Inheritance

Joined Table

SQLAlchemy includes a feature where a model class hierarchy is implemented by giving each class it’s own table. For example, an Employee might have an id, name and type. Then a Manager model might be defined that derives from Employee and adds the manager_data column.

See also

Joined Table Inheritance

OpenAlchemy documentation for joined table inheritance.

SQLAlchemy joined table inheritance documentation

Documentation for SQLAlchemy joined table inheritance.

The following example defines joined tabled inheritance where Employee is the parent and Manager and Engineer derive from it:

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openapi: "3.0.0"

info:
  title: Test Schema
  description: API to illustrate OpenAlchemy joined inheritance.
  version: "0.1"

paths:
  /employee:
    get:
      summary: Used to retrieve all employees.
      responses:
        200:
          description: Return all employees from the database.
          content:
            application/json:
              schema:
                type: array
                items:
                  "$ref": "#/components/schemas/Employee"
  /manager:
    get:
      summary: Used to retrieve all managers.
      responses:
        200:
          description: Return all managers from the database.
          content:
            application/json:
              schema:
                type: array
                items:
                  "$ref": "#/components/schemas/Manager"
  /engineer:
    get:
      summary: Used to retrieve all engineers.
      responses:
        200:
          description: Return all engineers from the database.
          content:
            application/json:
              schema:
                type: array
                items:
                  "$ref": "#/components/schemas/Engineer"

components:
  schemas:
    Employee:
      description: Person that works for a company.
      type: object
      x-tablename: employee
      properties:
        id:
          type: integer
          description: Unique identifier for the employee.
          example: 0
          x-primary-key: true
        name:
          type: string
          description: The name of the employee.
          example: David Andersson
        type:
          type: string
          description: The type of the employee.
          example: employee
      x-kwargs:
        __mapper_args__:
          polymorphic_on: type
          polymorphic_identity: employee
    Manager:
      allOf:
        - $ref: "#/components/schemas/Employee"
        - x-inherits: true
          x-tablename: manager
          type: object
          properties:
            id:
              type: integer
              description: Unique identifier for the manager.
              example: 0
              x-primary-key: true
              x-foreign-key: employee.id
            manager_data:
              type: string
              description: Data for the manager.
              example: some data
          x-kwargs:
            __mapper_args__:
              polymorphic_identity: manager
    Engineer:
      allOf:
        - $ref: "#/components/schemas/Employee"
        - x-inherits: true
          x-tablename: engineer
          type: object
          properties:
            id:
              type: integer
              description: Unique identifier for the engineer.
              example: 0
              x-primary-key: true
              x-foreign-key: employee.id
            engineer_info:
              type: string
              description: Information for the manager.
              example: some info
          x-kwargs:
            __mapper_args__:
              polymorphic_identity: engineer

The following file uses OpenAlchemy to generate the SQLAlchemy models:

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from open_alchemy import init_yaml

init_yaml("joined-example-spec.yml", models_filename="joined_models_auto.py")

The SQLAlchemy models generated by OpenAlchemy are equivalent to the following traditional models file:

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import sqlalchemy as sa
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()


class Employee(Base):
    """Person that works for a company."""

    __tablename__ = "employee"
    id = sa.Column(sa.Integer, primary_key=True)
    name = sa.Column(sa.String)
    type = sa.Column(sa.String)

    __mapper_args__ = {"polymorphic_on": type, "polymorphic_identity": "employee"}


class Manager(Employee):
    """Person that works for a company."""

    __tablename__ = "manager"
    id = sa.Column(sa.Integer, sa.ForeignKey("employee.id"), primary_key=True)
    manager_data = sa.Column(sa.String)

    __mapper_args__ = {"polymorphic_identity": "manager"}


class Engineer(Employee):
    """Person that works for a company."""

    __tablename__ = "engineer"
    id = sa.Column(sa.Integer, sa.ForeignKey("employee.id"), primary_key=True)
    engineer_info = sa.Column(sa.String)

    __mapper_args__ = {"polymorphic_identity": "engineer"}

OpenAlchemy will generate the following typed models:

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"""Autogenerated SQLAlchemy models based on OpenAlchemy models."""
# pylint: disable=no-member,super-init-not-called,unused-argument

import typing

import sqlalchemy
from sqlalchemy import orm

from open_alchemy import models

Base = models.Base  # type: ignore


class EmployeeDict(typing.TypedDict, total=False):
    """TypedDict for properties that are not required."""

    id: typing.Optional[int]
    name: typing.Optional[str]
    type: typing.Optional[str]


class TEmployee(typing.Protocol):
    """
    SQLAlchemy model protocol.

