Add Grid.gridify() constructor

- the purpose of this constructor method is to generate all `Pixel`s
  for a `Grid` that have at least one `Address` assigned to them
- fix missing `UniqueConstraint` in `Grid` class => it was not possible
  to create two `Grid`s with the same `.side_length` in different cities
- change the `City.viewport` property into two separate `City.southwest`
  and `City.northeast` properties; also add `City.total_x` and
  `City.total_y` properties for convenience

src/urban_meal_delivery/db/addresses.py

@@ -1,6 +1,5 @@
 """Provide the ORM's `Address` model."""
 
-
 import sqlalchemy as sa
 from sqlalchemy import orm
 from sqlalchemy.dialects import postgresql
@@ -93,7 +92,7 @@ class Address(meta.Base):
     def location(self) -> utils.Location:
         """The location of the address.
 
-        The returned `Location` object relates to `.city.viewport.southwest`.
+        The returned `Location` object relates to `.city.southwest`.
 
         See also the `.x` and `.y` properties that are shortcuts for
         `.location.x` and `.location.y`.
@@ -103,7 +102,7 @@ class Address(meta.Base):
         """
         if not hasattr(self, '_location'):  # noqa:WPS421  note:b1f68d24
             self._location = utils.Location(self.latitude, self.longitude)
-            self._location.relate_to(self.city.as_xy_origin)
+            self._location.relate_to(self.city.southwest)
         return self._location
 
     @property

src/urban_meal_delivery/db/cities.py

@@ -1,6 +1,5 @@
 """Provide the ORM's `City` model."""
 
-from typing import Dict
 
 import sqlalchemy as sa
 from sqlalchemy import orm
@@ -69,30 +68,45 @@ class City(meta.Base):
         return self._center
 
     @property
-    def viewport(self) -> Dict[str, utils.Location]:
-        """Google Maps viewport of the city.
+    def northeast(self) -> utils.Location:
+        """The city's northeast corner of the Google Maps viewport.
 
         Implementation detail: This property is cached as none of the
         underlying attributes to calculate the value are to be changed.
         """
-        if not hasattr(self, '_viewport'):  # noqa:WPS421  note:d334120e
-            self._viewport = {
-                'northeast': utils.Location(
-                    self._northeast_latitude, self._northeast_longitude,
-                ),
-                'southwest': utils.Location(
-                    self._southwest_latitude, self._southwest_longitude,
-                ),
-            }
+        if not hasattr(self, '_northeast'):  # noqa:WPS421  note:d334120e
+            self._northeast = utils.Location(
+                self._northeast_latitude, self._northeast_longitude,
+            )
 
-        return self._viewport
+        return self._northeast
 
     @property
-    def as_xy_origin(self) -> utils.Location:
-        """The southwest corner of the `.viewport`.
+    def southwest(self) -> utils.Location:
+        """The city's southwest corner of the Google Maps viewport.
 
-        This property serves, for example, as the `other` argument to the
-        `Location.relate_to()` method when representing an `Address`
-        in the x-y plane.
+        Implementation detail: This property is cached as none of the
+        underlying attributes to calculate the value are to be changed.
         """
-        return self.viewport['southwest']
+        if not hasattr(self, '_southwest'):  # noqa:WPS421  note:d334120e
+            self._southwest = utils.Location(
+                self._southwest_latitude, self._southwest_longitude,
+            )
+
+        return self._southwest
+
+    @property
+    def total_x(self) -> int:
+        """The horizontal distance from the city's west to east end in meters.
+
+        The city borders refer to the Google Maps viewport.
+        """
+        return self.northeast.easting - self.southwest.easting
+
+    @property
+    def total_y(self) -> int:
+        """The vertical distance from the city's south to north end in meters.
+
+        The city borders refer to the Google Maps viewport.
+        """
+        return self.northeast.northing - self.southwest.northing

src/urban_meal_delivery/db/grids.py

@@ -1,8 +1,11 @@
 """Provide the ORM's `Grid` model."""
 
+from __future__ import annotations
+
 import sqlalchemy as sa
 from sqlalchemy import orm
 
+from urban_meal_delivery import db
 from urban_meal_delivery.db import meta
 
 
@@ -27,6 +30,9 @@ class Grid(meta.Base):
         sa.ForeignKeyConstraint(
             ['city_id'], ['cities.id'], onupdate='RESTRICT', ondelete='RESTRICT',
         ),
+        # Each `Grid`, characterized by its `.side_length`,
+        # may only exists once for a given `.city`.
+        sa.UniqueConstraint('city_id', 'side_length'),
         # Needed by a `ForeignKeyConstraint` in `address_pixel_association`.
         sa.UniqueConstraint('id', 'city_id'),
     )
@@ -46,3 +52,45 @@ class Grid(meta.Base):
     def pixel_area(self) -> float:
         """The area of a `Pixel` on the grid in square kilometers."""
         return (self.side_length ** 2) / 1_000_000  # noqa:WPS432
+
+    @classmethod
+    def gridify(cls, city: db.City, side_length: int) -> db.Grid:
+        """Create a fully populated `Grid` for a `city`.
+
+        The created `Grid` contains only the `Pixel`s for which
+        there is at least one `Address` in it.
+
+        Args:
+            city: city for which the grid is created
+            side_length: the length of a square `Pixel`'s side
+
+        Returns:
+            grid: including `grid.pixels` with the associated `city.addresses`
+        """
+        grid = cls(city=city, side_length=side_length)
+
+        # Create `Pixel` objects covering the entire `city`.
+        # Note: `+1` so that `city.northeast` corner is on the grid.
+        possible_pixels = [
+            db.Pixel(n_x=n_x, n_y=n_y)
+            for n_x in range((city.total_x // side_length) + 1)
+            for n_y in range((city.total_y // side_length) + 1)
+        ]
+
+        # For convenient lookup by `.n_x`-`.n_y` coordinates.
+        pixel_map = {(pixel.n_x, pixel.n_y): pixel for pixel in possible_pixels}
+
+        for address in city.addresses:
+            # Determine which `pixel` the `address` belongs to.
+            n_x = address.x // side_length
+            n_y = address.y // side_length
+            pixel = pixel_map[n_x, n_y]
+
+            # Create an association between the `address` and `pixel`.
+            assoc = db.AddressPixelAssociation(address=address, pixel=pixel)
+            pixel.addresses.append(assoc)
+
+        # Only keep `pixel`s that contain at least one `Address`.
+        grid.pixels = [pixel for pixel in pixel_map.values() if pixel.addresses]
+
+        return grid