rebase-migration: Make it possible to handle chains of migrations.

Previously, the rebase_migration command was not able to rebase
chains of migrations in a single app.

This commit introduces a new flag -- "new" which basically is used
for the first migration you create, and it wipes out your migration
history in max_migration.txt, and writes up that first migration in
the first line. Any further migrations added without the flag are
simply added under each other in the max_migration.txt.

This would allow the rebase_migration command to access the
chain of migrations that need to be rebased in a commit, and
will rebase them accordingly.

Fixes adamchainz#27

src/django_linear_migrations/apps.py

@@ -188,41 +188,28 @@ def check_max_migration_files(
             )
             continue
 
-        max_migration_txt_lines = max_migration_txt.read_text().strip().splitlines()
-        if len(max_migration_txt_lines) > 1:
-            errors.append(
-                Error(
-                    id="dlm.E002",
-                    msg=f"{app_label}'s max_migration.txt contains multiple lines.",
-                    hint=(
-                        "This may be the result of a git merge. Fix the file"
-                        + " to contain only the name of the latest migration,"
-                        + " or maybe use the 'rebase-migration' command."
-                    ),
+        migration_txt_lines = max_migration_txt.read_text().strip().splitlines()
+        for migration_name in migration_txt_lines:
+            if migration_name not in migration_details.names:
+                errors.append(
+                    Error(
+                        id="dlm.E003",
+                        msg=(
+                            f"{app_label}'s max_migration.txt points to"
+                            + f" non-existent migration {migration_name!r}."
+                        ),
+                        hint=(
+                            "Edit the max_migration.txt to contain the latest"
+                            + " migration's name."
+                        ),
+                    )
                 )
-            )
-            continue
-
-        max_migration_name = max_migration_txt_lines[0]
-        if max_migration_name not in migration_details.names:
-            errors.append(
-                Error(
-                    id="dlm.E003",
-                    msg=(
-                        f"{app_label}'s max_migration.txt points to"
-                        + f" non-existent migration {max_migration_name!r}."
-                    ),
-                    hint=(
-                        "Edit the max_migration.txt to contain the latest"
-                        + " migration's name."
-                    ),
-                )
-            )
             continue
 
         real_max_migration_name = [
             name for gp_app_label, name in graph_plan if gp_app_label == app_label
         ][-1]
+        max_migration_name = migration_txt_lines[-1]
         if max_migration_name != real_max_migration_name:
             errors.append(
                 Error(

src/django_linear_migrations/management/commands/makemigrations.py

@@ -1,6 +1,9 @@
 from __future__ import annotations
 
+from typing import Any
+
 import django
+from django.core.management.base import CommandParser
 from django.core.management.commands.makemigrations import Command as BaseCommand
 from django.db.migrations import Migration
 
@@ -9,6 +12,18 @@
 
 
 class Command(BaseCommand):
+    def add_arguments(self, parser: CommandParser) -> None:
+        super().add_arguments(parser)
+        parser.add_argument(
+            "--new",
+            action="store_true",
+            help="Create and register the migration as the first migration of the commit.",
+        )
+
+    def handle(self, *app_labels: str, **options: Any) -> None:
+        self.first_migration = options["new"]
+        super().handle(*app_labels, **options)
+
     if django.VERSION >= (4, 2):
 
         def write_migration_files(
@@ -22,7 +37,7 @@ def write_migration_files(
                 changes,
                 update_previous_migration_paths,
             )
-            _post_write_migration_files(self.dry_run, changes)
+            self._post_write_migration_files(self.dry_run, changes)
 
     else:
 
@@ -31,25 +46,33 @@ def write_migration_files(  # type: ignore[misc,override]
             changes: dict[str, list[Migration]],
         ) -> None:
             super().write_migration_files(changes)
-            _post_write_migration_files(self.dry_run, changes)
+            self._post_write_migration_files(self.dry_run, changes)
 
+    def _post_write_migration_files(
+        self, dry_run: bool, changes: dict[str, list[Migration]]
+    ) -> None:
+        if dry_run:
+            return
 
-def _post_write_migration_files(
-    dry_run: bool, changes: dict[str, list[Migration]]
-) -> None:
-    if dry_run:
-        return
+        first_party_app_labels = {
+            app_config.label for app_config in first_party_app_configs()
+        }
 
-    first_party_app_labels = {
-        app_config.label for app_config in first_party_app_configs()
-    }
+        for app_label, app_migrations in changes.items():
+            if app_label not in first_party_app_labels:
+                continue
 
-    for app_label, app_migrations in changes.items():
-        if app_label not in first_party_app_labels:
-            continue
+            # Reload required as we've generated changes
+            migration_details = MigrationDetails(app_label, do_reload=True)
+            max_migration_name = app_migrations[-1].name
+            max_migration_txt = migration_details.dir / "max_migration.txt"
 
-        # Reload required as we've generated changes
-        migration_details = MigrationDetails(app_label, do_reload=True)
-        max_migration_name = app_migrations[-1].name
-        max_migration_txt = migration_details.dir / "max_migration.txt"
-        max_migration_txt.write_text(max_migration_name + "\n")
+            if self.first_migration:
+                max_migration_txt.write_text(max_migration_name + "\n")
+                self.first_migration = False
+                continue
+
+            current_version_migrations = max_migration_txt.read_text()
+            max_migration_txt.write_text(
+                current_version_migrations + max_migration_name + "\n"
+            )

src/django_linear_migrations/management/commands/rebase_migration.py

@@ -46,27 +46,43 @@ def handle(self, *args: Any, app_label: str, **options: Any) -> None:
         if not max_migration_txt.exists():
             raise CommandError(f"{app_label} does not have a max_migration.txt.")
 
-        migration_names = find_migration_names(
-            max_migration_txt.read_text().splitlines()
-        )
+        migration_names = find_migration_names(max_migration_txt.read_text())
         if migration_names is None:
             raise CommandError(
                 f"{app_label}'s max_migration.txt does not seem to contain a"
                 + " merge conflict."
             )
-        merged_migration_name, rebased_migration_name = migration_names
-        if merged_migration_name not in migration_details.names:
-            raise CommandError(
-                f"Parsed {merged_migration_name!r} as the already-merged"
-                + f" migration name from {app_label}'s max_migration.txt, but"
-                + " this migration does not exist."
-            )
-        if rebased_migration_name not in migration_details.names:
-            raise CommandError(
-                f"Parsed {rebased_migration_name!r} as the rebased migration"
-                + f" name from {app_label}'s max_migration.txt, but this"
-                + " migration does not exist."
-            )
+
+        merged_migration_names, rebased_migration_names = migration_names
+
+        for merged_migration_name in merged_migration_names:
+            if merged_migration_name not in migration_details.names:
+                raise CommandError(
+                    f"Parsed {merged_migration_name!r} as the already-merged"
+                    + f" migration name from {app_label}'s max_migration.txt, but"
+                    + " this migration does not exist."
+                )
+
+        for rebased_migration_name in rebased_migration_names:
+            if rebased_migration_name not in migration_details.names:
+                raise CommandError(
+                    f"Parsed {rebased_migration_name!r} as the rebased migration"
+                    + f" name from {app_label}'s max_migration.txt, but this"
+                    + " migration does not exist."
+                )
+
+        self.last_migration_name = merged_migration_names[-1]
+
+        first_migration = True
+        for rebased_migration_name in rebased_migration_names:
+            self.rebase_migration(app_label, rebased_migration_name, first_migration)
+            first_migration = False
+
+    def rebase_migration(
+        self, app_label: str, rebased_migration_name: str, first_migration: bool
+    ) -> None:
+        migration_details = MigrationDetails(app_label)
+        max_migration_txt = migration_details.dir / "max_migration.txt"
 