    Person that works for a company.

    Attrs:
        id: Unique identifier for the employee.
        name: The name of the employee.
        type: The type of the employee.

    """

    # SQLAlchemy properties
    __table__: sqlalchemy.Table
    __tablename__: str
    query: orm.Query

    # Model properties
    id: "sqlalchemy.Column[typing.Optional[int]]"
    name: "sqlalchemy.Column[typing.Optional[str]]"
    type: "sqlalchemy.Column[typing.Optional[str]]"

    def __init__(
        self,
        id: typing.Optional[int] = None,
        name: typing.Optional[str] = None,
        type: typing.Optional[str] = None,
    ) -> None:
        """
        Construct.

        Args:
            id: Unique identifier for the employee.
            name: The name of the employee.
            type: The type of the employee.

        """
        ...

    @classmethod
    def from_dict(
        cls,
        id: typing.Optional[int] = None,
        name: typing.Optional[str] = None,
        type: typing.Optional[str] = None,
    ) -> "TEmployee":
        """
        Construct from a dictionary (eg. a POST payload).

        Args:
            id: Unique identifier for the employee.
            name: The name of the employee.
            type: The type of the employee.

        Returns:
            Model instance based on the dictionary.

        """
        ...

    @classmethod
    def from_str(cls, value: str) -> "TEmployee":
        """
        Construct from a JSON string (eg. a POST payload).

        Returns:
            Model instance based on the JSON string.

        """
        ...

    def to_dict(self) -> EmployeeDict:
        """
        Convert to a dictionary (eg. to send back for a GET request).

        Returns:
            Dictionary based on the model instance.

        """
        ...

    def to_str(self) -> str:
        """
        Convert to a JSON string (eg. to send back for a GET request).

        Returns:
            JSON string based on the model instance.

        """
        ...


Employee: typing.Type[TEmployee] = models.Employee  # type: ignore


class ManagerDict(typing.TypedDict, total=False):
    """TypedDict for properties that are not required."""

    id: typing.Optional[int]
    manager_data: typing.Optional[str]
    name: typing.Optional[str]
    type: typing.Optional[str]


class TManager(typing.Protocol):
    """
    SQLAlchemy model protocol.

    Person that works for a company.

    Attrs:
        id: Unique identifier for the manager.
        manager_data: Data for the manager.
        name: The name of the employee.
        type: The type of the employee.

    """

    # SQLAlchemy properties
    __table__: sqlalchemy.Table
    __tablename__: str
    query: orm.Query

    # Model properties
    id: "sqlalchemy.Column[typing.Optional[int]]"
    manager_data: "sqlalchemy.Column[typing.Optional[str]]"
    name: "sqlalchemy.Column[typing.Optional[str]]"
    type: "sqlalchemy.Column[typing.Optional[str]]"

    def __init__(
        self,
        id: typing.Optional[int] = None,
        manager_data: typing.Optional[str] = None,
        name: typing.Optional[str] = None,
        type: typing.Optional[str] = None,
    ) -> None:
        """
        Construct.

        Args:
            id: Unique identifier for the manager.
            manager_data: Data for the manager.
            name: The name of the employee.
            type: The type of the employee.

        """
        ...

    @classmethod
    def from_dict(
        cls,
        id: typing.Optional[int] = None,
        manager_data: typing.Optional[str] = None,
        name: typing.Optional[str] = None,
        type: typing.Optional[str] = None,
    ) -> "TManager":
        """
        Construct from a dictionary (eg. a POST payload).

        Args:
            id: Unique identifier for the manager.
            manager_data: Data for the manager.
            name: The name of the employee.
            type: The type of the employee.

        Returns:
            Model instance based on the dictionary.

        """
        ...

    @classmethod
    def from_str(cls, value: str) -> "TManager":
        """
        Construct from a JSON string (eg. a POST payload).

        Returns:
            Model instance based on the JSON string.

        """
        ...

    def to_dict(self) -> ManagerDict:
        """
        Convert to a dictionary (eg. to send back for a GET request).

        Returns:
            Dictionary based on the model instance.

        """
        ...

    def to_str(self) -> str:
        """
        Convert to a JSON string (eg. to send back for a GET request).

        Returns:
            JSON string based on the model instance.

        """
        ...


