Fixed #27845 -- Allowed both right and left optimizations of operations.

Thanks Raphael Gaschignard for the suggestion.

django/db/migrations/optimizer.py

@@ -42,21 +42,29 @@ class MigrationOptimizer:
         """Inner optimization loop."""
         new_operations = []
         for i, operation in enumerate(operations):
+            right = True  # Should we reduce on the right or on the left.
             # Compare it to each operation after it
             for j, other in enumerate(operations[i + 1:]):
                 in_between = operations[i + 1:i + j + 1]
                 result = operation.reduce(other, in_between, app_label)
                 if isinstance(result, list):
-                    # Add operations optimized through, optimized operations,
-                    # and the remaining ones.
+                    if right:
                         new_operations.extend(in_between)
                         new_operations.extend(result)
-                    new_operations.extend(operations[i + j + 2:])
-                    return new_operations
-                if not result:
-                    # We can't optimize across `other`.
+                    elif all(op.reduce(other, [], app_label) is True for op in in_between):
+                        # Perform a left reduction if all of the in-between
+                        # operations can optimize through other.
+                        new_operations.extend(result)
+                        new_operations.extend(in_between)
+                    else:
+                        # Otherwise keep trying.
                         new_operations.append(operation)
                         break
+                    new_operations.extend(operations[i + j + 2:])
+                    return new_operations
+                elif not result:
+                    # Can't perform a right reduction.
+                    right = False
             else:
                 new_operations.append(operation)
         return new_operations

tests/migrations/test_optimizer.py

@@ -270,8 +270,6 @@ class OptimizerTests(SimpleTestCase):
             app_label="testapp",
         )
 
-        # This could be optimized a bit more but it generates a valid set of
-        # operations.
         self.assertOptimizesTo(
             [
                 migrations.CreateModel('Book', [('name', models.CharField(max_length=255))]),
@@ -284,18 +282,12 @@ class OptimizerTests(SimpleTestCase):
                 migrations.DeleteModel('Person'),
             ],
             [
-                migrations.CreateModel('Person', [('name', models.CharField(max_length=255))]),
-                migrations.CreateModel('Book', [
-                    ('name', models.CharField(max_length=255)),
-                    ('author', models.ForeignKey('test_app.Person', models.CASCADE)),
-                ]),
+                migrations.CreateModel('Book', [('name', models.CharField(max_length=255))]),
                 migrations.CreateModel('Reviewer', [('name', models.CharField(max_length=255))]),
                 migrations.CreateModel('Review', [
                     ('book', models.ForeignKey('test_app.Book', models.CASCADE)),
                     ('reviewer', models.ForeignKey('test_app.Reviewer', models.CASCADE)),
                 ]),
-                migrations.RemoveField('book', 'author'),
-                migrations.DeleteModel('Person'),
             ],
         )