id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
25850	Migrations ignore inconsistent history silently	Shai Berger	nobody	"Doing some bad things, it is possible to get to a state where a migration is recorded as executed without all of its dependencies being executed. For example, if we have migrations for the same app developed on separate branches, and the person who merges them decides to resolve the conflict not by adding a ""merge migration"" as suggested by Django, but by editing the dependencies on migrations which were already run.

One should not do that, of course, but if one does do that, Django should refuse to do anything with these migrations until the situation is resolved. Currently, it just happily ignores the unfulfilled dependencies, and will migrate forwards and even cheerfully create new migrations. We don't even have an `InconsistentMigrationHistory` exception.

A failing test draft (add at the end of `tests/migrations/test_executor.py`):
{{{
    def test_reject_holes_in_history(self):
        """"""
        If the history is inconsistent with the sources, cry foul
        a: 1 <--- 2 <--- 3
        If a2 is applied already and a1 is not, and we're asked to migrate to
        a3, then something is very wrong and we should not proceed
        """"""
        a1_impl = FakeMigration('a1')
        a1 = ('a', '1')
        a2_impl = FakeMigration('a2')
        a2 = ('a', '2')
        a3_impl = FakeMigration('a3')
        a3 = ('a', '3')
        graph = MigrationGraph()
        graph.add_node(a1, a1_impl)
        graph.add_node(a2, a2_impl)
        graph.add_node(a3, a3_impl)
        graph.add_dependency(None, a2, a1)
        graph.add_dependency(None, a3, a2)

        executor = MigrationExecutor(None)
        executor.loader = FakeLoader(graph, {a2})

        with self.assertRaises(Exception):
            plan = executor.migration_plan({a3})
 }}}"	Cleanup/optimization	closed	Migrations	dev	Normal	fixed			Accepted	1	0	0	0	0	0