Manager: typing.Type[TManager] = models.Manager  # type: ignore


class EngineerDict(typing.TypedDict, total=False):
    """TypedDict for properties that are not required."""

    id: typing.Optional[int]
    engineer_info: typing.Optional[str]
    name: typing.Optional[str]
    type: typing.Optional[str]


class TEngineer(typing.Protocol):
    """
    SQLAlchemy model protocol.

    Person that works for a company.

    Attrs:
        id: Unique identifier for the engineer.
        engineer_info: Information for the manager.
        name: The name of the employee.
        type: The type of the employee.

    """

    # SQLAlchemy properties
    __table__: sqlalchemy.Table
    __tablename__: str
    query: orm.Query

    # Model properties
    id: "sqlalchemy.Column[typing.Optional[int]]"
    engineer_info: "sqlalchemy.Column[typing.Optional[str]]"
    name: "sqlalchemy.Column[typing.Optional[str]]"
    type: "sqlalchemy.Column[typing.Optional[str]]"

    def __init__(
        self,
        id: typing.Optional[int] = None,
        engineer_info: typing.Optional[str] = None,
        name: typing.Optional[str] = None,
        type: typing.Optional[str] = None,
    ) -> None:
        """
        Construct.

        Args:
            id: Unique identifier for the engineer.
            engineer_info: Information for the manager.
            name: The name of the employee.
            type: The type of the employee.

        """
        ...

    @classmethod
    def from_dict(
        cls,
        id: typing.Optional[int] = None,
        engineer_info: typing.Optional[str] = None,
        name: typing.Optional[str] = None,
        type: typing.Optional[str] = None,
    ) -> "TEngineer":
        """
        Construct from a dictionary (eg. a POST payload).

        Args:
            id: Unique identifier for the engineer.
            engineer_info: Information for the manager.
            name: The name of the employee.
            type: The type of the employee.

        Returns:
            Model instance based on the dictionary.

        """
        ...

    @classmethod
    def from_str(cls, value: str) -> "TEngineer":
        """
        Construct from a JSON string (eg. a POST payload).

        Returns:
            Model instance based on the JSON string.

        """
        ...

    def to_dict(self) -> EngineerDict:
        """
        Convert to a dictionary (eg. to send back for a GET request).

        Returns:
            Dictionary based on the model instance.

        """
        ...

    def to_str(self) -> str:
        """
        Convert to a JSON string (eg. to send back for a GET request).

        Returns:
            JSON string based on the model instance.

        """
        ...


Engineer: typing.Type[TEngineer] = models.Engineer  # type: ignore

Single Table

Single table inheritance is very similar to Joined Table Inheritance with the difference that all classes are linked to the same table and there is no foreign key relationship between the models.

See also

Single Table Inheritance

OpenAlchemy documentation for single table inheritance.

SQLAlchemy single table inheritance documentation

Documentation for SQLAlchemy single table inheritance.

The following example defines single tabled inheritance where Employee is the parent and Manager and Engineer derive from it:

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openapi: "3.0.0"

info:
  title: Test Schema
  description: API to illustrate OpenAlchemy single inheritance.
  version: "0.1"

paths:
  /employee:
    get:
      summary: Used to retrieve all employees.
      responses:
        200:
          description: Return all employees from the database.
          content:
            application/json:
              schema:
                type: array
                items:
                  "$ref": "#/components/schemas/Employee"
  /manager:
    get:
      summary: Used to retrieve all managers.
      responses:
        200:
          description: Return all managers from the database.
          content:
            application/json:
              schema:
                type: array
                items:
                  "$ref": "#/components/schemas/Manager"
  /engineer:
    get:
      summary: Used to retrieve all engineers.
      responses:
        200:
          description: Return all engineers from the database.
          content:
            application/json:
              schema:
                type: array
                items:
                  "$ref": "#/components/schemas/Engineer"

components:
  schemas:
    Employee:
      description: Person that works for a company.
      type: object
      x-tablename: employee
      properties:
        id:
          type: integer
          description: Unique identifier for the employee.
          example: 0
          x-primary-key: true
        name:
          type: string
          description: The name of the employee.
          example: David Andersson
        type:
          type: string
          description: The type of the employee.
          example: employee
      x-kwargs:
        __mapper_args__:
          polymorphic_on: type
          polymorphic_identity: employee
    Manager:
      allOf:
        - $ref: "#/components/schemas/Employee"
        - x-inherits: true
          type: object
          properties:
            manager_data:
              type: string
              description: Data for the manager.
              example: some data
          x-kwargs:
            __mapper_args__:
              polymorphic_identity: manager
    Engineer:
      allOf:
        - $ref: "#/components/schemas/Employee"
        - x-inherits: true
          type: object
          properties:
            engineer_info:
              type: string
              description: Information for the manager.
              example: some info
          x-kwargs:
            __mapper_args__:
              polymorphic_identity: engineer

