collect domain event from aggregate roots when command handler is executed

Author: pgorecki

poetry.lock

@@ -28,11 +28,11 @@ click = "8.0.4"
 httpx = "^0.23.1"
 requests = "^2.28.1"
 bcrypt = "^4.0.1"
+mypy = "^1.4.1"
 
 [tool.poetry.dev-dependencies]
 poethepoet = "^0.10.0"
 pytest-cov = "^2.12.1"
-mypy = "^0.910"
 vulture = "^2.7"
 
 [build-system]

pyproject.toml

@@ -21,9 +21,7 @@ def place_bid(
     command: PlaceBidCommand, listing_repository: ListingRepository
 ) -> CommandResult:
     bidder = Bidder(id=command.bidder_id)
-    bid = Bid(bidder=bidder, max_price=Money(command.amount))
+    bid = Bid(bidder=bidder, max_price=Money(amount=command.amount))
 
     listing = listing_repository.get_by_id(command.listing_id)
     listing.place_bid(bid)
-
-    return CommandResult.success()

src/modules/bidding/application/command/place_bid.py

@@ -19,6 +19,4 @@ def retract_bid(
     bidder = Bidder(id=command.bidder_id)
 
     listing: Listing = listing_repository.get_by_id(id=command.listing_id)
-    event = listing.retract_bid_of(bidder)
-
-    return CommandResult.success(event=event)
+    listing.retract_bid_of(bidder)

src/modules/bidding/application/command/retract_bid.py

@@ -0,0 +1,4 @@
+from .notify_outbid_winner import notify_outbid_winner
+from .when_listing_is_published_start_auction import (
+    when_listing_is_published_start_auction,
+)

src/modules/bidding/application/event/__init__.py

@@ -0,0 +1,8 @@
+from modules.bidding.application import bidding_module
+from modules.bidding.domain.events import BidWasPlaced
+from seedwork.infrastructure.logging import logger
+
+
+@bidding_module.domain_event_handler
+def notify_outbid_winner(event: BidWasPlaced):
+    logger.info(f"Message from a handler: Listing {event.listing_id} was published")

src/modules/bidding/application/event/notify_outbid_winner.py

@@ -20,4 +20,3 @@ def when_listing_is_published_start_auction(
         ends_at=datetime.now() + timedelta(days=7),
     )
     listing_repository.add(listing)
-    return EventResult.success()

src/modules/bidding/application/event.py …en_listing_is_published_start_auction.pysrc/modules/bidding/application/event.py renamed to src/modules/bidding/application/event/when_listing_is_published_start_auction.py src/modules/bidding/application/event.py renamed to src/modules/bidding/application/event/when_listing_is_published_start_auction.py

@@ -2,10 +2,16 @@
 from datetime import datetime, timedelta
 from typing import Optional
 
+from modules.bidding.domain.events import (
+    BidWasPlaced,
+    BidWasRetracted,
+    HighestBidderWasOutbid,
+    ListingWasCancelled,
+)
 from modules.bidding.domain.rules import (
     BidCanBeRetracted,
     ListingCanBeCancelled,
-    PlacedBidMustBeGreaterOrEqualThanNextMinimumBid,
+    PriceOfPlacedBidMustBeGreaterOrEqualThanNextMinimumPrice,
 )
 from modules.bidding.domain.value_objects import Bid, Bidder, Seller
 from seedwork.domain.entities import AggregateRoot
@@ -26,18 +32,6 @@ class ListingCannotBeCancelled(DomainException):
     ...
 
