django/tests/migrations/test_writer.py

import datetime
import decimal
import enum
import functools
import math
import os
import re
import uuid
from unittest import mock

import custom_migration_operations.more_operations
import custom_migration_operations.operations

from django import get_version
from django.conf import settings
from django.core.validators import EmailValidator, RegexValidator
from django.db import migrations, models
from django.db.migrations.writer import (
    MigrationWriter, OperationWriter, SettingsReference,
)
from django.test import SimpleTestCase
from django.utils.deconstruct import deconstructible
from django.utils.functional import SimpleLazyObject
from django.utils.timezone import get_default_timezone, get_fixed_timezone, utc
from django.utils.translation import gettext_lazy as _
from django.utils.version import PY36

from .models import FoodManager, FoodQuerySet


class Money(decimal.Decimal):
    def deconstruct(self):
        return (
            '%s.%s' % (self.__class__.__module__, self.__class__.__name__),
            [str(self)],
            {}
        )


class TestModel1:
    def upload_to(self):
        return '/somewhere/dynamic/'
    thing = models.FileField(upload_to=upload_to)


class OperationWriterTests(SimpleTestCase):

    def test_empty_signature(self):
        operation = custom_migration_operations.operations.TestOperation()
        buff, imports = OperationWriter(operation, indentation=0).serialize()
        self.assertEqual(imports, {'import custom_migration_operations.operations'})
        self.assertEqual(
            buff,
            'custom_migration_operations.operations.TestOperation(\n'
            '),'
        )

    def test_args_signature(self):
        operation = custom_migration_operations.operations.ArgsOperation(1, 2)
        buff, imports = OperationWriter(operation, indentation=0).serialize()
        self.assertEqual(imports, {'import custom_migration_operations.operations'})
        self.assertEqual(
            buff,
            'custom_migration_operations.operations.ArgsOperation(\n'
            '    arg1=1,\n'
            '    arg2=2,\n'
            '),'
        )

    def test_kwargs_signature(self):
        operation = custom_migration_operations.operations.KwargsOperation(kwarg1=1)
        buff, imports = OperationWriter(operation, indentation=0).serialize()
        self.assertEqual(imports, {'import custom_migration_operations.operations'})
        self.assertEqual(
            buff,
            'custom_migration_operations.operations.KwargsOperation(\n'
            '    kwarg1=1,\n'
            '),'
        )

    def test_args_kwargs_signature(self):
        operation = custom_migration_operations.operations.ArgsKwargsOperation(1, 2, kwarg2=4)
        buff, imports = OperationWriter(operation, indentation=0).serialize()
        self.assertEqual(imports, {'import custom_migration_operations.operations'})
        self.assertEqual(
            buff,
            'custom_migration_operations.operations.ArgsKwargsOperation(\n'
            '    arg1=1,\n'
            '    arg2=2,\n'
            '    kwarg2=4,\n'
            '),'
        )

    def test_nested_args_signature(self):
        operation = custom_migration_operations.operations.ArgsOperation(
            custom_migration_operations.operations.ArgsOperation(1, 2),
            custom_migration_operations.operations.KwargsOperation(kwarg1=3, kwarg2=4)
        )
        buff, imports = OperationWriter(operation, indentation=0).serialize()
        self.assertEqual(imports, {'import custom_migration_operations.operations'})
        self.assertEqual(
            buff,
            'custom_migration_operations.operations.ArgsOperation(\n'
            '    arg1=custom_migration_operations.operations.ArgsOperation(\n'
            '        arg1=1,\n'
            '        arg2=2,\n'
            '    ),\n'
            '    arg2=custom_migration_operations.operations.KwargsOperation(\n'
            '        kwarg1=3,\n'
            '        kwarg2=4,\n'
            '    ),\n'
            '),'
        )

    def test_multiline_args_signature(self):
        operation = custom_migration_operations.operations.ArgsOperation("test\n    arg1", "test\narg2")
        buff, imports = OperationWriter(operation, indentation=0).serialize()
        self.assertEqual(imports, {'import custom_migration_operations.operations'})
        self.assertEqual(
            buff,
            "custom_migration_operations.operations.ArgsOperation(\n"
            "    arg1='test\\n    arg1',\n"
            "    arg2='test\\narg2',\n"
            "),"
        )

    def test_expand_args_signature(self):
        operation = custom_migration_operations.operations.ExpandArgsOperation([1, 2])
        buff, imports = OperationWriter(operation, indentation=0).serialize()
        self.assertEqual(imports, {'import custom_migration_operations.operations'})
        self.assertEqual(
            buff,
            'custom_migration_operations.operations.ExpandArgsOperation(\n'
            '    arg=[\n'
            '        1,\n'
            '        2,\n'
            '    ],\n'
            '),'
        )

    def test_nested_operation_expand_args_signature(self):
        operation = custom_migration_operations.operations.ExpandArgsOperation(
            arg=[
                custom_migration_operations.operations.KwargsOperation(
                    kwarg1=1,
                    kwarg2=2,
                ),
            ]
        )
        buff, imports = OperationWriter(operation, indentation=0).serialize()
        self.assertEqual(imports, {'import custom_migration_operations.operations'})
        self.assertEqual(
            buff,
            'custom_migration_operations.operations.ExpandArgsOperation(\n'
            '    arg=[\n'
            '        custom_migration_operations.operations.KwargsOperation(\n'
            '            kwarg1=1,\n'
            '            kwarg2=2,\n'
            '        ),\n'
            '    ],\n'
            '),'
        )


class WriterTests(SimpleTestCase):
    """
    Tests the migration writer (makes migration files from Migration instances)
    """

    def safe_exec(self, string, value=None):
        d = {}
        try:
            exec(string, globals(), d)
        except Exception as e:
            if value:
                self.fail("Could not exec %r (from value %r): %s" % (string.strip(), value, e))
            else:
                self.fail("Could not exec %r: %s" % (string.strip(), e))
        return d

    def serialize_round_trip(self, value):
        string, imports = MigrationWriter.serialize(value)
        return self.safe_exec("%s\ntest_value_result = %s" % ("\n".join(imports), string), value)['test_value_result']

    def assertSerializedEqual(self, value):
        self.assertEqual(self.serialize_round_trip(value), value)

    def assertSerializedResultEqual(self, value, target):
        self.assertEqual(MigrationWriter.serialize(value), target)

    def assertSerializedFieldEqual(self, value):
        new_value = self.serialize_round_trip(value)
        self.assertEqual(value.__class__, new_value.__class__)
        self.assertEqual(value.max_length, new_value.max_length)
        self.assertEqual(value.null, new_value.null)
        self.assertEqual(value.unique, new_value.unique)

    def test_serialize_numbers(self):
        self.assertSerializedEqual(1)
        self.assertSerializedEqual(1.2)
        self.assertTrue(math.isinf(self.serialize_round_trip(float("inf"))))
        self.assertTrue(math.isinf(self.serialize_round_trip(float("-inf"))))
        self.assertTrue(math.isnan(self.serialize_round_trip(float("nan"))))

        self.assertSerializedEqual(decimal.Decimal('1.3'))
        self.assertSerializedResultEqual(
            decimal.Decimal('1.3'),
            ("Decimal('1.3')", {'from decimal import Decimal'})
        )

        self.assertSerializedEqual(Money('1.3'))
        self.assertSerializedResultEqual(
            Money('1.3'),
            ("migrations.test_writer.Money('1.3')", {'import migrations.test_writer'})
        )

    def test_serialize_constants(self):
        self.assertSerializedEqual(None)
        self.assertSerializedEqual(True)
        self.assertSerializedEqual(False)

    def test_serialize_strings(self):
        self.assertSerializedEqual(b"foobar")
        string, imports = MigrationWriter.serialize(b"foobar")
        self.assertEqual(string, "b'foobar'")
        self.assertSerializedEqual("föobár")
        string, imports = MigrationWriter.serialize("foobar")
        self.assertEqual(string, "'foobar'")

    def test_serialize_multiline_strings(self):
        self.assertSerializedEqual(b"foo\nbar")
        string, imports = MigrationWriter.serialize(b"foo\nbar")
        self.assertEqual(string, "b'foo\\nbar'")
        self.assertSerializedEqual("föo\nbár")
        string, imports = MigrationWriter.serialize("foo\nbar")
        self.assertEqual(string, "'foo\\nbar'")

    def test_serialize_collections(self):
        self.assertSerializedEqual({1: 2})
        self.assertSerializedEqual(["a", 2, True, None])
        self.assertSerializedEqual({2, 3, "eighty"})
        self.assertSerializedEqual({"lalalala": ["yeah", "no", "maybe"]})
        self.assertSerializedEqual(_('Hello'))

    def test_serialize_builtin_types(self):
        self.assertSerializedEqual([list, tuple, dict, set, frozenset])
        self.assertSerializedResultEqual(
            [list, tuple, dict, set, frozenset],
            ("[list, tuple, dict, set, frozenset]", set())
        )

    def test_serialize_lazy_objects(self):
        pattern = re.compile(r'^foo$')
        lazy_pattern = SimpleLazyObject(lambda: pattern)
        self.assertEqual(self.serialize_round_trip(lazy_pattern), pattern)

    def test_serialize_enums(self):
        class TextEnum(enum.Enum):
            A = 'a-value'
            B = 'value-b'

        class BinaryEnum(enum.Enum):
            A = b'a-value'
            B = b'value-b'

        class IntEnum(enum.IntEnum):
            A = 1
            B = 2

        self.assertSerializedResultEqual(
            TextEnum.A,
            ("migrations.test_writer.TextEnum('a-value')", {'import migrations.test_writer'})
        )
        self.assertSerializedResultEqual(
            BinaryEnum.A,
            ("migrations.test_writer.BinaryEnum(b'a-value')", {'import migrations.test_writer'})
        )
        self.assertSerializedResultEqual(
            IntEnum.B,
            ("migrations.test_writer.IntEnum(2)", {'import migrations.test_writer'})
        )

        field = models.CharField(default=TextEnum.B, choices=[(m.value, m) for m in TextEnum])
        string = MigrationWriter.serialize(field)[0]
        self.assertEqual(
            string,
            "models.CharField(choices=["
            "('a-value', migrations.test_writer.TextEnum('a-value')), "
            "('value-b', migrations.test_writer.TextEnum('value-b'))], "
            "default=migrations.test_writer.TextEnum('value-b'))"
        )
        field = models.CharField(default=BinaryEnum.B, choices=[(m.value, m) for m in BinaryEnum])
        string = MigrationWriter.serialize(field)[0]
        self.assertEqual(
            string,
            "models.CharField(choices=["
            "(b'a-value', migrations.test_writer.BinaryEnum(b'a-value')), "
            "(b'value-b', migrations.test_writer.BinaryEnum(b'value-b'))], "
            "default=migrations.test_writer.BinaryEnum(b'value-b'))"
        )
        field = models.IntegerField(default=IntEnum.A, choices=[(m.value, m) for m in IntEnum])
        string = MigrationWriter.serialize(field)[0]
        self.assertEqual(
            string,
            "models.IntegerField(choices=["
            "(1, migrations.test_writer.IntEnum(1)), "
            "(2, migrations.test_writer.IntEnum(2))], "
            "default=migrations.test_writer.IntEnum(1))"
        )

    def test_serialize_uuid(self):
        self.assertSerializedEqual(uuid.uuid1())
        self.assertSerializedEqual(uuid.uuid4())

        uuid_a = uuid.UUID('5c859437-d061-4847-b3f7-e6b78852f8c8')
        uuid_b = uuid.UUID('c7853ec1-2ea3-4359-b02d-b54e8f1bcee2')
        self.assertSerializedResultEqual(
            uuid_a,
            ("uuid.UUID('5c859437-d061-4847-b3f7-e6b78852f8c8')", {'import uuid'})
        )
        self.assertSerializedResultEqual(
            uuid_b,
            ("uuid.UUID('c7853ec1-2ea3-4359-b02d-b54e8f1bcee2')", {'import uuid'})
        )

        field = models.UUIDField(choices=((uuid_a, 'UUID A'), (uuid_b, 'UUID B')), default=uuid_a)
        string = MigrationWriter.serialize(field)[0]
        self.assertEqual(
            string,
            "models.UUIDField(choices=["
            "(uuid.UUID('5c859437-d061-4847-b3f7-e6b78852f8c8'), 'UUID A'), "
            "(uuid.UUID('c7853ec1-2ea3-4359-b02d-b54e8f1bcee2'), 'UUID B')], "
            "default=uuid.UUID('5c859437-d061-4847-b3f7-e6b78852f8c8'))"
        )

    def test_serialize_functions(self):
        with self.assertRaisesMessage(ValueError, 'Cannot serialize function: lambda'):
            self.assertSerializedEqual(lambda x: 42)
        self.assertSerializedEqual(models.SET_NULL)
        string, imports = MigrationWriter.serialize(models.SET(42))
        self.assertEqual(string, 'models.SET(42)')
        self.serialize_round_trip(models.SET(42))

    def test_serialize_datetime(self):
        self.assertSerializedEqual(datetime.datetime.utcnow())
        self.assertSerializedEqual(datetime.datetime.utcnow)
        self.assertSerializedEqual(datetime.datetime.today())
        self.assertSerializedEqual(datetime.datetime.today)
        self.assertSerializedEqual(datetime.date.today())
        self.assertSerializedEqual(datetime.date.today)
        self.assertSerializedEqual(datetime.datetime.now().time())
        self.assertSerializedEqual(datetime.datetime(2014, 1, 1, 1, 1, tzinfo=get_default_timezone()))
        self.assertSerializedEqual(datetime.datetime(2013, 12, 31, 22, 1, tzinfo=get_fixed_timezone(180)))
        self.assertSerializedResultEqual(
            datetime.datetime(2014, 1, 1, 1, 1),
            ("datetime.datetime(2014, 1, 1, 1, 1)", {'import datetime'})
        )
        self.assertSerializedResultEqual(
            datetime.datetime(2012, 1, 1, 1, 1, tzinfo=utc),
            (
                "datetime.datetime(2012, 1, 1, 1, 1, tzinfo=utc)",
                {'import datetime', 'from django.utils.timezone import utc'},
            )
        )

    def test_serialize_fields(self):
        self.assertSerializedFieldEqual(models.CharField(max_length=255))
        self.assertSerializedResultEqual(
            models.CharField(max_length=255),
            ("models.CharField(max_length=255)", {"from django.db import models"})
        )
        self.assertSerializedFieldEqual(models.TextField(null=True, blank=True))
        self.assertSerializedResultEqual(
            models.TextField(null=True, blank=True),
            ("models.TextField(blank=True, null=True)", {'from django.db import models'})
        )

    def test_serialize_settings(self):
        self.assertSerializedEqual(SettingsReference(settings.AUTH_USER_MODEL, "AUTH_USER_MODEL"))
        self.assertSerializedResultEqual(
            SettingsReference("someapp.model", "AUTH_USER_MODEL"),
            ("settings.AUTH_USER_MODEL", {"from django.conf import settings"})
        )

    def test_serialize_iterators(self):
        self.assertSerializedResultEqual(
            ((x, x * x) for x in range(3)),
            ("((0, 0), (1, 1), (2, 4))", set())
        )

    def test_serialize_compiled_regex(self):
        """
        Make sure compiled regex can be serialized.
        """
        regex = re.compile(r'^\w+$')
        self.assertSerializedEqual(regex)

    def test_serialize_class_based_validators(self):
        """
        Ticket #22943: Test serialization of class-based validators, including
        compiled regexes.
        """
        validator = RegexValidator(message="hello")
        string = MigrationWriter.serialize(validator)[0]
        self.assertEqual(string, "django.core.validators.RegexValidator(message='hello')")
        self.serialize_round_trip(validator)

        # Test with a compiled regex.
        validator = RegexValidator(regex=re.compile(r'^\w+$'))
        string = MigrationWriter.serialize(validator)[0]
        self.assertEqual(string, "django.core.validators.RegexValidator(regex=re.compile('^\\\\w+$'))")
        self.serialize_round_trip(validator)

        # Test a string regex with flag
        validator = RegexValidator(r'^[0-9]+$', flags=re.S)
        string = MigrationWriter.serialize(validator)[0]
        if PY36:
            self.assertEqual(string, "django.core.validators.RegexValidator('^[0-9]+$', flags=re.RegexFlag(16))")
        else:
            self.assertEqual(string, "django.core.validators.RegexValidator('^[0-9]+$', flags=16)")
        self.serialize_round_trip(validator)

        # Test message and code
        validator = RegexValidator('^[-a-zA-Z0-9_]+$', 'Invalid', 'invalid')
        string = MigrationWriter.serialize(validator)[0]
        self.assertEqual(string, "django.core.validators.RegexValidator('^[-a-zA-Z0-9_]+$', 'Invalid', 'invalid')")
        self.serialize_round_trip(validator)

