Migration VCS conflict detection and --merge for makemigrations

11 files changed, 341 insertions, 117 deletions

diff --git a/django/core/management/commands/makemigrations.py b/django/core/management/commands/makemigrations.py
index bb07160bb6..284898d8d4 100644
--- a/django/core/management/commands/makemigrations.py
+++ b/django/core/management/commands/makemigrations.py
@@ -1,21 +1,26 @@
 import sys
 import os
+import operator
 from optparse import make_option
 
-from django.core.management.base import BaseCommand
+from django.core.management.base import BaseCommand, CommandError
 from django.core.exceptions import ImproperlyConfigured
-from django.db import connections, DEFAULT_DB_ALIAS
+from django.db import connections, DEFAULT_DB_ALIAS, migrations
 from django.db.migrations.loader import MigrationLoader
-from django.db.migrations.autodetector import MigrationAutodetector, InteractiveMigrationQuestioner
+from django.db.migrations.autodetector import MigrationAutodetector
+from django.db.migrations.questioner import MigrationQuestioner, InteractiveMigrationQuestioner
 from django.db.migrations.state import ProjectState
 from django.db.migrations.writer import MigrationWriter
 from django.db.models.loading import cache
+from django.utils.six.moves import reduce
 
 
 class Command(BaseCommand):
     option_list = BaseCommand.option_list + (
         make_option('--dry-run', action='store_true', dest='dry_run', default=False,
             help="Just show what migrations would be made; don't actually write them."),
+        make_option('--merge', action='store_true', dest='merge', default=False,
+            help="Enable fixing of migration conflicts."),
     )
 
     help = "Creates new migration(s) for apps."
@@ -26,6 +31,7 @@ class Command(BaseCommand):
         self.verbosity = int(options.get('verbosity'))
         self.interactive = options.get('interactive')
         self.dry_run = options.get('dry_run', False)
+        self.merge = options.get('merge', False)
 
         # Make sure the app they asked for exists
         app_labels = set(app_labels)
@@ -44,6 +50,26 @@ class Command(BaseCommand):
         # (makemigrations doesn't look at the database state).
         loader = MigrationLoader(connections[DEFAULT_DB_ALIAS])
 
+        # Before anything else, see if there's conflicting apps and drop out
+        # hard if there are any and they don't want to merge
+        conflicts = loader.detect_conflicts()
+        if conflicts and not self.merge:
+            name_str = "; ".join(
+                "%s in %s" % (", ".join(names), app)
+                for app, names in conflicts.items()
+            )
+            raise CommandError("Conflicting migrations detected (%s).\nTo fix them run 'python manage.py makemigrations --merge'" % name_str)
+
+        # If they want to merge and there's nothing to merge, then politely exit
+        if self.merge and not conflicts:
+            self.stdout.write("No conflicts detected to merge.")
+            return
+
+        # If they want to merge and there is something to merge, then
+        # divert into the merge code
+        if self.merge and conflicts:
+            return self.handle_merge(loader, conflicts)
+
         # Detect changes
         autodetector = MigrationAutodetector(
             loader.graph.project_state(),
@@ -87,3 +113,68 @@ class Command(BaseCommand):
                     migration_string = writer.as_string()
                     with open(writer.path, "wb") as fh:
                         fh.write(migration_string)
+
+    def handle_merge(self, loader, conflicts):
+        """
+        Handles merging together conflicted migrations interactively,
+        if it's safe; otherwise, advises on how to fix it.
+        """
+        if self.interactive:
+            questioner = InteractiveMigrationQuestioner()
+        else:
+            questioner = MigrationQuestioner()
+        for app_label, migration_names in conflicts.items():
+            # Grab out the migrations in question, and work out their
+            # common ancestor.
+            merge_migrations = []
+            for migration_name in migration_names:
+                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:
+                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
+                    )
+            # 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.
+            if self.verbosity > 0:
+                self.stdout.write(self.style.MIGRATE_HEADING("Merging %s" % app_label))
+                for migration in merge_migrations:
+                    self.stdout.write(self.style.MIGRATE_LABEL("  Branch %s" % migration.name))
+                    for operation in migration.merged_operations:
+                        self.stdout.write("    - %s\n" % operation.describe())
+            if questioner.ask_merge(app_label):
+                # If they still want to merge it, then write out an empty
+                # file depending on the migrations needing merging.
+                numbers = [
+                    MigrationAutodetector.parse_number(migration.name)
+                    for migration in merge_migrations
+                ]
+                try:
+                    biggest_number = max([x for x in numbers if x is not None])
+                except ValueError:
+                    biggest_number = 1
+                subclass = type("Migration", (migrations.Migration, ), {
+                    "dependencies": [(app_label, migration.name) for migration in merge_migrations],
+                })
+                new_migration = subclass("%04i_merge" % (biggest_number + 1), app_label)
+                writer = MigrationWriter(new_migration)
+                with open(writer.path, "wb") as fh:
+                    fh.write(writer.as_string())
+                if self.verbosity > 0:
+                    self.stdout.write("\nCreated new merge migration %s" % writer.path)
diff --git a/django/core/management/commands/migrate.py b/django/core/management/commands/migrate.py
index 3e796b655f..093c8a79d0 100644
--- a/django/core/management/commands/migrate.py
+++ b/django/core/management/commands/migrate.py
@@ -62,6 +62,16 @@ class Command(BaseCommand):
         # Work out which apps have migrations and which do not
         executor = MigrationExecutor(connection, self.migration_progress_callback)
 
