Commit 54d3dcbc authored by Brad Walker's avatar Brad Walker Committed by Tim Graham
Browse files

Removed reduce() usage in makemigrations; refs #23796. 

A lambda all_items_equal() replaced a reduce() that was broken for potential
3+-way merges. A reduce(operator.eq, ...) accumulates bools and can't
generically check equality of all items in a sequence:

>>> bool(reduce(operator.eq, [('migrations', '0001_initial')] * 3))
False

The code now counts the number of common ancestors to calculate slice offsets
for the branches. Each branch shares the same number of common ancestors.

The common_ancestor for loop statement had incomplete branch coverage.
parent cfa26f29

django/core/management/commands/makemigrations.py

+11 −17
Original line number Diff line number Diff line 
from itertools import takewhile

import sys

import os

import operator



from django.apps import apps

from django.core.management.base import BaseCommand, CommandError
@@ -11,7 +11,7 @@ from django.db.migrations.questioner import MigrationQuestioner, InteractiveMigr 
from django.db.migrations.state import ProjectState

from django.db.migrations.writer import MigrationWriter

from django.utils.six import iteritems

from django.utils.six.moves import reduce

from django.utils.six.moves import zip





class Command(BaseCommand):
@@ -187,24 +187,18 @@ class Command(BaseCommand): 
                migration = loader.get_migration(app_label, migration_name)

                migration.ancestry = loader.graph.forwards_plan((app_label, migration_name))

                merge_migrations.append(migration)

            common_ancestor = None

            for level in zip(*[m.ancestry for m in merge_migrations]):

                if reduce(operator.eq, level):

                    common_ancestor = level[0]

                else:

                    break

            if common_ancestor is None:

            all_items_equal = lambda seq: all(item == seq[0] for item in seq[1:])

            merge_migrations_generations = zip(*[m.ancestry for m in merge_migrations])

            common_ancestor_count = sum(1 for common_ancestor_generation

                                        in takewhile(all_items_equal, merge_migrations_generations))

            if not common_ancestor_count:

                raise ValueError("Could not find common ancestor of %s" % migration_names)

            # Now work out the operations along each divergent branch

            for migration in merge_migrations:

                migration.branch = migration.ancestry[

                    (migration.ancestry.index(common_ancestor) + 1):

                ]

                migration.merged_operations = []

                for node_app, node_name in migration.branch:

                    migration.merged_operations.extend(

                        loader.get_migration(node_app, node_name).operations

                    )

                migration.branch = migration.ancestry[common_ancestor_count:]

                migrations_ops = (loader.get_migration(node_app, node_name).operations

                                  for node_app, node_name in migration.branch)

                migration.merged_operations = sum(migrations_ops, [])

            # In future, this could use some of the Optimizer code

            # (can_optimize_through) to automatically see if they're

            # mergeable. For now, we always just prompt the user.