Aggregate 7 - Pydantic and orjson

This example shows another variation of the Dog aggregate class used in the tutorial and module docs.

Similar to the previous example, the model is expressed in a functional style. In contrast to the previous example, this example uses Pydantic to define immutable aggregate and event classes, rather than defining them as Python frozen data classes. This has implications for the persistence layer.

The application class in this example uses its own persistence classes PydanticMapper and OrjsonTranscoder. Pydantic is responsible for converting domain model objects to object types that orjson can serialise, and for reconstructing model objects from JSON objects that have been deserialised by orjson.

One advantage of using Pydantic here is that any custom value objects will be automatically reconstructed without needing to define the transcoding classes that would be needed when using the library’s default JSONTranscoder. This is demonstrated in the example below with the Trick class, which is used in both aggregate events and aggregate state, and which is reconstructed from serialised string values, representing only the name of the trick, from both recorded aggregate events and from recorded snapshots.

Domain model

from __future__ import annotations

from datetime import datetime, timezone
from functools import singledispatch
from typing import Any, Callable, Dict, Iterable, Optional, Tuple, TypeVar
from uuid import UUID, uuid4

from pydantic import BaseModel

from eventsourcing.utils import get_topic


class DomainEvent(BaseModel):
    originator_id: UUID
    originator_version: int
    timestamp: datetime

    class Config:
        frozen = True


def create_timestamp() -> datetime:
    return datetime.now(tz=timezone.utc)


class Aggregate(BaseModel):
    id: UUID
    version: int
    created_on: datetime
    modified_on: datetime

    class Config:
        frozen = True


class Snapshot(DomainEvent):
    topic: str
    state: Dict[str, Any]

    @classmethod
    def take(cls, aggregate: Aggregate) -> Snapshot:
        return Snapshot(
            originator_id=aggregate.id,
            originator_version=aggregate.version,
            timestamp=create_timestamp(),
            topic=get_topic(type(aggregate)),
            state=aggregate.model_dump(),
        )


TAggregate = TypeVar("TAggregate", bound=Aggregate)
MutatorFunction = Callable[..., Optional[TAggregate]]


def aggregate_projector(
    mutator: MutatorFunction[TAggregate],
) -> Callable[[Optional[TAggregate], Iterable[DomainEvent]], Optional[TAggregate]]:
    def project_aggregate(
        aggregate: Optional[TAggregate], events: Iterable[DomainEvent]
    ) -> Optional[TAggregate]:
        for event in events:
            aggregate = mutator(event, aggregate)
        return aggregate

    return project_aggregate


class Trick(BaseModel):
    name: str


class Dog(Aggregate):
    name: str
    tricks: Tuple[Trick, ...]


class DogRegistered(DomainEvent):
    name: str


class TrickAdded(DomainEvent):
    trick: Trick


def register_dog(name: str) -> DomainEvent:
    return DogRegistered(
        originator_id=uuid4(),
        originator_version=1,
        timestamp=create_timestamp(),
        name=name,
    )


def add_trick(dog: Dog, trick: Trick) -> DomainEvent:
    return TrickAdded(
        originator_id=dog.id,
        originator_version=dog.version + 1,
        timestamp=create_timestamp(),
        trick=trick,
    )


@singledispatch
def mutate_dog(event: DomainEvent, dog: Optional[Dog]) -> Optional[Dog]:
    """Mutates aggregate with event."""


@mutate_dog.register
def _(event: DogRegistered, _: None) -> Dog:
    return Dog(
        id=event.originator_id,
        version=event.originator_version,
        created_on=event.timestamp,
        modified_on=event.timestamp,
        name=event.name,
        tricks=(),
    )


@mutate_dog.register
def _(event: TrickAdded, dog: Dog) -> Dog:
    return Dog(
        id=dog.id,
        version=event.originator_version,
        created_on=dog.created_on,
        modified_on=event.timestamp,
        name=dog.name,
        tricks=dog.tricks + (event.trick,),
    )


@mutate_dog.register
def _(event: Snapshot, _: None) -> Dog:
    return Dog(
        id=event.state["id"],
        version=event.state["version"],
        created_on=event.state["created_on"],
        modified_on=event.state["modified_on"],
        name=event.state["name"],
        tricks=event.state["tricks"],
    )


project_dog = aggregate_projector(mutate_dog)

Application

from __future__ import annotations

from typing import Any, Dict
from uuid import UUID

from eventsourcing.application import Application
from eventsourcing.examples.aggregate7.domainmodel import (
    Snapshot,
    Trick,
    add_trick,
    project_dog,
    register_dog,
)
from eventsourcing.examples.aggregate7.persistence import (
    OrjsonTranscoder,
    PydanticMapper,
)
from eventsourcing.persistence import Mapper, Transcoder


class DogSchool(Application):
    is_snapshotting_enabled = True
    snapshot_class = Snapshot

    def register_dog(self, name: str) -> UUID:
        event = register_dog(name)
        self.save(event)
        return event.originator_id

    def add_trick(self, dog_id: UUID, trick: str) -> None:
        dog = self.repository.get(dog_id, projector_func=project_dog)
        self.save(add_trick(dog, Trick(name=trick)))

    def get_dog(self, dog_id: UUID) -> Dict[str, Any]:
        dog = self.repository.get(dog_id, projector_func=project_dog)
        return {"name": dog.name, "tricks": dog.tricks}

    def construct_mapper(self) -> Mapper:
        return self.factory.mapper(
            transcoder=self.construct_transcoder(),
            mapper_class=PydanticMapper,
        )

    def construct_transcoder(self) -> Transcoder:
        return OrjsonTranscoder()

Persistence

from __future__ import annotations

from typing import Any, Dict, cast

import orjson
from pydantic import BaseModel

from eventsourcing.domain import DomainEventProtocol
from eventsourcing.persistence import (
    Mapper,
    ProgrammingError,
    StoredEvent,
    Transcoder,
    Transcoding,
)
from eventsourcing.utils import get_topic, resolve_topic


class PydanticMapper(Mapper):
    def to_stored_event(self, domain_event: DomainEventProtocol) -> StoredEvent:
        topic = get_topic(domain_event.__class__)
        event_state = cast(BaseModel, domain_event).model_dump()
        stored_state = self.transcoder.encode(event_state)
        if self.compressor:
            stored_state = self.compressor.compress(stored_state)
        if self.cipher:
            stored_state = self.cipher.encrypt(stored_state)
        return StoredEvent(
            originator_id=domain_event.originator_id,
            originator_version=domain_event.originator_version,
            topic=topic,
            state=stored_state,
        )

    def to_domain_event(self, stored: StoredEvent) -> DomainEventProtocol:
        stored_state = stored.state
        if self.cipher:
            stored_state = self.cipher.decrypt(stored_state)
        if self.compressor:
            stored_state = self.compressor.decompress(stored_state)
        event_state: Dict[str, Any] = self.transcoder.decode(stored_state)
        cls = resolve_topic(stored.topic)
        return cls(**event_state)


class OrjsonTranscoder(Transcoder):
    def encode(self, obj: Any) -> bytes:
        return orjson.dumps(obj)

    def decode(self, data: bytes) -> Any:
        return orjson.loads(data)

    def register(self, transcoding: Transcoding) -> None:
        raise ProgrammingError("Please use Pydantic BaseModel")  # pragma: no cover

Test case

from __future__ import annotations

from typing import Tuple
from unittest import TestCase

from eventsourcing.examples.aggregate7.application import DogSchool
from eventsourcing.examples.aggregate7.domainmodel import Trick, project_dog


class TestDogSchool(TestCase):
    def test_dog_school(self) -> None:
        # Construct application object.
        school = DogSchool()

        # Evolve application state.
        dog_id = school.register_dog("Fido")
        school.add_trick(dog_id, "roll over")
        school.add_trick(dog_id, "play dead")

        # Query application state.
        dog = school.get_dog(dog_id)
        self.assertEqual(dog["name"], "Fido")
        self.assertEqualTricks(dog["tricks"], ("roll over", "play dead"))

        # Select notifications.
        notifications = school.notification_log.select(start=1, limit=10)
        assert len(notifications) == 3

        # Take snapshot.
        school.take_snapshot(dog_id, version=3, projector_func=project_dog)
        dog = school.get_dog(dog_id)
        self.assertEqual(dog["name"], "Fido")
        self.assertEqualTricks(dog["tricks"], ("roll over", "play dead"))

        # Continue with snapshotted aggregate.
        school.add_trick(dog_id, "fetch ball")
        dog = school.get_dog(dog_id)
        self.assertEqual(dog["name"], "Fido")
        self.assertEqualTricks(dog["tricks"], ("roll over", "play dead", "fetch ball"))

    def assertEqualTricks(
        self, actual: Tuple[Trick, ...], expected: Tuple[str, ...]
    ) -> None:
        self.assertEqual(len(actual), len(expected))
        for i, trick in enumerate(actual):
            self.assertIsInstance(trick, Trick)
            self.assertEqual(trick.name, expected[i])