Add ORM models for the pixel grids

- add Grid, Pixel, and AddressPixelAssociation ORM models
- each Grid belongs to a City an is characterized by the side_length
  of all the square Pixels contained in it
- Pixels aggregate Addresses => many-to-many relationship (that is
  modeled with SQLAlchemy's Association Pattern to implement a couple
  of constraints)

tests/db/test_pixels.py

@@ -0,0 +1,90 @@
+"""Test the ORM's `Pixel` model."""
+# pylint:disable=no-self-use
+
+import pytest
+import sqlalchemy as sqla
+from sqlalchemy import exc as sa_exc
+
+from urban_meal_delivery import db
+
+
+class TestSpecialMethods:
+    """Test special methods in `Pixel`."""
+
+    def test_create_pixel(self, pixel):
+        """Test instantiation of a new `Pixel` object."""
+        assert pixel is not None
+
+    def test_text_representation(self, pixel):
+        """`Pixel` has a non-literal text representation."""
+        result = repr(pixel)
+
+        assert result == f'<Pixel: ({pixel.n_x}, {pixel.n_y})>'
+
+
+@pytest.mark.db
+@pytest.mark.no_cover
+class TestConstraints:
+    """Test the database constraints defined in `Pixel`."""
+
+    def test_insert_into_database(self, db_session, pixel):
+        """Insert an instance into the (empty) database."""
+        assert db_session.query(db.Pixel).count() == 0
+
+        db_session.add(pixel)
+        db_session.commit()
+
+        assert db_session.query(db.Pixel).count() == 1
+
+    def test_delete_a_referenced_grid(self, db_session, pixel):
+        """Remove a record that is referenced with a FK."""
+        db_session.add(pixel)
+        db_session.commit()
+
+        # Must delete without ORM as otherwise an UPDATE statement is emitted.
+        stmt = sqla.delete(db.Grid).where(db.Grid.id == pixel.grid.id)
+
+        with pytest.raises(
+            sa_exc.IntegrityError, match='fk_pixels_to_grids_via_grid_id',
+        ):
+            db_session.execute(stmt)
+
+    def test_negative_n_x(self, db_session, pixel):
+        """Insert an instance with invalid data."""
+        pixel.n_x = -1
+        db_session.add(pixel)
+
+        with pytest.raises(sa_exc.IntegrityError, match='n_x_is_positive'):
+            db_session.commit()
+
+    def test_negative_n_y(self, db_session, pixel):
+        """Insert an instance with invalid data."""
+        pixel.n_y = -1
+        db_session.add(pixel)
+
+        with pytest.raises(sa_exc.IntegrityError, match='n_y_is_positive'):
+            db_session.commit()
+
+    def test_non_unique_coordinates_within_a_grid(self, db_session, pixel):
+        """Insert an instance with invalid data."""
+        another_pixel = db.Pixel(grid=pixel.grid, n_x=pixel.n_x, n_y=pixel.n_y)
+        db_session.add(another_pixel)
+
+        with pytest.raises(sa_exc.IntegrityError, match='duplicate key value'):
+            db_session.commit()
+
+
+class TestProperties:
+    """Test properties in `Pixel`."""
+
+    def test_side_length(self, pixel):
+        """Test `Pixel.side_length` property."""
+        result = pixel.side_length
+
+        assert result == 1_000
+
+    def test_area(self, pixel):
+        """Test `Pixel.area` property."""
+        result = pixel.area
+
+        assert result == 1.0