Aggregate 8 - Pydantic with declarative syntax

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

Similar to the first example, the aggregate is expressed using the library’s declarative syntax. And similar to the previous example, the model events are defined using Pydantic.

The application class in this example uses the persistence classes PydanticMapper and OrjsonTranscoder, just like the previous example. 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. The application class also uses the custom Snapshot class, which is defined as a Pydantic model.

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.

Domain model

from datetime import datetime
from typing import Any, Dict, List
from uuid import UUID

from pydantic import BaseModel

from eventsourcing.domain import (
    Aggregate as BaseAggregate,

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

    class Config:
        allow_mutation = False

class Aggregate(BaseAggregate):
    class Event(DomainEvent, CanMutateAggregate):

    class Created(Event, CanInitAggregate):
        originator_topic: str

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

class Dog(Aggregate):
    def __init__(self, name: str) -> None: = name
        self.tricks: List[str] = []

    def add_trick(self, trick: str) -> None:


from typing import Any, Dict
from uuid import UUID

from eventsourcing.application import Application
from eventsourcing.examples.aggregate8.domainmodel import Dog, Snapshot
from eventsourcing.examples.aggregate8.persistence import (
from eventsourcing.persistence import Mapper, Transcoder

class DogSchool(Application):
    env = {
    snapshot_class = Snapshot

    def register_dog(self, name: str) -> UUID:
        dog = Dog(name)

    def add_trick(self, dog_id: UUID, trick: str) -> None:
        dog: Dog = self.repository.get(dog_id)

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

    def construct_mapper(self) -> Mapper:
        return self.factory.mapper(

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


from typing import Any, Dict, cast

import orjson
from pydantic import BaseModel

from eventsourcing.domain import DomainEventProtocol
from eventsourcing.persistence import (
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).dict()
        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(

    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 unittest import TestCase

from eventsourcing.examples.aggregate8.application import DogSchool

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)
        assert dog["name"] == "Fido"
        assert dog["tricks"] == ("roll over", "play dead")

        # Select notifications.
        notifications =, limit=10)
        assert len(notifications) == 3

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

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