Make Grid.gridify() use only pickup addresses

- ensure a `Restaurant` only has one unique `Order.pickup_address`
- rework `Grid.gridify()` so that only pickup addresses are assigned
  into `Pixel`s
- include database migrations to ensure the data adhere to these
  tighter constraints

src/urban_meal_delivery/console/gridify.py

@@ -8,7 +8,7 @@ from urban_meal_delivery.console import decorators
 
 
 @click.command()
-@decorators.db_revision('888e352d7526')
+@decorators.db_revision('e86290e7305e')
 def gridify() -> None:  # pragma: no cover  note:b1f68d24
     """Create grids for all cities.
 

src/urban_meal_delivery/db/addresses.py

@@ -52,7 +52,7 @@ class Address(meta.Base):
 
     # Relationships
     city = orm.relationship('City', back_populates='addresses')
-    restaurant = orm.relationship('Restaurant', back_populates='address', uselist=False)
+    restaurants = orm.relationship('Restaurant', back_populates='address')
     orders_picked_up = orm.relationship(
         'Order',
         back_populates='pickup_address',

src/urban_meal_delivery/db/grids.py

@@ -54,11 +54,12 @@ class Grid(meta.Base):
         return round((self.side_length ** 2) / 1_000_000, 1)
 
     @classmethod
-    def gridify(cls, city: db.City, side_length: int) -> db.Grid:
+    def gridify(cls, city: db.City, side_length: int) -> db.Grid:  # noqa:WPS210
         """Create a fully populated `Grid` for a `city`.
 
-        The `Grid` contains only `Pixel`s that have at least one `Address`.
-        `Address` objects outside the `city`'s viewport are discarded.
+        The `Grid` contains only `Pixel`s that have at least one
+        `Order.pickup_address`. `Address` objects outside the `.city`'s
+        viewport are discarded.
 
         Args:
             city: city for which the grid is created
@@ -72,7 +73,14 @@ class Grid(meta.Base):
         # `Pixel`s grouped by `.n_x`-`.n_y` coordinates.
         pixels = {}
 
-        for address in city.addresses:
+        pickup_addresses = (  # noqa:ECE:001
+            db.session.query(db.Address)
+            .join(db.Order, db.Address.id == db.Order.pickup_address_id)
+            .filter(db.Address.city == city)
+            .all()
+        )
+
+        for address in pickup_addresses:
             # Check if an `address` is not within the `city`'s viewport, ...
             not_within_city_viewport = (
                 address.x < 0

src/urban_meal_delivery/db/orders.py

@@ -79,12 +79,6 @@ class Order(meta.Base):  # noqa:WPS214
         sa.ForeignKeyConstraint(
             ['customer_id'], ['customers.id'], onupdate='RESTRICT', ondelete='RESTRICT',
         ),
-        sa.ForeignKeyConstraint(
-            ['restaurant_id'],
-            ['restaurants.id'],
-            onupdate='RESTRICT',
-            ondelete='RESTRICT',
-        ),
         sa.ForeignKeyConstraint(
             ['courier_id'], ['couriers.id'], onupdate='RESTRICT', ondelete='RESTRICT',
         ),
@@ -94,6 +88,14 @@ class Order(meta.Base):  # noqa:WPS214
             onupdate='RESTRICT',
             ondelete='RESTRICT',
         ),
+        sa.ForeignKeyConstraint(
+            # This foreign key ensures that there is only
+            # one `.pickup_address` per `.restaurant`
+            ['restaurant_id', 'pickup_address_id'],
+            ['restaurants.id', 'restaurants.address_id'],
+            onupdate='RESTRICT',
+            ondelete='RESTRICT',
+        ),
         sa.ForeignKeyConstraint(
             ['delivery_address_id'],
             ['addresses.id'],
@@ -302,7 +304,11 @@ class Order(meta.Base):  # noqa:WPS214
 
     # Relationships
     customer = orm.relationship('Customer', back_populates='orders')
-    restaurant = orm.relationship('Restaurant', back_populates='orders')
+    restaurant = orm.relationship(
+        'Restaurant',
+        back_populates='orders',
+        primaryjoin='Restaurant.id == Order.restaurant_id',
+    )
     courier = orm.relationship('Courier', back_populates='orders')
     pickup_address = orm.relationship(
         'Address',

src/urban_meal_delivery/db/pixels.py

@@ -12,7 +12,7 @@ class Pixel(meta.Base):
     Square pixels aggregate `Address` objects within a `City`.
     Every `Address` belongs to exactly one `Pixel` in a `Grid`.
 
-    Every `Pixel` has a unique "coordinate" within the `Grid`.
+    Every `Pixel` has a unique `n_x`-`n_y` coordinate within the `Grid`.
     """
 
     __tablename__ = 'pixels'

src/urban_meal_delivery/db/restaurants.py

@@ -34,10 +34,12 @@ class Restaurant(meta.Base):
             '0 <= estimated_prep_duration AND estimated_prep_duration <= 2400',
             name='realistic_estimated_prep_duration',
         ),
+        # Needed by a `ForeignKeyConstraint` in `Order`.
+        sa.UniqueConstraint('id', 'address_id'),
     )
 
     # Relationships
-    address = orm.relationship('Address', back_populates='restaurant')
+    address = orm.relationship('Address', back_populates='restaurants')
     orders = orm.relationship('Order', back_populates='restaurant')
 
     def __repr__(self) -> str: