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

django/db/migrations/writer.py

@@ -158,7 +158,7 @@ class MigrationWriter(object):
             "replaces_str": "",
         }
 
-        imports = {"from django.db import migrations, models"}
+        imports = set()
 
         # Deconstruct operations
         operations = []
@@ -188,7 +188,15 @@ class MigrationWriter(object):
                 migration_imports.add(line.split("import")[1].strip())
                 imports.remove(line)
                 self.needs_manual_porting = True
-        imports.discard("from django.db import models")
+
+        # 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])

tests/migrations/test_writer.py

@@ -512,6 +512,22 @@ class WriterTests(TestCase):
             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