+        # Before anything else, see if there's conflicting apps and drop out
+        # hard if there are any
+        conflicts = executor.loader.detect_conflicts()
+        if conflicts:
+            name_str = "; ".join(
+                "%s in %s" % (", ".join(names), app)
+                for app, names in conflicts.items()
+            )
+            raise CommandError("Conflicting migrations detected (%s).\nTo fix them run 'python manage.py makemigrations --merge'" % name_str)
+
         # If they supplied command line arguments, work out what they mean.
         run_syncdb = False
         target_app_labels_only = True
diff --git a/django/db/migrations/autodetector.py b/django/db/migrations/autodetector.py
index f8ec49ab0a..86ddd3c3d2 100644
--- a/django/db/migrations/autodetector.py
+++ b/django/db/migrations/autodetector.py
@@ -1,13 +1,8 @@
-import importlib
-import os
 import re
-import sys
 
 from django.db.migrations import operations
 from django.db.migrations.migration import Migration
-from django.db.models.loading import cache
-from django.utils import datetime_safe
-from django.utils.six.moves import input
+from django.db.migrations.questioner import MigrationQuestioner
 
 
 class MigrationAutodetector(object):
@@ -369,108 +364,3 @@ class MigrationAutodetector(object):
         if re.match(r"^\d+_", name):
             return int(name.split("_")[0])
         return None
-
-
-class MigrationQuestioner(object):
-    """
-    Gives the autodetector responses to questions it might have.
-    This base class has a built-in noninteractive mode, but the
-    interactive subclass is what the command-line arguments will use.
-    """
-
-    def __init__(self, defaults=None):
-        self.defaults = defaults or {}
-
-    def ask_initial(self, app_label):
-        "Should we create an initial migration for the app?"
-        return self.defaults.get("ask_initial", False)
-
-    def ask_not_null_addition(self, field_name, model_name):
-        "Adding a NOT NULL field to a model"
-        # None means quit
-        return None
-
-    def ask_rename(self, model_name, old_name, new_name, field_instance):
-        "Was this field really renamed?"
-        return self.defaults.get("ask_rename", False)
-
-
-class InteractiveMigrationQuestioner(MigrationQuestioner):
-
-    def __init__(self, specified_apps=set()):
-        self.specified_apps = specified_apps
-
-    def _boolean_input(self, question, default=None):
-        result = input("%s " % question)
-        if not result and default is not None:
-            return default
-        while len(result) < 1 or result[0].lower() not in "yn":
-            result = input("Please answer yes or no: ")
-        return result[0].lower() == "y"
-
-    def _choice_input(self, question, choices):
-        print(question)
-        for i, choice in enumerate(choices):
-            print(" %s) %s" % (i + 1, choice))
-        result = input("Select an option: ")
-        while True:
-            try:
-                value = int(result)
-                if 0 < value <= len(choices):
-                    return value
-            except ValueError:
-                pass
-            result = input("Please select a valid option: ")
-
-    def ask_initial(self, app_label):
-        "Should we create an initial migration for the app?"
-        # If it was specified on the command line, definitely true
-        if app_label in self.specified_apps:
-            return True
-        # Otherwise, we look to see if it has a migrations module
-        # without any Python files in it, apart from __init__.py.
-        # Apps from the new app template will have these; the python
-        # file check will ensure we skip South ones.
-        models_module = cache.get_app(app_label)
-        migrations_import_path = "%s.migrations" % models_module.__package__
-        try:
-            migrations_module = importlib.import_module(migrations_import_path)
-        except ImportError:
-            return False
-        else:
-            filenames = os.listdir(os.path.dirname(migrations_module.__file__))
-            return not any(x.endswith(".py") for x in filenames if x != "__init__.py")
-
-    def ask_not_null_addition(self, field_name, model_name):
-        "Adding a NOT NULL field to a model"
-        choice = self._choice_input(
-            "You are trying to add a non-nullable field '%s' to %s without a default;\n" % (field_name, model_name) +
-            "we can't do that (the database needs something to populate existing rows).\n" +
-            "Please select a fix:",
-            [
-                "Provide a one-off default now (will be set on all existing rows)",
-                "Quit, and let me add a default in models.py",
-            ]
-        )
-        if choice == 2:
-            sys.exit(3)
-        else:
-            print("Please enter the default value now, as valid Python")
-            print("The datetime module is available, so you can do e.g. datetime.date.today()")
-            while True:
-                code = input(">>> ")
-                if not code:
-                    print("Please enter some code, or 'exit' (with no quotes) to exit.")
-                elif code == "exit":
-                    sys.exit(1)
-                else:
-                    try:
-                        return eval(code, {}, {"datetime": datetime_safe})
-                    except (SyntaxError, NameError) as e:
-                        print("Invalid input: %s" % e)
-                    else:
-                        break
-
-    def ask_rename(self, model_name, old_name, new_name, field_instance):
-        "Was this field really renamed?"
-        return self._boolean_input("Did you rename %s.%s to %s.%s (a %s)? [y/N]" % (model_name, old_name, model_name, new_name, field_instance.__class__.__name__), False)
diff --git a/django/db/migrations/loader.py b/django/db/migrations/loader.py
index 598c582fa0..f2510b8368 100644
--- a/django/db/migrations/loader.py
+++ b/django/db/migrations/loader.py
@@ -187,6 +187,20 @@ class MigrationLoader(object):
             for parent in migration.dependencies:
                 self.graph.add_dependency(key, parent)
 
+    def detect_conflicts(self):
+        """
+        Looks through the loaded graph and detects any conflicts - apps
+        with more than one leaf migration. Returns a dict of the app labels
+        that conflict with the migration names that conflict.
+        """
+        seen_apps = {}
+        conflicting_apps = set()
+        for app_label, migration_name in self.graph.leaf_nodes():
+            if app_label in seen_apps:
+                conflicting_apps.add(app_label)
+            seen_apps.setdefault(app_label, set()).add(migration_name)
+        return dict((app_label, seen_apps[app_label]) for app_label in conflicting_apps)
+
 
