Introduce Seat.sorting_rank (#1499)

* Introduce Seat.sorting_rank

* Fix comments

* Comments, for real

src/pretix/base/models/seating.py

@@ -40,7 +40,7 @@ class SeatingPlan(LoggedModel):
     layout = models.TextField(validators=[SeatingPlanLayoutValidator()])
 
     Category = namedtuple('Categrory', 'name')
-    RawSeat = namedtuple('Seat', 'name guid number row category zone')
+    RawSeat = namedtuple('Seat', 'name guid number row category zone sorting_rank')
 
     def __str__(self):
         return self.name
@@ -60,16 +60,36 @@ class SeatingPlan(LoggedModel):
         ]
 
     def iter_all_seats(self):
-        for z in self.layout_data['zones']:
-            for r in z['rows']:
-                for s in r['seats']:
+        # This returns all seats in a plan and assignes each of them a rank. The rank is used for sorting lists of
+        # seats later. The rank does not say anything about the *quality* of a seat, and is only meant as a heuristic
+        # to make it easier for humas to process lists of seats. The current algorithm assumes that there are less
+        # than 10'000 zones, less than 10'000 rows in every zone and less than 10'000 seats in every row.
+        # Respectively, no row/seat numbers may be numeric with a value of 10'000 or more. The resulting ranks
+        # *will* have gaps. We chose this way over just sorting the seats and continuously enumerating them as an
+        # optimization, because this way we do not need to update the rank of very seat if we change a plan a little.
+        for zi, z in enumerate(self.layout_data['zones']):
+            for ri, r in enumerate(z['rows']):
+                try:
+                    row_rank = int(r['row_number'])
+                except ValueError:
+                    row_rank = ri
+                for si, s in enumerate(r['seats']):
+                    try:
+                        seat_rank = int(s['seat_number'])
+                    except ValueError:
+                        seat_rank = si
+                    rank = (
+                        10000 * 10000 * zi + 10000 * row_rank + seat_rank
+                    )
+
                     yield self.RawSeat(
                         number=s['seat_number'],
                         guid=s['seat_guid'],
                         name='{} {}'.format(r['row_number'], s['seat_number']),  # TODO: Zone? Variable scheme?
                         row=r['row_number'],
                         zone=z['name'],
-                        category=s['category']
+                        category=s['category'],
+                        sorting_rank=rank
                     )
 
 
@@ -98,6 +118,10 @@ class Seat(models.Model):
     seat_guid = models.CharField(max_length=190, db_index=True)
     product = models.ForeignKey('Item', null=True, blank=True, related_name='seats', on_delete=models.CASCADE)
     blocked = models.BooleanField(default=False)
+    sorting_rank = models.BigIntegerField(default=0)
+
+    class Meta:
+        ordering = ['sorting_rank', 'seat_guid']
 
     def __str__(self):
         parts = []