How to re-use SQLAlchemy models across multiple tables

Posted: April 22 2021

Source Code

When using SQLAlchemy declarative models you might find yourself wanting to re-use a model for multiple tables, this post will show you how to do it.

A new base model

A lot of tables share common columns, like an id, last_updated, etc... To handle this we can declare a new base model that all others inherit from:

def snake_caser(text):
    Converts camelCase/PascalCase to snake_case
    return re.sub(r'(?<!^)(?=[A-Z])', '_', text).lower()

class CustomBase(db.Base):
    This is our customised base table
    It includes a table name generator that converts the ClassName to class_name
    an ID primary key column and a basic repr() method

    Any models that inherit from this will automatically get these things
    __abstract__ = True

    def __tablename__(cls):
        return snake_caser(cls.__name__)

    id = Column(Integer, primary_key=True)

    def __repr__(self):
        return f'<{self.__class__.__name__} {}>'

The key part to this is __abstract__ = True, this tells SQLAlchemy that this isn't a real table (this is documented here).

This base also includes an id column, a basic repr method and a __tablename__ declared attribute (more on this in a bit) that takes the class name and converts it to snake case.

The re-usable models

Now we can create our abstract base for our real tables:

class UserBase(CustomBase):
    __abstract__ = True

    label = ''

    # Columns
    name = Column(String(40))
    email = Column(String(120))

    # Relationship
    def posts(cls):
        return relationship(f'Post{cls.label}', backref=cls.__tablename__, lazy=True)

class PostBase(CustomBase):
    __abstract__ = True

    label = ''

    # Columns
    body = Column(Text(240))
    private = Column(Boolean)

    def user_id(cls):
        return Column(Integer, ForeignKey(f'user_{snake_caser(cls.label)}.id'))

These are both similar to the original CustomBase but have dynamic foreign keys and relationships based on the label property.

These are declared using the @declared_attr decorator which let's us use methods as attributes, which let's us dynamically create the mappings (documented here).

Now we can declare our real models, inheriting from UserBase and PostBase and all they need is a label property:

# ProductA Models
class UserProductA(UserBase):
    This extends the UserBase model so is a copy of that but with the table name user_product_a

    We can overwrite or extend however we want but the only the we NEED is to define the label
    The label should be cased the same as the class name
    label = 'ProductA'

class PostProductA(PostBase):
    label = 'ProductA'

It's as easy as that, if you add a label property to the child models everything will just work ™️.

UserProductA will have a relationship with PostProductA and PostProductA will have a foreign key to user_product_a.