         rebased_migration_filename = f"{rebased_migration_name}.py"
         rebased_migration_path = migration_details.dir / rebased_migration_filename
@@ -136,7 +152,7 @@ def handle(self, *args: Any, app_label: str, **options: Any) -> None:
                     ast.Tuple(
                         elts=[
                             ast.Constant(app_label),
-                            ast.Constant(merged_migration_name),
+                            ast.Constant(self.last_migration_name),
                         ]
                     )
                 )
@@ -152,16 +168,23 @@ def handle(self, *args: Any, app_label: str, **options: Any) -> None:
 
         new_content = before_deps + ast_unparse(new_dependencies) + after_deps
 
-        merged_number, _merged_rest = merged_migration_name.split("_", 1)
+        last_merged_number, _merged_rest = self.last_migration_name.split("_", 1)
         _rebased_number, rebased_rest = rebased_migration_name.split("_", 1)
-        new_number = int(merged_number) + 1
+        new_number = int(last_merged_number) + 1
         new_name = str(new_number).zfill(4) + "_" + rebased_rest
         new_path_parts = rebased_migration_path.parts[:-1] + (f"{new_name}.py",)
         new_path = Path(*new_path_parts)
 
         rebased_migration_path.rename(new_path)
         new_path.write_text(new_content)
-        max_migration_txt.write_text(f"{new_name}\n")
+
+        if first_migration:
+            max_migration_txt.write_text(f"{new_name}\n")
+        else:
+            current_version_migrations = max_migration_txt.read_text()
+            max_migration_txt.write_text(current_version_migrations + f"{new_name}\n")
+
+        self.last_migration_name = new_name
 
         black_path = shutil.which("black")
         if black_path:  # pragma: no cover
@@ -176,19 +199,45 @@ def handle(self, *args: Any, app_label: str, **options: Any) -> None:
         )
 
 
-def find_migration_names(max_migration_lines: list[str]) -> tuple[str, str] | None:
-    lines = max_migration_lines
-    if len(lines) <= 1:
+def find_migration_names(
+    current_version_migrations: str,
+) -> tuple[list[str], list[str]] | None:
+    migrations_lines = current_version_migrations.strip().splitlines()
+
+    if len(migrations_lines) <= 1:
         return None
-    if not lines[0].startswith("<<<<<<<"):
+    if not migrations_lines[0].startswith("<<<<<<<"):
         return None
-    if not lines[-1].startswith(">>>>>>>"):
+    if not migrations_lines[-1].startswith(">>>>>>>"):
         return None
-    migration_names = (lines[1].strip(), lines[-2].strip())
+
+    merged_migration_names = []
+    rebased_migration_names = []
+
+    index = 0
+    while index < len(migrations_lines):
+        if migrations_lines[index].startswith("<<<<<<<"):
+            index += 1
+            while not migrations_lines[index].startswith("======="):
+                if migrations_lines[index] == "|||||||":
+                    while not migrations_lines[index].startswith("======="):
+                        index += 1
+                else:
+                    merged_migration_names.append(migrations_lines[index])
+                    index += 1
+
+            index += 1
+
+        else:
+            while not migrations_lines[index].startswith(">>>>>>>"):
+                rebased_migration_names.append(migrations_lines[index])
+                index += 1
+            break
+
     if is_merge_in_progress():
         # During the merge 'ours' and 'theirs' are swapped in comparison with rebase
-        migration_names = (migration_names[1], migration_names[0])
-    return migration_names
+        return (rebased_migration_names, merged_migration_names)
+    return (merged_migration_names, rebased_migration_names)
 
 
 def is_merge_in_progress() -> bool:

tests/test_checks.py

@@ -64,25 +64,14 @@ def test_dlm_E001(self):
         assert result[0].id == "dlm.E001"
         assert result[0].msg == "testapp's max_migration.txt does not exist."
 
