refactor bidding logic

Author: pgorecki

src/modules/bidding/README.md

@@ -10,8 +10,8 @@ System will let you know (by email) if someone outbids you, and you can decide i
 Sometimes you can be automatically outbid (if some other buyer sets his maximum limit higher that yours).
 
 For example:
-1. Alice wants to sell X. She sets the initial price for X item as 10 USD. 
-2. Bob wants to place a bid. The minimum amount is 10 USD (the ask price), and he places the bid of 15 USD. As the only bidder, he is the winner and the current price for X is 10 USD.
+1. Alice wants to sell X. She sets the ask price for this item as 10 USD. 
+2. Bob wants to place a bid on X. The minimum amount is 10 USD (the ask price), and he places the bid of 15 USD. As the only bidder, he is the winner and the current price for X is 10 USD.
 3. Alice and Bob are notified by email.
 4. Charlie places his bid, but now the minimum price he can bid is 11 USD, so he decides to bid 12 USD. As this is not enough to outbid Bob, he is not the winner, and the price is increased to 12 USD.
 5. Charlie places another bid, this time with the amount of 20 USD. As this is more than Bob's maximum limit, Charlie is the winner and the price is increased to 16 USD.

src/modules/bidding/application/event.py

@@ -15,7 +15,7 @@ def when_listing_is_published_start_auction(
     listing = Listing(
         id=event.listing_id,
         seller=Seller(id=event.seller_id),
-        initial_price=event.ask_price,
+        ask_price=event.ask_price,
         starts_at=datetime.now(),
         ends_at=datetime.now() + timedelta(days=7),
     )

src/modules/bidding/domain/entities.py

@@ -5,7 +5,7 @@
 from modules.bidding.domain.rules import (
     BidCanBeRetracted,
     ListingCanBeCancelled,
-    PlacedBidMustBeGreaterThanCurrentWinningBid,
+    PlacedBidMustBeGreaterOrEqualThanNextMinimumBid,
 )
 from modules.bidding.domain.value_objects import Bid, Bidder, Seller
 from seedwork.domain.entities import AggregateRoot
@@ -41,35 +41,30 @@ class ListingCancelledEvent(DomainEvent):
 @dataclass(kw_only=True)
 class Listing(AggregateRoot):
     seller: Seller
-    initial_price: Money
+    ask_price: Money
     starts_at: datetime
     ends_at: datetime
     bids: list[Bid] = field(default_factory=list)
 
     @property
     def current_price(self) -> Money:
-        highest_price = self.initial_price
-        second_highest_price = self.initial_price
+        """The current price is the price buyers are competing against"""
+        if len(self.bids) < 2:
+            return self.ask_price
 
-        if len(self.bids) == 0:
-            return self.initial_price
+        sorted_prices = sorted([bid.max_price for bid in self.bids], reverse=True)
+        return sorted_prices[1]
 
-        if len(self.bids) == 1:
-            return min(self.bids[0].max_price, self.initial_price)
-
-        for bid in self.bids:
-            if bid.max_price > highest_price:
-                second_highest_price = highest_price
-                highest_price = bid.max_price
-
-        return second_highest_price + Money(1, currency=self.initial_price.currency)
+    @property
+    def next_minimum_price(self) -> Money:
+        return self.current_price + Money(1, currency=self.ask_price.currency)
 
     # public commands
     def place_bid(self, bid: Bid) -> type[DomainEvent]:
         """Public method"""
         self.check_rule(
-            PlacedBidMustBeGreaterThanCurrentWinningBid(
-                bid=bid, current_price=self.current_price
+            PlacedBidMustBeGreaterOrEqualThanNextMinimumBid(
+                current_price=bid.max_price, next_minimum_price=self.next_minimum_price
             )
         )
 

src/modules/bidding/domain/rules.py

@@ -2,22 +2,21 @@
 
 from pydantic import Field
 
-from modules.bidding.domain.value_objects import Bid
 from seedwork.domain.rules import BusinessRule
 from seedwork.domain.value_objects import Money
 
 
-class PlacedBidMustBeGreaterThanCurrentWinningBid(BusinessRule):
-    __message = "Placed bid must be greater than {current_price}"
+class PlacedBidMustBeGreaterOrEqualThanNextMinimumBid(BusinessRule):
+    __message = "Placed bid must be greater or equal than {next_minimum_price}"
 
