upper"  [attr-defined]

Adding support to Flask-SQLAlchemy

I have recently begun to use type annotations more heavily in my code (especially for class and function signatures) so I was excited to try out SQLAlchemy 2.0. But then I realized that almost all of my usage of SQLAlchemy 2.0 was inside Flask apps, using the Flask-SQLAlchemy extension, and at the time, it did not support SQLAlchemy 2.0. What's a girl to do? Add support for it, of course!

I experimented with several ways to support SQLAlchemy 2.0 and eventually settled on a proposal that would be compatible with (hopefully all) the ways to customize SQLAlchemy 2.0 base classes. You can can choose for their base class to inherit from DeclarativeBase or DeclarativeBaseNoMeta, and you can add on MappedAsDataclass if they'd like to use dataclass-like data models.

A few examples:

class Base(DeclarativeBase):
db = SQLAlchemy(model_class=Base)

class Todo(db.Model):
     id: Mapped[int] = mapped_column(primary_key=True)
     title: Mapped[str] = mapped_column(nullable=True)
class Base(DeclarativeBase, MappedAsDataclass):
db = SQLAlchemy(model_class=Base)
class Todo(db.Model):
    id: Mapped[int] = mapped_column(init=False, primary_key=True)
    title: Mapped[str] = mapped_column(default=None)

The pull request was rather large, since we decided to default the documentation to 2.0 style classes, plus I parameterized every test to check all the possible base classes. Thanks to helpful reviews from the community (especially lead Flask maintainer David Lord), we were able to merge the PR and release SQLAlchemy 2.0 support on September 11th.

Porting Flask apps to SQLAlchemy 2.0

Since the release, I've been happily porting sample Flask applications over to use the new style models in SQLAlchemy 2.0, and also using the opportunity to make sure our code doesn't use the legacy way of querying data as well.

Here are a few pull requests that show the changes needed:

Of course, as those are samples, there wasn't a lot of code to change. In a complex production codebase, it will be a much bigger change to upgrade all your models. Hopefully you have tests written before making the change, so you can ensure they're made in a backwards compatible way.

Additional resources

As you're upgrading your models to new-style models, make sure you look through both the SQLAlchemy docs and the Flask-SQLAlchemy docs for examples of what you're trying to accomplish. You can even search through each GitHub repository for additional examples, as some situations that aren't in the docs are still covered in unit tests. The SQLAlchemy docs can be daunting in their scope, so I recommend bookmarking their ORM quickstart and Migration cheatsheet.

In addition to those docs, check out this great summary from Miguel Grinberg on the 2.0 changes. If you prefer learning via video, check out my video series about SQLAlchemy 2.0 on the VS Code channel.

If you do run into any issues with porting your Flask app to SQLAlchemy 2.0, try to figure out first if it's a Flask-SQLAlchemy issue or a core SQLAlchemy issue. Many of the Flask-SQLAlchemy issue reports are in fact just SQLAlchemy issues. You can discuss SQLAlchemy issues in their GitHub discussions and discuss Flask-SQLAlchemy issues in our GitHub discussions or Discord.

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