A bit of an autodetector and a bit of a writer

django/db/migrations/autodetector.py

+from django.db.migrations import operations
+from django.db.migrations.migration import Migration
+
+
+class AutoDetector(object):
+    """
+    Takes a pair of ProjectStates, and compares them to see what the
+    first would need doing to make it match the second (the second
+    usually being the project's current state).
+
+    Note that this naturally operates on entire projects at a time,
+    as it's likely that changes interact (for example, you can't
+    add a ForeignKey without having a migration to add the table it
+    depends on first). A user interface may offer single-app detection
+    if it wishes, with the caveat that it may not always be possible.
+    """
+
+    def __init__(self, from_state, to_state):
+        self.from_state = from_state
+        self.to_state = to_state
+
+    def changes(self):
+        """
+        Returns a set of migration plans which will achieve the
+        change from from_state to to_state.
+        """
+        # We'll store migrations as lists by app names for now
+        self.migrations = {}
+        # Stage one: Adding models.
+        added_models = set(self.to_state.keys()) - set(self.from_state.keys())
+        for app_label, model_name in added_models:
+            model_state = self.to_state[app_label, model_name]
+            self.add_to_migration(
+                app_label,
+                operations.CreateModel(
+                    model_state.name,
+                    model_state.fields,
+                    model_state.options,
+                    model_state.bases,
+                )
+            )
+        # Removing models
+        removed_models = set(self.from_state.keys()) - set(self.to_state.keys())
+        for app_label, model_name in removed_models:
+            model_state = self.from_state[app_label, model_name]
+            self.add_to_migration(
+                app_label,
+                operations.DeleteModel(
+                    model_state.name,
+                )
+            )
+        # Alright, now sort out and return the migrations
+        for app_label, migrations in self.migrations.items():
+            for m1, m2 in zip(migrations, migrations[1:]):
+                m2.dependencies.append((app_label, m1.name))
+        # Flatten and return
+        result = set()
+        for app_label, migrations in self.migrations.items():
+            for migration in migrations:
+                subclass = type("Migration", (Migration,), migration)
+                instance = subclass(migration['name'], app_label)
+                result.append(instance)
+        return result
+
+    def add_to_migration(self, app_label, operation):
+        migrations = self.migrations.setdefault(app_label, [])
+        if not migrations:
+            migrations.append({"name": "temp-%i" % len(migrations) + 1, "operations": [], "dependencies": []})
+        migrations[-1].operations.append(operation)

django/db/migrations/operations/base.py

@@ -15,6 +15,24 @@ class Operation(object):
     # Some operations are impossible to reverse, like deleting data.
     reversible = True
 
+    def __new__(cls, *args, **kwargs):
+        # We capture the arguments to make returning them trivial
+        self = object.__new__(cls)
+        self._constructor_args = (args, kwargs)
+        return self
+
+    def deconstruct(self):
+        """
+        Returns a 3-tuple of class import path (or just name if it lives
+        under django.db.migrations), positional arguments, and keyword
+        arguments.
+        """
+        return (
+            self.__class__.__name__,
+            self._constructor_args[0],
+            self._constructor_args[1],
+        )
+
     def state_forwards(self, app_label, state):
         """
         Takes the state from the previous migration, and mutates it

