Fixed #24514 -- Made migration writer omit models import if it's unused.

            "replaces_str": "",

        }

        imports = {"from django.db import migrations, models"}

        imports = set()

        # Deconstruct operations

        operations = []

                migration_imports.add(line.split("import")[1].strip())

                imports.remove(line)

                self.needs_manual_porting = True

        # django.db.migrations is always used, but models import may not be.

        # If models import exists, merge it with migrations import.

        if "from django.db import models" in imports:

            imports.discard("from django.db import models")

            imports.add("from django.db import migrations, models")

        else:

            imports.add("from django.db import migrations")

        # Sort imports by the package / module to be imported (the part after

        # "from" in "from ... import ..." or after "import" in "import ...").

        sorted_imports = sorted(imports, key=lambda i: i.split()[1])

            output

        )

    def test_models_import_omitted(self):

        """

        django.db.models shouldn't be imported if unused.

        """

        migration = type(str("Migration"), (migrations.Migration,), {

            "operations": [

                migrations.AlterModelOptions(

                    name='model',

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

                    ),

            ]

        })

        writer = MigrationWriter(migration)

        output = writer.as_string().decode('utf-8')

        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