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_grids.py

@@ -0,0 +1,60 @@
+"""Test the ORM's `Grid` 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 `Grid`."""
+
+    def test_create_grid(self, grid):
+        """Test instantiation of a new `Grid` object."""
+        assert grid is not None
+
+    def test_text_representation(self, grid):
+        """`Grid` has a non-literal text representation."""
+        result = repr(grid)
+
+        assert result == f'<Grid: {grid.pixel_area}>'
+
+
+@pytest.mark.db
+@pytest.mark.no_cover
+class TestConstraints:
+    """Test the database constraints defined in `Grid`."""
+
+    def test_insert_into_database(self, db_session, grid):
+        """Insert an instance into the (empty) database."""
+        assert db_session.query(db.Grid).count() == 0
+
+        db_session.add(grid)
+        db_session.commit()
+
+        assert db_session.query(db.Grid).count() == 1
+
+    def test_delete_a_referenced_city(self, db_session, grid):
+        """Remove a record that is referenced with a FK."""
+        db_session.add(grid)
+        db_session.commit()
+
+        # Must delete without ORM as otherwise an UPDATE statement is emitted.
+        stmt = sqla.delete(db.City).where(db.City.id == grid.city.id)
+
+        with pytest.raises(
+            sa_exc.IntegrityError, match='fk_grids_to_cities_via_city_id',
+        ):
+            db_session.execute(stmt)
+
+
+class TestProperties:
+    """Test properties in `Grid`."""
+
+    def test_pixel_area(self, grid):
+        """Test `Grid.pixel_area` property."""
+        result = grid.pixel_area
+
+        assert result == 1.0