 class BadMigrationError(Exception):
     """
diff --git a/django/db/migrations/questioner.py b/django/db/migrations/questioner.py
new file mode 100644
index 0000000000..2ae74d4ef6
--- /dev/null
+++ b/django/db/migrations/questioner.py
@@ -0,0 +1,122 @@
+import importlib
+import os
+import sys
+
+from django.db.models.loading import cache
+from django.utils import datetime_safe
+from django.utils.six.moves import input
+from django.core.exceptions import ImproperlyConfigured
+
+
+class MigrationQuestioner(object):
+    """
+    Gives the autodetector responses to questions it might have.
+    This base class has a built-in noninteractive mode, but the
+    interactive subclass is what the command-line arguments will use.
+    """
+
+    def __init__(self, defaults=None, specified_apps=None):
+        self.defaults = defaults or {}
+        self.specified_apps = specified_apps or set()
+
+    def ask_initial(self, app_label):
+        "Should we create an initial migration for the app?"
+        # If it was specified on the command line, definitely true
+        if app_label in self.specified_apps:
+            return True
+        # Otherwise, we look to see if it has a migrations module
+        # without any Python files in it, apart from __init__.py.
+        # Apps from the new app template will have these; the python
+        # file check will ensure we skip South ones.
+        try:
+            models_module = cache.get_app(app_label)
+        except ImproperlyConfigured:  # It's a fake app
+            return self.defaults.get("ask_initial", False)
+        migrations_import_path = "%s.migrations" % models_module.__package__
+        try:
+            migrations_module = importlib.import_module(migrations_import_path)
+        except ImportError:
+            return self.defaults.get("ask_initial", False)
+        else:
+            filenames = os.listdir(os.path.dirname(migrations_module.__file__))
+            return not any(x.endswith(".py") for x in filenames if x != "__init__.py")
+
+    def ask_not_null_addition(self, field_name, model_name):
+        "Adding a NOT NULL field to a model"
+        # None means quit
+        return None
+
+    def ask_rename(self, model_name, old_name, new_name, field_instance):
+        "Was this field really renamed?"
+        return self.defaults.get("ask_rename", False)
+
+    def ask_merge(self, app_label):
+        "Do you really want to merge these migrations?"
+        return self.defaults.get("ask_merge", False)
+
+
+class InteractiveMigrationQuestioner(MigrationQuestioner):
+
+    def _boolean_input(self, question, default=None):
+        result = input("%s " % question)
+        if not result and default is not None:
+            return default
+        while len(result) < 1 or result[0].lower() not in "yn":
+            result = input("Please answer yes or no: ")
+        return result[0].lower() == "y"
+
+    def _choice_input(self, question, choices):
+        print(question)
+        for i, choice in enumerate(choices):
+            print(" %s) %s" % (i + 1, choice))
+        result = input("Select an option: ")
+        while True:
+            try:
+                value = int(result)
+                if 0 < value <= len(choices):
+                    return value
+            except ValueError:
+                pass
+            result = input("Please select a valid option: ")
+
+    def ask_not_null_addition(self, field_name, model_name):