The following file uses OpenAlchemy to generate the SQLAlchemy models:

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from open_alchemy import init_yaml

init_yaml("single-example-spec.yml", models_filename="single_models_auto.py")

The SQLAlchemy models generated by OpenAlchemy are equivalent to the following traditional models file:

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import sqlalchemy as sa
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()


class Employee(Base):
    """Person that works for a company."""

    __tablename__ = "employee"
    id = sa.Column(sa.Integer, primary_key=True)
    name = sa.Column(sa.String)
    type = sa.Column(sa.String)

    __mapper_args__ = {"polymorphic_on": type, "polymorphic_identity": "employee"}


class Manager(Employee):
    """Person that works for a company."""

    manager_data = sa.Column(sa.String)

    __mapper_args__ = {"polymorphic_identity": "manager"}


class Engineer(Employee):
    """Person that works for a company."""

    engineer_info = sa.Column(sa.String)

    __mapper_args__ = {"polymorphic_identity": "engineer"}

OpenAlchemy will generate the following typed models:

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"""Autogenerated SQLAlchemy models based on OpenAlchemy models."""
# pylint: disable=no-member,super-init-not-called,unused-argument

import typing

import sqlalchemy
from sqlalchemy import orm

from open_alchemy import models

Base = models.Base  # type: ignore


class EmployeeDict(typing.TypedDict, total=False):
    """TypedDict for properties that are not required."""

    id: typing.Optional[int]
    name: typing.Optional[str]
    type: typing.Optional[str]


class TEmployee(typing.Protocol):
    """
    SQLAlchemy model protocol.

    Person that works for a company.

    Attrs:
        id: Unique identifier for the employee.
        name: The name of the employee.
        type: The type of the employee.

    """

    # SQLAlchemy properties
    __table__: sqlalchemy.Table
    __tablename__: str
    query: orm.Query

    # Model properties
    id: "sqlalchemy.Column[typing.Optional[int]]"
    name: "sqlalchemy.Column[typing.Optional[str]]"
    type: "sqlalchemy.Column[typing.Optional[str]]"

    def __init__(
        self,
        id: typing.Optional[int] = None,
        name: typing.Optional[str] = None,
        type: typing.Optional[str] = None,
    ) -> None:
        """
        Construct.

        Args:
            id: Unique identifier for the employee.
            name: The name of the employee.
            type: The type of the employee.

        """
        ...

    @classmethod
    def from_dict(
        cls,
        id: typing.Optional[int] = None,
        name: typing.Optional[str] = None,
        type: typing.Optional[str] = None,
    ) -> "TEmployee":
        """
        Construct from a dictionary (eg. a POST payload).

        Args:
            id: Unique identifier for the employee.
            name: The name of the employee.
            type: The type of the employee.

        Returns:
            Model instance based on the dictionary.

        """
        ...

    @classmethod
    def from_str(cls, value: str) -> "TEmployee":
        """
        Construct from a JSON string (eg. a POST payload).

        Returns:
            Model instance based on the JSON string.

        """
        ...

    def to_dict(self) -> EmployeeDict:
        """
        Convert to a dictionary (eg. to send back for a GET request).

        Returns:
            Dictionary based on the model instance.

        """
        ...

    def to_str(self) -> str:
        """
        Convert to a JSON string (eg. to send back for a GET request).

        Returns:
            JSON string based on the model instance.

        """
        ...


Employee: typing.Type[TEmployee] = models.Employee  # type: ignore


class ManagerDict(typing.TypedDict, total=False):
    """TypedDict for properties that are not required."""

    manager_data: typing.Optional[str]
    id: typing.Optional[int]
    name: typing.Optional[str]
    type: typing.Optional[str]


class TManager(typing.Protocol):
    """
    SQLAlchemy model protocol.

    Person that works for a company.

    Attrs:
        manager_data: Data for the manager.
        id: Unique identifier for the employee.
        name: The name of the employee.
        type: The type of the employee.