        # Test with a subclass.
        validator = EmailValidator(message="hello")
        string = MigrationWriter.serialize(validator)[0]
        self.assertEqual(string, "django.core.validators.EmailValidator(message='hello')")
        self.serialize_round_trip(validator)

        validator = deconstructible(path="migrations.test_writer.EmailValidator")(EmailValidator)(message="hello")
        string = MigrationWriter.serialize(validator)[0]
        self.assertEqual(string, "migrations.test_writer.EmailValidator(message='hello')")

        validator = deconstructible(path="custom.EmailValidator")(EmailValidator)(message="hello")
        with self.assertRaisesMessage(ImportError, "No module named 'custom'"):
            MigrationWriter.serialize(validator)

        validator = deconstructible(path="django.core.validators.EmailValidator2")(EmailValidator)(message="hello")
        with self.assertRaisesMessage(ValueError, "Could not find object EmailValidator2 in django.core.validators."):
            MigrationWriter.serialize(validator)

    def test_serialize_empty_nonempty_tuple(self):
        """
        Ticket #22679: makemigrations generates invalid code for (an empty
        tuple) default_permissions = ()
        """
        empty_tuple = ()
        one_item_tuple = ('a',)
        many_items_tuple = ('a', 'b', 'c')
        self.assertSerializedEqual(empty_tuple)
        self.assertSerializedEqual(one_item_tuple)
        self.assertSerializedEqual(many_items_tuple)

    def test_serialize_builtins(self):
        string, imports = MigrationWriter.serialize(range)
        self.assertEqual(string, 'range')
        self.assertEqual(imports, set())

    def test_serialize_unbound_method_reference(self):
        """An unbound method used within a class body can be serialized."""
        self.serialize_round_trip(TestModel1.thing)

    def test_serialize_local_function_reference(self):
        """A reference in a local scope can't be serialized."""
        class TestModel2:
            def upload_to(self):
                return "somewhere dynamic"
            thing = models.FileField(upload_to=upload_to)

        with self.assertRaisesMessage(ValueError, 'Could not find function upload_to in migrations.test_writer'):
            self.serialize_round_trip(TestModel2.thing)

    def test_serialize_managers(self):
        self.assertSerializedEqual(models.Manager())
        self.assertSerializedResultEqual(
            FoodQuerySet.as_manager(),
            ('migrations.models.FoodQuerySet.as_manager()', {'import migrations.models'})
        )
        self.assertSerializedEqual(FoodManager('a', 'b'))
        self.assertSerializedEqual(FoodManager('x', 'y', c=3, d=4))

    def test_serialize_frozensets(self):
        self.assertSerializedEqual(frozenset())
        self.assertSerializedEqual(frozenset("let it go"))

    def test_serialize_set(self):
        self.assertSerializedEqual(set())
        self.assertSerializedResultEqual(set(), ('set()', set()))
        self.assertSerializedEqual({'a'})
        self.assertSerializedResultEqual({'a'}, ("{'a'}", set()))

    def test_serialize_timedelta(self):
        self.assertSerializedEqual(datetime.timedelta())
        self.assertSerializedEqual(datetime.timedelta(minutes=42))

    def test_serialize_functools_partial(self):
        value = functools.partial(datetime.timedelta, 1, seconds=2)
        result = self.serialize_round_trip(value)
        self.assertEqual(result.func, value.func)
        self.assertEqual(result.args, value.args)
        self.assertEqual(result.keywords, value.keywords)

    def test_serialize_functools_partialmethod(self):
        value = functools.partialmethod(datetime.timedelta, 1, seconds=2)
        result = self.serialize_round_trip(value)
        self.assertIsInstance(result, functools.partialmethod)
        self.assertEqual(result.func, value.func)
        self.assertEqual(result.args, value.args)
        self.assertEqual(result.keywords, value.keywords)

    def test_serialize_type_none(self):
        self.assertSerializedEqual(type(None))

    def test_simple_migration(self):
        """
        Tests serializing a simple migration.
        """
        fields = {
            'charfield': models.DateTimeField(default=datetime.datetime.utcnow),
            'datetimefield': models.DateTimeField(default=datetime.datetime.utcnow),
        }

        options = {
            'verbose_name': 'My model',
            'verbose_name_plural': 'My models',
        }