-    bid: Bid
     current_price: Money
+    next_minimum_price: Money
 
     def is_broken(self) -> bool:
-        return self.bid.max_price <= self.current_price
+        return self.current_price < self.next_minimum_price
 
     def get_message(self) -> str:
-        return self.__message.format(current_price=self.current_price)
+        return self.__message.format(next_minimum_price=self.next_minimum_price)
 
 
 class BidCanBeRetracted(BusinessRule):

src/modules/bidding/infrastructure/listing_repository.py

@@ -77,7 +77,7 @@ def model_to_entity(self, instance: ListingModel) -> Listing:
         return Listing(
             id=deserialize_id(instance.id),
             seller=Seller(id=deserialize_id(d["seller_id"])),
-            initial_price=deserialize_money(d["initial_price"]),
+            ask_price=deserialize_money(d["ask_price"]),
             starts_at=deserialize_datetime(d["starts_at"]),
             ends_at=deserialize_datetime(d["ends_at"]),
         )
@@ -88,7 +88,7 @@ def entity_to_model(self, entity: Listing) -> ListingModel:
             data={
                 "starts_at": serialize_datetime(entity.starts_at),
                 "ends_at": serialize_datetime(entity.ends_at),
-                "initial_price": serialize_money(entity.initial_price),
+                "ask_price": serialize_money(entity.ask_price),
                 "seller_id": serialize_id(entity.seller.id),
                 "bids": [serialize_bid(b) for b in entity.bids],
             },

src/modules/bidding/tests/domain/test_bidding.py

@@ -14,7 +14,7 @@ def test_listing_initial_price():
     listing = Listing(
         id=Listing.next_id(),
         seller=seller,
-        initial_price=Money(10),
+        ask_price=Money(10),
         starts_at=datetime.utcnow(),
         ends_at=datetime.utcnow(),
     )
@@ -30,7 +30,7 @@ def test_place_one_bid():
     listing = Listing(
         id=Listing.next_id(),
         seller=seller,
-        initial_price=Money(10),
+        ask_price=Money(10),
         starts_at=datetime.utcnow(),
         ends_at=datetime.utcnow(),
     )
@@ -40,22 +40,77 @@ def test_place_one_bid():
 
 
 @pytest.mark.unit
-def test_place_two_bids():
+def test_place_two_bids_second_buyer_outbids():
     now = datetime.utcnow()
-    seller = Seller(id=UUID.v4())
-    bidder1 = Bidder(id=UUID.v4())
-    bidder2 = Bidder(id=UUID.v4())
+    seller = Seller(id=UUID(int=1))
+    bidder1 = Bidder(id=UUID(int=2))
+    bidder2 = Bidder(id=UUID(int=3))
     listing = Listing(
-        id=Listing.next_id(),
+        id=UUID(int=4),
         seller=seller,
-        initial_price=Money(10),
+        ask_price=Money(10),
         starts_at=datetime.utcnow(),
         ends_at=datetime.utcnow(),
     )
-    listing.place_bid(Bid(max_price=Money(15), bidder=bidder1, placed_at=now))
-    listing.place_bid(Bid(max_price=Money(30), bidder=bidder2, placed_at=now))
+    assert listing.current_price == Money(10)
+    assert listing.next_minimum_price == Money(11)
+
+    # bidder1 places a bid
+    listing.place_bid(Bid(bidder=bidder1, max_price=Money(15), placed_at=now))
+    assert listing.current_price == Money(10)
+    assert listing.next_minimum_price == Money(11)
+
+    # bidder2 successfully outbids bidder1
+    listing.place_bid(Bid(bidder=bidder2, max_price=Money(30), placed_at=now))
+    assert listing.current_price == Money(15)
+    assert listing.next_minimum_price == Money(16)
     assert listing.winning_bid == Bid(Money(30), bidder=bidder2, placed_at=now)
-    assert listing.current_price == Money(16)
+
+
+@pytest.mark.unit
+def test_place_two_bids_second_buyer_fails_to_outbid():
+    now = datetime.utcnow()
+    seller = Seller(id=UUID(int=1))
+    bidder1 = Bidder(id=UUID(int=2))
+    bidder2 = Bidder(id=UUID(int=3))
+    listing = Listing(
+        id=UUID(int=4),
+        seller=seller,
+        ask_price=Money(10),
+        starts_at=datetime.utcnow(),
+        ends_at=datetime.utcnow(),
+    )
+
+    # bidder1 places a bid
+    listing.place_bid(Bid(bidder=bidder1, max_price=Money(30), placed_at=now))
+    assert listing.current_price == Money(10)
+    assert listing.next_minimum_price == Money(11)
+
+    # bidder2 tries to outbid bidder1...
+    listing.place_bid(Bid(bidder=bidder2, max_price=Money(20), placed_at=now))
+
+    # ...but he fails. bidder1 is still a winner, but current price changes
+    assert listing.winning_bid == Bid(Money(30), bidder=bidder1, placed_at=now)
+    assert listing.current_price == Money(20)
+
+
+@pytest.mark.unit
+def test_place_two_bids_second_buyer_fails_to_outbid_with_same_amount():
+    now = datetime.utcnow()
+    seller = Seller(id=UUID(int=1))
+    bidder1 = Bidder(id=UUID(int=2))
+    bidder2 = Bidder(id=UUID(int=3))
+    listing = Listing(
+        id=UUID(int=4),
+        seller=seller,
+        ask_price=Money(10),
+        starts_at=datetime.utcnow(),
+        ends_at=datetime.utcnow(),
+    )
+    listing.place_bid(Bid(bidder=bidder1, max_price=Money(30), placed_at=now))
+    listing.place_bid(Bid(bidder=bidder2, max_price=Money(30), placed_at=now))
+    assert listing.winning_bid == Bid(Money(30), bidder=bidder1, placed_at=now)
+    assert listing.current_price == Money(30)
 
