Define the core model

The domain drive design approach is core domain model centric. The core model contains only the business logic of the app, nothing more.

Initial model

The first step is to define a model, at the moment, we don’t need to create a complete model, we can start by creating model to register books. So let start that simple.

In a domain/model.py we can store the following model.

from jeepito import Field, Model


class Book(Model):
    id: str = Field(...)
    title: str = Field(...)
    author: str = Field(...)
    isbn: str = Field(...)

Why there is a domain module ?

In traditional apps, we use to create a model module at the root of the project, so, it is legitimate to ask why we create a domain namespace. The domain module contains all the business logic, and it is the namespace where we only with the language of the business domain. In domain driven design, it is named the Ubiquitous Language

In a Django app, for example, the model contains database details, such as forein keys, index and also contains some GUI details.

In the domain module, this is not the proper place for that. Says differently, you can’t use an ORM to describe your domain model.

Note

Behind the scene, jeepito use pydantic, the Field is just a reexport of the pydantic Field class.

Defining commands

In a domain/messages.py we define the format of model updates.

Let start by a command to create a book.

from jeepito import Command, Field, Metadata


class RegisterBook(Command):
    id: str = Field(...)
    isbn: str = Field(...)
    title: str = Field(...)
    author: str = Field(...)
    metadata: Metadata = Metadata(name="register_book", schema_version=1)

The command field describe the data payload of the command, it represent the contract of the command. Every update that introduce a Breaking Change of the command are discouraged, in that case, a new command is created with the change updated.

Note

For example, if we want to transform the author by a list of authors, it is a breaking change, and a new version of the command has to be created.

from jeepito import Command, Field, Metadata


class RegisterBookV1(Command):
    """Initial version of the command authorize only one author per book."""

    id: str = Field(...)
    isbn: str = Field(...)
    title: str = Field(...)
    author: str = Field(...)
    metadata: Metadata = Metadata(name="register_book", schema_version=1)


class RegisterBookV2(Command):
    id: str = Field(...)
    isbn: str = Field(...)
    title: str = Field(...)
    authors: list[str] = Field(...)
    metadata: Metadata = Metadata(name="register_book", schema_version=2)