-    def test_dlm_E002(self):
-        (self.migrations_dir / "__init__.py").touch()
-        (self.migrations_dir / "0001_initial.py").write_text(empty_migration)
-        (self.migrations_dir / "max_migration.txt").write_text("line1\nline2\n")
-
-        result = check_max_migration_files()
-
-        assert len(result) == 1
-        assert result[0].id == "dlm.E002"
-        assert result[0].msg == "testapp's max_migration.txt contains multiple lines."
-
     def test_dlm_E003(self):
         (self.migrations_dir / "__init__.py").touch()
         (self.migrations_dir / "0001_initial.py").write_text(empty_migration)
         (self.migrations_dir / "max_migration.txt").write_text("0001_start\n")
 
         result = check_max_migration_files()
 
-        assert len(result) == 1
+        assert len(result) == 2
         assert result[0].id == "dlm.E003"
         assert result[0].msg == (
             "testapp's max_migration.txt points to non-existent migration"

tests/test_makemigrations.py

@@ -40,15 +40,15 @@ def test_dry_run(self):
         assert not max_migration_txt.exists()
 
     def test_creates_max_migration_txt(self):
-        out, err, returncode = self.call_command("testapp")
+        out, err, returncode = self.call_command("testapp", "--new")
 
         assert returncode == 0
         max_migration_txt = self.migrations_dir / "max_migration.txt"
         assert max_migration_txt.read_text() == "0001_initial\n"
 
     @unittest.skipUnless(django.VERSION >= (4, 2), "--update added in Django 4.2")
     def test_update(self):
-        self.call_command("testapp")
+        self.call_command("testapp", "--new")
         max_migration_txt = self.migrations_dir / "max_migration.txt"
         assert max_migration_txt.read_text() == "0001_initial\n"
 
@@ -59,10 +59,12 @@ class Meta:
         out, err, returncode = self.call_command("--update", "testapp")
         assert returncode == 0
         max_migration_txt = self.migrations_dir / "max_migration.txt"
-        assert max_migration_txt.read_text() == "0001_initial_updated\n"
+        assert max_migration_txt.read_text() == "0001_initial\n0001_initial_updated\n"
 
     def test_creates_max_migration_txt_given_name(self):
-        out, err, returncode = self.call_command("testapp", "--name", "brand_new")
+        out, err, returncode = self.call_command(
+            "testapp", "--name", "brand_new", "--new"
+        )
 
         assert returncode == 0
         max_migration_txt = self.migrations_dir / "max_migration.txt"
@@ -89,7 +91,36 @@ class Migration(migrations.Migration):
 
         assert returncode == 0
         max_migration_txt = self.migrations_dir / "max_migration.txt"
-        assert max_migration_txt.read_text() == "0002_create_book\n"
+        assert max_migration_txt.read_text() == "0001_initial\n0002_create_book\n"
+
+    def test_create_max_migration_txt_with_multiple_migrations(self):
+        max_migration_txt = self.migrations_dir / "max_migration.txt"
+        (self.migrations_dir / "__init__.py").touch()
+
+        out, err, returncode = self.call_command("testapp", "--name", "first", "--new")
+
+        assert returncode == 0
+        assert max_migration_txt.read_text() == "0001_first\n"
+
+        # Creating a second migration on without the `new` flag keeps
+        # the first migration, while updates the last migration in the
+        # "max_migration.txt"
+        out, err, returncode = self.call_command(
+            "testapp", "--empty", "--name", "second"
+        )
+
+        assert returncode == 0
+        assert max_migration_txt.read_text() == "0001_first\n0002_second\n"
+
+        # Creating a third migration on without the `new` flag keeps
+        # the first migration, while updates the last migration in the
+        # "max_migration.txt"
+        out, err, returncode = self.call_command(
+            "testapp", "--empty", "--name", "third"
+        )
+
+        assert returncode == 0
+        assert max_migration_txt.read_text() == "0001_first\n0002_second\n0003_third\n"
 
     @override_settings(FIRST_PARTY_APPS=[])
     def test_skips_creating_max_migration_txt_for_non_first_party_app(self):