 
 @pytest.mark.unit
@@ -66,7 +121,7 @@ def test_place_two_bids_by_same_bidder():
     listing = Listing(
         id=Listing.next_id(),
         seller=seller,
-        initial_price=Money(10),
+        ask_price=Money(10),
         starts_at=datetime.utcnow(),
         ends_at=datetime.utcnow(),
     )
@@ -85,7 +140,7 @@ def test_cannot_place_bid_if_listing_ended():
     listing = Listing(
         id=Listing.next_id(),
         seller=seller,
-        initial_price=Money(10),
+        ask_price=Money(10),
         starts_at=datetime.utcnow(),
         ends_at=datetime.utcnow(),
     )
@@ -96,7 +151,7 @@ def test_cannot_place_bid_if_listing_ended():
     )
     with pytest.raises(
         BusinessRuleValidationException,
-        match="PlacedBidMustBeGreaterThanCurrentWinningBid",
+        match="PlacedBidMustBeGreaterOrEqualThanNextMinimumBid",
     ):
         listing.place_bid(bid)
 
@@ -108,7 +163,7 @@ def test_retract_bid():
     listing = Listing(
         id=Listing.next_id(),
         seller=seller,
-        initial_price=Money(10),
+        ask_price=Money(10),
         starts_at=datetime.utcnow(),
         ends_at=datetime.utcnow(),
     )
@@ -129,7 +184,7 @@ def test_cancel_listing():
     listing = Listing(
         id=Listing.next_id(),
         seller=seller,
-        initial_price=Money(10),
+        ask_price=Money(10),
         starts_at=now,
         ends_at=now + timedelta(days=10),
     )
@@ -147,7 +202,7 @@ def test_can_cancel_listing_with_bids():
     listing = Listing(
         id=Listing.next_id(),
         seller=seller,
-        initial_price=Money(10),
+        ask_price=Money(10),
         starts_at=now,
         ends_at=now + timedelta(days=10),
     )
@@ -171,7 +226,7 @@ def test_cannot_cancel_listing_with_bids():
     listing = Listing(
         id=Listing.next_id(),
         seller=seller,
-        initial_price=Money(10),
+        ask_price=Money(10),
         starts_at=now,
         ends_at=now + timedelta(hours=1),
     )

src/modules/bidding/tests/infrastructure/test_listing_repository.py

@@ -23,7 +23,7 @@ def test_listing_data_mapper_maps_entity_to_model():
     listing = Listing(
         id=UUID(int=1),
         seller=Seller(id=UUID(int=2)),
-        initial_price=Money(100, "PLN"),
+        ask_price=Money(100, "PLN"),
         starts_at=datetime.datetime(2020, 12, 1),
         ends_at=datetime.datetime(2020, 12, 31),
         bids=[
@@ -42,7 +42,7 @@ def test_listing_data_mapper_maps_entity_to_model():
         id=UUID(int=1),
         data={
             "seller_id": "00000000-0000-0000-0000-000000000002",
-            "initial_price": {
+            "ask_price": {
                 "amount": 100,
                 "currency": "PLN",
             },
@@ -70,7 +70,7 @@ def test_listing_data_mapper_maps_model_to_entity():
         id=UUID(int=1),
         data={
             "seller_id": "00000000-0000-0000-0000-000000000002",
-            "initial_price": {
+            "ask_price": {
                 "amount": 100,
                 "currency": "PLN",
             },
@@ -85,7 +85,7 @@ def test_listing_data_mapper_maps_model_to_entity():
     expected = Listing(
         id=UUID(int=1),
         seller=Seller(id=UUID("00000000000000000000000000000002")),
-        initial_price=Money(100, "PLN"),
+        ask_price=Money(100, "PLN"),
         starts_at=datetime.datetime(2020, 12, 1),
         ends_at=datetime.datetime(2020, 12, 31),
     )
@@ -97,7 +97,7 @@ def test_listing_persistence(db_session):
     original = Listing(
         id=Listing.next_id(),
         seller=Seller(id=uuid.uuid4()),
-        initial_price=Money(100, "PLN"),
+        ask_price=Money(100, "PLN"),
         starts_at=datetime.datetime(2020, 12, 1),
         ends_at=datetime.datetime(2020, 12, 31),
     )