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

tests/db/test_cities.py

@@ -3,7 +3,6 @@
 
 import pytest
 
-from tests.db.utils import test_locations as consts
 from urban_meal_delivery import db
 from urban_meal_delivery.db import utils
 
@@ -55,33 +54,44 @@ class TestProperties:
 
         assert result1 is result2
 
-    def test_viewport_overall(self, city):
-        """Test `City.viewport` property."""
-        result = city.viewport
-
-        assert isinstance(result, dict)
-        assert len(result) == 2
-
-    @pytest.mark.parametrize('corner', ['northeast', 'southwest'])
-    def test_viewport_corners(self, city, city_data, corner):
-        """Test `City.viewport` property."""
-        result = city.viewport[corner]
+    def test_northeast(self, city, city_data):
+        """Test `City.northeast` property."""
+        result = city.northeast
 
         assert isinstance(result, utils.Location)
-        assert result.latitude == pytest.approx(city_data[f'_{corner}_latitude'])
-        assert result.longitude == pytest.approx(city_data[f'_{corner}_longitude'])
+        assert result.latitude == pytest.approx(city_data['_northeast_latitude'])
+        assert result.longitude == pytest.approx(city_data['_northeast_longitude'])
 
-    def test_viewport_is_cached(self, city):
-        """Test `City.viewport` property."""
-        result1 = city.viewport
-        result2 = city.viewport
+    def test_northeast_is_cached(self, city):
+        """Test `City.northeast` property."""
+        result1 = city.northeast
+        result2 = city.northeast
 
         assert result1 is result2
 
-    def test_city_as_xy_origin(self, city):
-        """Test `City.as_xy_origin` property."""
-        result = city.as_xy_origin
+    def test_southwest(self, city, city_data):
+        """Test `City.southwest` property."""
+        result = city.southwest
 
-        assert result.zone == consts.ZONE
-        assert consts.MIN_EASTING < result.easting < consts.MAX_EASTING
-        assert consts.MIN_NORTHING < result.northing < consts.MAX_NORTHING
+        assert isinstance(result, utils.Location)
+        assert result.latitude == pytest.approx(city_data['_southwest_latitude'])
+        assert result.longitude == pytest.approx(city_data['_southwest_longitude'])
+
+    def test_southwest_is_cached(self, city):
+        """Test `City.southwest` property."""
+        result1 = city.southwest
+        result2 = city.southwest
+
+        assert result1 is result2
+
+    def test_total_x(self, city):
+        """Test `City.total_x` property."""
+        result = city.total_x
+
+        assert result > 18_000
+
+    def test_total_y(self, city):
+        """Test `City.total_y` property."""
+        result = city.total_y
+
+        assert result > 9_000

tests/db/test_grids.py

@@ -49,6 +49,18 @@ class TestConstraints:
         ):
             db_session.execute(stmt)
 
+    def test_two_grids_with_identical_side_length(self, db_session, grid):
+        """Insert a record that violates a unique constraint."""
+        db_session.add(grid)
+        db_session.commit()
+
+        # Create a `Grid` with the same `.side_length` in the same `.city`.
+        another_grid = db.Grid(city=grid.city, side_length=grid.side_length)
+        db_session.add(another_grid)
+
+        with pytest.raises(sa_exc.IntegrityError, match='duplicate key value'):
+            db_session.commit()
+
 
 class TestProperties:
     """Test properties in `Grid`."""
@@ -58,3 +70,86 @@ class TestProperties:
         result = grid.pixel_area
 
         assert result == 1.0
+
+
+class TestGridification:
+    """Test the `Grid.gridify()` constructor."""
+
+    def test_one_pixel_covering_entire_city_without_addresses(self, city):
+        """At the very least, there must be one `Pixel` ...
+
+        ... if the `side_length` is greater than both the
+        horizontal and vertical distances of the viewport.
+        """
+        city.addresses = []
+
+        # `+1` as otherwise there would be a second pixel in one direction.
+        side_length = max(city.total_x, city.total_y) + 1
+
+        result = db.Grid.gridify(city=city, side_length=side_length)
+
+        assert isinstance(result, db.Grid)
+        assert len(result.pixels) == 0  # noqa:WPS507
+
+    def test_one_pixel_covering_entire_city_with_one_address(self, city, address):
+        """At the very least, there must be one `Pixel` ...
+
+        ... if the `side_length` is greater than both the
+        horizontal and vertical distances of the viewport.
+        """
+        city.addresses = [address]
+
+        # `+1` as otherwise there would be a second pixel in one direction.
+        side_length = max(city.total_x, city.total_y) + 1
+
+        result = db.Grid.gridify(city=city, side_length=side_length)
+
+        assert isinstance(result, db.Grid)
+        assert len(result.pixels) == 1
+
+    def test_four_pixels_with_two_addresses(self, city, make_address):
+        """Two `Address` objects in distinct `Pixel` objects."""
+        # Create two `Address` objects in distinct `Pixel`s.
+        city.addresses = [
+            # One `Address` in the lower-left `Pixel`, ...
+            make_address(latitude=48.8357377, longitude=2.2517412),
+            # ... and another one in the upper-right one.
+            make_address(latitude=48.8898312, longitude=2.4357622),
+        ]
+
+        side_length = max(city.total_x // 2, city.total_y // 2) + 1
+
+        # By assumption of the test data.
+        n_pixels_x = (city.total_x // side_length) + 1
+        n_pixels_y = (city.total_y // side_length) + 1
+        assert n_pixels_x * n_pixels_y == 4
+
+        # Create a `Grid` with at most four `Pixel`s.
+        result = db.Grid.gridify(city=city, side_length=side_length)
+
+        assert isinstance(result, db.Grid)
+        assert len(result.pixels) == 2
+
+    @pytest.mark.db
+    @pytest.mark.no_cover
+    @pytest.mark.parametrize('side_length', [250, 500, 1_000, 2_000, 4_000, 8_000])
+    def test_make_random_grids(self, db_session, city, make_address, side_length):
+        """With 100 random `Address` objects, a grid must have ...
+
+        ... between 1 and a deterministic number of `Pixel` objects.
+
+        This test creates confidence that the created `Grid`
+        objects adhere to the database constraints.
+        """
+        city.addresses = [make_address() for _ in range(100)]
+
+        n_pixels_x = (city.total_x // side_length) + 1
+        n_pixels_y = (city.total_y // side_length) + 1
+
+        result = db.Grid.gridify(city=city, side_length=side_length)
+
+        assert isinstance(result, db.Grid)
+        assert 1 <= len(result.pixels) <= n_pixels_x * n_pixels_y
+
+        db_session.add(result)
+        db_session.commit()

tests/db/test_orders.py

@@ -425,7 +425,7 @@ class TestProperties:
 
 @pytest.mark.db
 @pytest.mark.no_cover
-def test_make_random_orders(  # noqa:C901,WPS211,WPS210,WPS213,WPS231
+def test_make_random_orders(  # noqa:C901,WPS211,WPS213,WPS231
     db_session, make_address, make_courier, make_restaurant, make_order,
 ):
     """Sanity check the all the `make_*` fixtures.

tests/db/utils/test_locations.py

@@ -35,7 +35,7 @@ def faraway_location():
 @pytest.fixture
 def origin(city):
     """A `Location` object based off the one and only `city`."""
-    obj = city.as_xy_origin
+    obj = city.southwest
 
     assert obj.zone == ZONE  # sanity check