+        "Adding a NOT NULL field to a model"
+        choice = self._choice_input(
+            "You are trying to add a non-nullable field '%s' to %s without a default;\n" % (field_name, model_name) +
+            "we can't do that (the database needs something to populate existing rows).\n" +
+            "Please select a fix:",
+            [
+                "Provide a one-off default now (will be set on all existing rows)",
+                "Quit, and let me add a default in models.py",
+            ]
+        )
+        if choice == 2:
+            sys.exit(3)
+        else:
+            print("Please enter the default value now, as valid Python")
+            print("The datetime module is available, so you can do e.g. datetime.date.today()")
+            while True:
+                code = input(">>> ")
+                if not code:
+                    print("Please enter some code, or 'exit' (with no quotes) to exit.")
+                elif code == "exit":
+                    sys.exit(1)
+                else:
+                    try:
+                        return eval(code, {}, {"datetime": datetime_safe})
+                    except (SyntaxError, NameError) as e:
+                        print("Invalid input: %s" % e)
+                    else:
+                        break
+
+    def ask_rename(self, model_name, old_name, new_name, field_instance):
+        "Was this field really renamed?"
+        return self._boolean_input("Did you rename %s.%s to %s.%s (a %s)? [y/N]" % (model_name, old_name, model_name, new_name, field_instance.__class__.__name__), False)
+
+    def ask_merge(self, app_label):
+        return self._boolean_input(
+            "\nMerging will only work if the operations printed above do not conflict\n" +
+            "with each other (working on different fields or models)\n" +
+            "Do you want to merge these migration branches? [y/N]",
+            False,
+        )
diff --git a/tests/migrations/test_autodetector.py b/tests/migrations/test_autodetector.py
index a3db6e1e45..42e18c69cc 100644
--- a/tests/migrations/test_autodetector.py
+++ b/tests/migrations/test_autodetector.py
@@ -1,6 +1,7 @@
 # encoding: utf8
 from django.test import TestCase
-from django.db.migrations.autodetector import MigrationAutodetector, MigrationQuestioner
+from django.db.migrations.autodetector import MigrationAutodetector
+from django.db.migrations.questioner import MigrationQuestioner
 from django.db.migrations.state import ProjectState, ModelState
 from django.db.migrations.graph import MigrationGraph
 from django.db import models
@@ -63,7 +64,7 @@ class AutodetectorTests(TestCase):
         # Use project state to make a new migration change set
         before = self.make_project_state([])
         after = self.make_project_state([self.author_empty, self.other_pony, self.other_stable, self.third_thing])
-        autodetector = MigrationAutodetector(before, after, MigrationQuestioner({"ask_initial": True}))
+        autodetector = MigrationAutodetector(before, after, MigrationQuestioner(defaults={"ask_initial": True}))
         changes = autodetector._detect_changes()
         # Run through arrange_for_graph
         graph = MigrationGraph()
diff --git a/tests/migrations/test_commands.py b/tests/migrations/test_commands.py
index dbed522dd5..48fb68b03d 100644
--- a/tests/migrations/test_commands.py
+++ b/tests/migrations/test_commands.py
@@ -6,7 +6,7 @@ import copy
 import os
 import shutil
 