    """

    # SQLAlchemy properties
    __table__: sqlalchemy.Table
    __tablename__: str
    query: orm.Query

    # Model properties
    manager_data: "sqlalchemy.Column[typing.Optional[str]]"
    id: "sqlalchemy.Column[typing.Optional[int]]"
    name: "sqlalchemy.Column[typing.Optional[str]]"
    type: "sqlalchemy.Column[typing.Optional[str]]"

    def __init__(
        self,
        manager_data: typing.Optional[str] = None,
        id: typing.Optional[int] = None,
        name: typing.Optional[str] = None,
        type: typing.Optional[str] = None,
    ) -> None:
        """
        Construct.

        Args:
            manager_data: Data for the manager.
            id: Unique identifier for the employee.
            name: The name of the employee.
            type: The type of the employee.

        """
        ...

    @classmethod
    def from_dict(
        cls,
        manager_data: typing.Optional[str] = None,
        id: typing.Optional[int] = None,
        name: typing.Optional[str] = None,
        type: typing.Optional[str] = None,
    ) -> "TManager":
        """
        Construct from a dictionary (eg. a POST payload).

        Args:
            manager_data: Data for the manager.
            id: Unique identifier for the employee.
            name: The name of the employee.
            type: The type of the employee.

        Returns:
            Model instance based on the dictionary.

        """
        ...

    @classmethod
    def from_str(cls, value: str) -> "TManager":
        """
        Construct from a JSON string (eg. a POST payload).

        Returns:
            Model instance based on the JSON string.

        """
        ...

    def to_dict(self) -> ManagerDict:
        """
        Convert to a dictionary (eg. to send back for a GET request).

        Returns:
            Dictionary based on the model instance.

        """
        ...

    def to_str(self) -> str:
        """
        Convert to a JSON string (eg. to send back for a GET request).

        Returns:
            JSON string based on the model instance.

        """
        ...


Manager: typing.Type[TManager] = models.Manager  # type: ignore


class EngineerDict(typing.TypedDict, total=False):
    """TypedDict for properties that are not required."""

    engineer_info: typing.Optional[str]
    id: typing.Optional[int]
    name: typing.Optional[str]
    type: typing.Optional[str]


class TEngineer(typing.Protocol):
    """
    SQLAlchemy model protocol.

    Person that works for a company.

    Attrs:
        engineer_info: Information for the manager.
        id: Unique identifier for the employee.
        name: The name of the employee.
        type: The type of the employee.

    """

    # SQLAlchemy properties
    __table__: sqlalchemy.Table
    __tablename__: str
    query: orm.Query

    # Model properties
    engineer_info: "sqlalchemy.Column[typing.Optional[str]]"
    id: "sqlalchemy.Column[typing.Optional[int]]"
    name: "sqlalchemy.Column[typing.Optional[str]]"
    type: "sqlalchemy.Column[typing.Optional[str]]"

    def __init__(
        self,
        engineer_info: typing.Optional[str] = None,
        id: typing.Optional[int] = None,
        name: typing.Optional[str] = None,
        type: typing.Optional[str] = None,
    ) -> None:
        """
        Construct.

        Args:
            engineer_info: Information for the manager.
            id: Unique identifier for the employee.
            name: The name of the employee.
            type: The type of the employee.

        """
        ...

    @classmethod
    def from_dict(
        cls,
        engineer_info: typing.Optional[str] = None,
        id: typing.Optional[int] = None,
        name: typing.Optional[str] = None,
        type: typing.Optional[str] = None,
    ) -> "TEngineer":
        """
        Construct from a dictionary (eg. a POST payload).

        Args:
            engineer_info: Information for the manager.
            id: Unique identifier for the employee.
            name: The name of the employee.
            type: The type of the employee.

        Returns:
            Model instance based on the dictionary.

        """
        ...

    @classmethod
    def from_str(cls, value: str) -> "TEngineer":
        """
        Construct from a JSON string (eg. a POST payload).

        Returns:
            Model instance based on the JSON string.

        """
        ...

    def to_dict(self) -> EngineerDict:
        """
        Convert to a dictionary (eg. to send back for a GET request).

        Returns:
            Dictionary based on the model instance.

        """
        ...

    def to_str(self) -> str:
        """
        Convert to a JSON string (eg. to send back for a GET request).

        Returns:
            JSON string based on the model instance.

        """
        ...


Engineer: typing.Type[TEngineer] = models.Engineer  # type: ignore

See also

Getting Started