Aggregate 6 - Functional style¶
This example shows another variation of the Dog
aggregate class used
in the tutorial and module docs.
Like in the previous example, this example defines immutable Aggregate
and
DomainEvent
base classes, as frozen dataclasses. However, this time the
aggregate class has no methods. All the functionality has been implemented
as module-level functions.
Like in the previous examples, the application code in this example must receive the domain events that are returned from the aggregate command methods. The aggregate projector function must also be supplied when getting an aggregate from the repository and when taking snapshots.
Domain model¶
from dataclasses import dataclass
from datetime import datetime, timezone
from functools import singledispatch
from typing import Callable, Iterable, Optional, Tuple, TypeVar, Union
from uuid import UUID, uuid4
from eventsourcing.domain import Snapshot
@dataclass(frozen=True)
class DomainEvent:
originator_id: UUID
originator_version: int
timestamp: datetime
def create_timestamp() -> datetime:
return datetime.now(tz=timezone.utc)
@dataclass(frozen=True)
class Aggregate:
id: UUID
version: int
created_on: datetime
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
@dataclass(frozen=True)
class Dog(Aggregate):
name: str
tricks: Tuple[str, ...]
@dataclass(frozen=True)
class DogRegistered(DomainEvent):
name: str
@dataclass(frozen=True)
class TrickAdded(DomainEvent):
trick: str
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: str) -> DomainEvent:
return TrickAdded(
originator_id=dog.id,
originator_version=dog.version + 1,
timestamp=create_timestamp(),
trick=trick,
)
@singledispatch
def mutate_dog(
event: Union[DomainEvent, Snapshot], 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,
name=event.name,
tricks=(),
)
@mutate_dog.register
def _(event: TrickAdded, dog: Dog) -> Dog:
return Dog(
id=dog.id,
version=event.originator_version,
created_on=event.timestamp,
name=dog.name,
tricks=dog.tricks + (event.trick,),
)
@mutate_dog.register(Snapshot)
def _(event: Snapshot, _: None) -> Dog:
return Dog(
id=event.state["id"],
version=event.state["version"],
created_on=event.state["created_on"],
name=event.state["name"],
tricks=tuple(event.state["tricks"]), # comes back from JSON as a list
)
project_dog = aggregate_projector(mutate_dog)
Application¶
from typing import Any, Dict
from uuid import UUID
from eventsourcing.application import Application
from eventsourcing.examples.aggregate6.domainmodel import (
add_trick,
project_dog,
register_dog,
)
class DogSchool(Application):
is_snapshotting_enabled = True
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))
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}
Test case¶
from unittest import TestCase
from eventsourcing.examples.aggregate6.application import DogSchool
from eventsourcing.examples.aggregate6.domainmodel import 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)
assert dog["name"] == "Fido"
assert 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)
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")