What is the difference between the declarative_base() and db.Model?

Question

The quickstart tutorial for the Flask-SQLAlchemy plugin instructs users to create table models inheriting the db.Model class, e.g.

app = Flask(__main__)
db = SQLAlchemy(app)
class Users(db.Model):
    __tablename__ = 'users'
    ...

However, the SQLAlchemy tutorial and the bottle-SQLAlchemy README both suggest that table models inherit a Base instantiated from declarative_base().

Base = declarative_base()
class Users(Base):
    __tablename__ = 'users'
    ...

What is the difference between these two approaches?

Asked By: drs

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Source

Answer 1

Looking in the Flask-SQLAlchemy source code the db.Model class is initialized as follows:

self.Model = self.make_declarative_base()

And here is the make_declarative_base() method:

def make_declarative_base(self):
    """Creates the declarative base."""
    base = declarative_base(cls=Model, name='Model',
                            metaclass=_BoundDeclarativeMeta)
    base.query = _QueryProperty(self)
    return base

The _BoundDeclarativeMeta metaclass is a subclass of SQLAlchemy’s DeclarativeMeta, it simply adds support for computing a default value for __tablename__ (the table name) and also to handle binds.

The base.query property enables Flask-SQLAlchemy based models to access a query object as Model.query instead of SQLAlchemy’s session.query(Model).

The _QueryProperty query class is also subclassed from SQLAlchemy’s query. The Flask-SQLAlchemy subclass adds three additional query methods that do not exist in SQLAlchemy: get_or_404(), first_or_404() and paginate().

I believe these are the only differences.

Answered By: Miguel Grinberg

What is the difference between the declarative_base() and db.Model?

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