        migration = type("Migration", (migrations.Migration,), {
            "operations": [
                migrations.CreateModel("MyModel", tuple(fields.items()), options, (models.Model,)),
                migrations.CreateModel("MyModel2", tuple(fields.items()), bases=(models.Model,)),
                migrations.CreateModel(
                    name="MyModel3", fields=tuple(fields.items()), options=options, bases=(models.Model,)
                ),
                migrations.DeleteModel("MyModel"),
                migrations.AddField("OtherModel", "datetimefield", fields["datetimefield"]),
            ],
            "dependencies": [("testapp", "some_other_one")],
        })
        writer = MigrationWriter(migration)
        output = writer.as_string()
        # We don't test the output formatting - that's too fragile.
        # Just make sure it runs for now, and that things look alright.
        result = self.safe_exec(output)
        self.assertIn("Migration", result)

    def test_migration_path(self):
        test_apps = [
            'migrations.migrations_test_apps.normal',
            'migrations.migrations_test_apps.with_package_model',
            'migrations.migrations_test_apps.without_init_file',
        ]

        base_dir = os.path.dirname(os.path.dirname(__file__))

        for app in test_apps:
            with self.modify_settings(INSTALLED_APPS={'append': app}):
                migration = migrations.Migration('0001_initial', app.split('.')[-1])
                expected_path = os.path.join(base_dir, *(app.split('.') + ['migrations', '0001_initial.py']))
                writer = MigrationWriter(migration)
                self.assertEqual(writer.path, expected_path)

    def test_custom_operation(self):
        migration = type("Migration", (migrations.Migration,), {
            "operations": [
                custom_migration_operations.operations.TestOperation(),
                custom_migration_operations.operations.CreateModel(),
                migrations.CreateModel("MyModel", (), {}, (models.Model,)),
                custom_migration_operations.more_operations.TestOperation()
            ],
            "dependencies": []
        })
        writer = MigrationWriter(migration)
        output = writer.as_string()
        result = self.safe_exec(output)
        self.assertIn("custom_migration_operations", result)
        self.assertNotEqual(
            result['custom_migration_operations'].operations.TestOperation,
            result['custom_migration_operations'].more_operations.TestOperation
        )

    def test_sorted_imports(self):
        """
        #24155 - Tests ordering of imports.
        """
        migration = type("Migration", (migrations.Migration,), {
            "operations": [
                migrations.AddField("mymodel", "myfield", models.DateTimeField(
                    default=datetime.datetime(2012, 1, 1, 1, 1, tzinfo=utc),
                )),
            ]
        })
        writer = MigrationWriter(migration)
        output = writer.as_string()
        self.assertIn(
            "import datetime\n"
            "from django.db import migrations, models\n"
            "from django.utils.timezone import utc\n",
            output
        )

    def test_migration_file_header_comments(self):
        """
        Test comments at top of file.
        """
        migration = type("Migration", (migrations.Migration,), {
            "operations": []
        })
        dt = datetime.datetime(2015, 7, 31, 4, 40, 0, 0, tzinfo=utc)
        with mock.patch('django.db.migrations.writer.now', lambda: dt):
            for include_header in (True, False):
                with self.subTest(include_header=include_header):
                    writer = MigrationWriter(migration, include_header)
                    output = writer.as_string()

                    self.assertEqual(
                        include_header,
                        output.startswith(
                            "# Generated by Django %s on 2015-07-31 04:40\n\n" % get_version()
                        )
                    )
                    if not include_header:
                        # Make sure the output starts with something that's not
                        # a comment or indentation or blank line
                        self.assertRegex(output.splitlines(keepends=True)[0], r"^[^#\s]+")

    def test_models_import_omitted(self):
        """
        django.db.models shouldn't be imported if unused.
        """
        migration = type("Migration", (migrations.Migration,), {
            "operations": [
                migrations.AlterModelOptions(
                    name='model',
                    options={'verbose_name': 'model', 'verbose_name_plural': 'models'},
                ),
            ]
        })
        writer = MigrationWriter(migration)
        output = writer.as_string()
        self.assertIn("from django.db import migrations\n", output)

    def test_deconstruct_class_arguments(self):
        # Yes, it doesn't make sense to use a class as a default for a
        # CharField. It does make sense for custom fields though, for example
        # an enumfield that takes the enum class as an argument.
        class DeconstructibleInstances:
            def deconstruct(self):
                return ('DeconstructibleInstances', [], {})

        string = MigrationWriter.serialize(models.CharField(default=DeconstructibleInstances))[0]
        self.assertEqual(string, "models.CharField(default=migrations.test_writer.DeconstructibleInstances)")