django/db/migrations/writer.py

+import datetime
+import types
+
+
+class MigrationWriter(object):
+    """
+    Takes a Migration instance and is able to produce the contents
+    of the migration file from it.
+    """
+
+    def __init__(self, migration):
+        self.migration = migration
+
+    def as_string(self):
+        """
+        Returns a string of the file contents.
+        """
+        items = {
+            "dependencies": repr(self.migration.dependencies),
+        }
+        imports = set()
+        # Deconstruct operations
+        operation_strings = []
+        for operation in self.migration.operations:
+            name, args, kwargs = operation.deconstruct()
+            arg_strings = []
+            for arg in args:
+                arg_string, arg_imports = self.serialize(arg)
+                arg_strings.append(arg_string)
+                imports.update(arg_imports)
+            for kw, arg in kwargs.items():
+                arg_string, arg_imports = self.serialize(arg)
+                imports.update(arg_imports)
+                arg_strings.append("%s = %s" % (kw, arg_string))
+            operation_strings.append("migrations.%s(%s\n        )" % (name, "".join("\n            %s," % arg for arg in arg_strings)))
+        items["operations"] = "[%s\n    ]" % "".join("\n        %s," % s for s in operation_strings)
+        # Format imports nicely
+        if not imports:
+            items["imports"] = ""
+        else:
+            items["imports"] = "\n".join(imports) + "\n"
+        return MIGRATION_TEMPLATE % items
+
+    @property
+    def filename(self):
+        return "%s.py" % self.migration.name
+
+    @classmethod
+    def serialize(cls, value):
+        """
+        Serializes the value to a string that's parsable by Python, along
+        with any needed imports to make that string work.
+        More advanced than repr() as it can encode things
+        like datetime.datetime.now.
+        """
+        # Sequences
+        if isinstance(value, (list, set, tuple)):
+            imports = set()
+            strings = []
+            for item in value:
+                item_string, item_imports = cls.serialize(item)
+                imports.update(item_imports)
+                strings.append(item_string)
+            if isinstance(value, set):
+                format = "set([%s])"
+            elif isinstance(value, tuple):
+                format = "(%s,)"
+            else:
+                format = "[%s]"
+            return format % (", ".join(strings)), imports
+        # Dictionaries
+        elif isinstance(value, dict):
+            imports = set()
+            strings = []
+            for k, v in value.items():
+                k_string, k_imports = cls.serialize(k)
+                v_string, v_imports = cls.serialize(v)
+                imports.update(k_imports)
+                imports.update(v_imports)
+                strings.append((k_string, v_string))
+            return "{%s}" % (", ".join(["%s: %s" % (k, v) for k, v in strings])), imports
+        # Datetimes
+        elif isinstance(value, (datetime.datetime, datetime.date)):
+            return repr(value), set(["import datetime"])
+        # Simple types
+        elif isinstance(value, (int, long, float, str, unicode, bool, types.NoneType)):
+            return repr(value), set()
+        # Functions
+        elif isinstance(value, (types.FunctionType, types.BuiltinFunctionType)):
+            # Special-cases, as these don't have im_class
+            special_cases = [
+                (datetime.datetime.now, "datetime.datetime.now", ["import datetime"]),
+                (datetime.datetime.utcnow, "datetime.datetime.utcnow", ["import datetime"]),
+                (datetime.date.today, "datetime.date.today", ["import datetime"]),
+            ]
+            for func, string, imports in special_cases:
+                if func == value:  # For some reason "utcnow is not utcnow"
+                    return string, set(imports)
+            # Method?
+            if hasattr(value, "im_class"):
+                klass = value.im_class
+                module = klass.__module__
+                return "%s.%s.%s" % (module, klass.__name__, value.__name__), set(["import %s" % module])
+            else:
+                module = value.__module__
+                if module is None:
+                    raise ValueError("Cannot serialize function %r: No module" % value)
+                return "%s.%s" % (module, value.__name__), set(["import %s" % module])
+        # Uh oh.
+        else:
+            raise ValueError("Cannot serialize: %r" % value)
+
+
+MIGRATION_TEMPLATE = """# encoding: utf8
+from django.db import models, migrations
+%(imports)s
+
+class Migration(migrations.Migration):
+
+    dependencies = %(dependencies)s
+
+    operations = %(operations)s
+"""

tests/migrations/test_operations.py

@@ -68,6 +68,12 @@ class OperationTests(TransactionTestCase):
         with connection.schema_editor() as editor:
             operation.database_backwards("test_crmo", editor, new_state, project_state)
         self.assertTableNotExists("test_crmo_pony")
+        # And deconstruction
+        definition = operation.deconstruct()
+        self.assertEqual(definition[0], "CreateModel")
+        self.assertEqual(len(definition[1]), 2)
+        self.assertEqual(len(definition[2]), 0)
+        self.assertEqual(definition[1][0], "Pony")
 
     def test_delete_model(self):
         """

tests/migrations/test_writer.py

+# encoding: utf8
+import datetime
+from django.test import TransactionTestCase
+from django.db.migrations.writer import MigrationWriter
+from django.db import migrations
+
+
+class WriterTests(TransactionTestCase):
+    """
+    Tests the migration writer (makes migration files from Migration instances)
+    """
+
+    def safe_exec(self, value, string):
+        l = {}
+        try:
+            exec(string, {}, l)
+        except:
+            self.fail("Could not serialize %r: failed to exec %r" % (value, string.strip()))
+        return l
+
+    def assertSerializedEqual(self, value):
+        string, imports = MigrationWriter.serialize(value)
+        new_value = self.safe_exec(value, "%s\ntest_value_result = %s" % ("\n".join(imports), string))['test_value_result']
+        self.assertEqual(new_value, value)
+
+    def assertSerializedIs(self, value):
+        string, imports = MigrationWriter.serialize(value)
+        new_value = self.safe_exec(value, "%s\ntest_value_result = %s" % ("\n".join(imports), string))['test_value_result']
+        self.assertIs(new_value, value)
+
+    def test_serialize(self):
+        """
+        Tests various different forms of the serializer.
+        This does not care about formatting, just that the parsed result is
+        correct, so we always exec() the result and check that.
+        """
+        # Basic values
+        self.assertSerializedEqual(1)
+        self.assertSerializedEqual(None)
+        self.assertSerializedEqual("foobar")
+        self.assertSerializedEqual(u"föobár")
+        self.assertSerializedEqual({1: 2})
+        self.assertSerializedEqual(["a", 2, True, None])
+        self.assertSerializedEqual(set([2, 3, "eighty"]))
+        self.assertSerializedEqual({"lalalala": ["yeah", "no", "maybe"]})
+        # Datetime stuff
+        self.assertSerializedEqual(datetime.datetime.utcnow())
+        self.assertSerializedEqual(datetime.datetime.utcnow)
+        self.assertSerializedEqual(datetime.date.today())
+        self.assertSerializedEqual(datetime.date.today)
+
+    def test_simple_migration(self):
+        """
+        Tests serializing a simple migration.
+        """
+        migration = type("Migration", (migrations.Migration,), {
+            "operations": [
+                migrations.DeleteModel("MyModel"),
+            ],
+            "dependencies": [("testapp", "some_other_one")],
+        })
+        writer = MigrationWriter(migration)
+        output = writer.as_string()
+        print output