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

Thanks Raphael Gaschignard for the suggestion.

1 files changed, 16 insertions, 8 deletions

diff --git a/django/db/migrations/optimizer.py b/django/db/migrations/optimizer.py
index 26ac88a221..7974dba66b 100644
--- a/django/db/migrations/optimizer.py
+++ b/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.
-                    new_operations.extend(in_between)
-                    new_operations.extend(result)
+                    if right:
+                        new_operations.extend(in_between)
+                        new_operations.extend(result)
+                    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
-                if not result:
-                    # We can't optimize across `other`.
-                    new_operations.append(operation)
-                    break
+                elif not result:
+                    # Can't perform a right reduction.
+                    right = False
             else:
                 new_operations.append(operation)
         return new_operations