-from django.core.management import call_command
+from django.core.management import call_command, CommandError
 from django.db.models.loading import cache
 from django.test.utils import override_settings
 from django.utils import six
@@ -72,6 +72,34 @@ class MigrateTests(MigrationTestBase):
         # Cleanup by unmigrating everything
         call_command("migrate", "migrations", "zero", verbosity=0)
 
+    @override_settings(MIGRATION_MODULES={"migrations": "migrations.test_migrations_conflict"})
+    def test_migrate_conflict_exit(self):
+        """
+        Makes sure that migrate exits if it detects a conflict.
+        """
+        with self.assertRaises(CommandError):
+            call_command("migrate", "migrations")
+
+    @override_settings(MIGRATION_MODULES={"migrations": "migrations.test_migrations_conflict"})
+    def test_makemigrations_conflict_exit(self):
+        """
+        Makes sure that makemigrations exits if it detects a conflict.
+        """
+        with self.assertRaises(CommandError):
+            call_command("makemigrations")
+
+    @override_settings(MIGRATION_MODULES={"migrations": "migrations.test_migrations_conflict"})
+    def test_makemigrations_merge_basic(self):
+        """
+        Makes sure that makemigrations doesn't error if you ask for
+        merge mode with a conflict present. Doesn't test writing of the merge
+        file, as that requires temp directories.
+        """
+        try:
+            call_command("makemigrations", merge=True, verbosity=0)
+        except CommandError:
+            self.fail("Makemigrations errored in merge mode with conflicts")
+
     @override_settings(MIGRATION_MODULES={"migrations": "migrations.test_migrations"})
     def test_sqlmigrate(self):
         """
diff --git a/tests/migrations/test_migrations_conflict/0001_initial.py b/tests/migrations/test_migrations_conflict/0001_initial.py
new file mode 100644
index 0000000000..344bebdfe3
--- /dev/null
+++ b/tests/migrations/test_migrations_conflict/0001_initial.py
@@ -0,0 +1,27 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    operations = [
+
+        migrations.CreateModel(
+            "Author",
+            [
+                ("id", models.AutoField(primary_key=True)),
+                ("name", models.CharField(max_length=255)),
+                ("slug", models.SlugField(null=True)),
+                ("age", models.IntegerField(default=0)),
+                ("silly_field", models.BooleanField(default=False)),
+            ],
+        ),
+
+        migrations.CreateModel(
+            "Tribble",
+            [
+                ("id", models.AutoField(primary_key=True)),
+                ("fluffy", models.BooleanField(default=True)),
+            ],
+        )
+
+    ]
diff --git a/tests/migrations/test_migrations_conflict/0002_conflicting_second.py b/tests/migrations/test_migrations_conflict/0002_conflicting_second.py
new file mode 100644
index 0000000000..15ea1f063a
--- /dev/null
+++ b/tests/migrations/test_migrations_conflict/0002_conflicting_second.py
@@ -0,0 +1,17 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [("migrations", "0001_initial")]
+
+    operations = [
+
+        migrations.CreateModel(
+            "Something",
+            [
+                ("id", models.AutoField(primary_key=True)),
+            ],
+        )
+
+    ]
diff --git a/tests/migrations/test_migrations_conflict/0002_second.py b/tests/migrations/test_migrations_conflict/0002_second.py
new file mode 100644
index 0000000000..ace9a83347
--- /dev/null
+++ b/tests/migrations/test_migrations_conflict/0002_second.py
@@ -0,0 +1,24 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [("migrations", "0001_initial")]
+
+    operations = [
+
+        migrations.DeleteModel("Tribble"),
+
+        migrations.RemoveField("Author", "silly_field"),
+
+        migrations.AddField("Author", "rating", models.IntegerField(default=0)),
+
+        migrations.CreateModel(
+            "Book",
+            [
+                ("id", models.AutoField(primary_key=True)),
+                ("author", models.ForeignKey("migrations.Author", null=True)),
+            ],
+        )
+
+    ]
diff --git a/tests/migrations/test_migrations_conflict/__init__.py b/tests/migrations/test_migrations_conflict/__init__.py
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/tests/migrations/test_migrations_conflict/__init__.py