 
-class BidPlacedEvent(DomainEvent):
-    ...
-
-
-class BidRetractedEvent(DomainEvent):
-    ...
-
-
-class ListingCancelledEvent(DomainEvent):
-    ...
-
-
 @dataclass(kw_only=True)
 class Listing(AggregateRoot[GenericUUID]):
     seller: Seller
@@ -61,34 +55,57 @@ def next_minimum_price(self) -> Money:
         return self.current_price + Money(amount=1, currency=self.ask_price.currency)
 
     # public commands
-    def place_bid(self, bid: Bid) -> DomainEvent:
+    def place_bid(self, bid: Bid):
         """Public method"""
         self.check_rule(
-            PlacedBidMustBeGreaterOrEqualThanNextMinimumBid(
+            PriceOfPlacedBidMustBeGreaterOrEqualThanNextMinimumPrice(
                 current_price=bid.max_price, next_minimum_price=self.next_minimum_price
             )
         )
 
+        previous_winner_id = self.highest_bid.bidder.id if self.highest_bid else None
+        current_winner_id = bid.bidder.id
+
         if self.has_bid_placed_by(bidder=bid.bidder):
             self._update_bid(bid)
         else:
             self._add_bid(bid)
 
-        return BidPlacedEvent(
-            listing_id=self.id, bidder=bid.bidder, max_price=bid.max_price
+        self.register_event(
+            BidWasPlaced(
+                listing_id=self.id,
+                bidder_id=bid.bidder.id,
+            )
         )
 
-    def retract_bid_of(self, bidder: Bidder) -> DomainEvent:
+        # if there was previous winner...
+        if previous_winner_id is not None and previous_winner_id != current_winner_id:
+            self.register_event(
+                HighestBidderWasOutbid(
+                    listing_id=self.id,
+                    outbid_bidder_id=previous_winner_id,
+                )
+            )
+
+    def retract_bid_of(self, bidder: Bidder):
         """Public method"""
         bid = self.get_bid_of(bidder)
         self.check_rule(
             BidCanBeRetracted(listing_ends_at=self.ends_at, bid_placed_at=bid.placed_at)
         )
 
         self._remove_bid_of(bidder=bidder)
-        return BidRetractedEvent(listing_id=self.id, bidder_id=bidder.id)
+        self.register_event(
+            BidWasRetracted(
+                listing_id=self.id,
+                retracting_bidder_id=bidder.id,
+                winning_bidder_id=self.highest_bid.bidder.id
+                if self.highest_bid
+                else None,
+            )
+        )
 
-    def cancel(self) -> DomainEvent:
+    def cancel(self):
         """
         Seller can cancel a listing (end a listing early). Listing must be eligible to cancelled,
         depending on time left and if bids have been placed.
@@ -100,14 +117,13 @@ def cancel(self) -> DomainEvent:
             )
         )
         self.ends_at = datetime.utcnow()
-        return ListingCancelledEvent(listing_id=self.id)
+        self.register_event(ListingWasCancelled(listing_id=self.id))
 
     def end(self) -> DomainEvent:
         """
         Ends listing.
         """
         raise NotImplementedError()
-        return []
 
     # public queries
     def get_bid_of(self, bidder: Bidder) -> Bid:
@@ -126,7 +142,7 @@ def has_bid_placed_by(self, bidder: Bidder) -> bool:
         return True
 
     @property
-    def winning_bid(self) -> Optional[Bid]:
+    def highest_bid(self) -> Optional[Bid]:
         try:
             highest_bid = max(self.bids, key=lambda bid: bid.max_price)
         except ValueError:

src/modules/bidding/domain/entities.py

@@ -1,2 +1,20 @@
-class PlacedBidIsGreaterThanCurrentWinningBid:
+from seedwork.domain.events import DomainEvent
+from seedwork.domain.value_objects import GenericUUID
+
+
+class BidWasPlaced(DomainEvent):
+    listing_id: GenericUUID
+    bidder_id: GenericUUID
+
+
+class HighestBidderWasOutbid(DomainEvent):
+    listing_id: GenericUUID
+    outbid_bidder_id: GenericUUID
+
+
+class BidWasRetracted(DomainEvent):
+    ...
+
+
+class ListingWasCancelled(DomainEvent):
     ...

src/modules/bidding/domain/events.py

@@ -6,7 +6,7 @@
 from seedwork.domain.value_objects import Money
 
 
-class PlacedBidMustBeGreaterOrEqualThanNextMinimumBid(BusinessRule):
+class PriceOfPlacedBidMustBeGreaterOrEqualThanNextMinimumPrice(BusinessRule):
     __message = "Placed bid must be greater or equal than {next_minimum_price}"
 
     current_price: Money