Fix #60 -- Adjust pre-AddField(db_default) for ForeignKey with callable defaults.

CHANGELOG.rst

@@ -5,6 +5,8 @@
 
 - Avoid unnecessary prompting for a default value on `ManyToManyField`
   removals. (#59)
+- Address a ``makemigration`` crash when adding a ``ForeignKey`` with a
+  callable ``default``. (#60)
 
 1.2.0
 =====

syzygy/operations.py

@@ -1,5 +1,6 @@
 from contextlib import contextmanager
 
+from django.db import models
 from django.db.migrations import operations
 from django.db.models.fields import NOT_PROVIDED
 from django.utils.functional import cached_property
@@ -201,7 +202,22 @@ def get_pre_add_field_operation(model_name, name, field, preserve_default=True):
                 "Fields with a db_default don't require a pre-deployment operation."
             )
         field = field.clone()
-        field.db_default = field.get_default()
+        if isinstance(field, models.ForeignKey):
+            # XXX: Replicate ForeignKey.get_default() logic in a way that
+            # doesn't require the field references to be pre-emptively
+            # resolved. This will need to be fixed upstream if we ever
+            # implement model state schema alterations.
+            # See  https://code.djangoproject.com/ticket/29898
+            field_default = super(models.ForeignKey, field).get_default()
+            if (
+                isinstance(field_default, models.Model)
+                and field_default._meta.label_lower == field.related_model.lower()
+            ):
+                target_field = field.to_fields[0] or "pk"
+                field_default = getattr(field_default, target_field)
+        else:
+            field_default = field.get_default()
+        field.db_default = field_default
         operation = operations.AddField(model_name, name, field, preserve_default)
         return operation
 

tests/models.py

@@ -3,3 +3,10 @@
 
 class Foo(models.Model):
     pass
+
+
+class Bar(models.Model):
+    name = models.CharField(max_length=100, unique=True)
+
+    class Meta:
+        managed = False

tests/test_autodetector.py

@@ -25,6 +25,8 @@
 )
 from syzygy.plan import get_migration_stage
 
+from .models import Bar
+
 
 class AutodetectorTestCase(TestCase):
     style = color_style()
@@ -53,7 +55,7 @@ def get_changes(
 
 
 class AutodetectorTests(AutodetectorTestCase):
-    def _test_field_addition(self, field):
+    def _test_field_addition(self, field, expected_db_default=None):
         from_model = ModelState("tests", "Model", [])
         to_model = ModelState("tests", "Model", [("field", field)])
         changes = self.get_changes([from_model], [to_model])["tests"]
@@ -64,7 +66,10 @@ def _test_field_addition(self, field):
         pre_operation = changes[0].operations[0]
         if field_db_default_supported:
             self.assertIsInstance(pre_operation, migrations.AddField)
-            self.assertEqual(pre_operation.field.db_default, field.default)
+            self.assertEqual(
+                pre_operation.field.db_default,
+                expected_db_default or field.get_default(),
+            )
         else:
             self.assertIsInstance(pre_operation, AddField)
         self.assertEqual(get_migration_stage(changes[1]), Stage.POST_DEPLOY)
@@ -81,10 +86,33 @@ def test_field_addition(self):
         fields = [
             models.IntegerField(default=42),
             models.IntegerField(null=True, default=42),
+            models.IntegerField(default=lambda: 42),
+            # Foreign keys with callable defaults should have their associated
+            # db_default generated with care.
+            (models.ForeignKey("tests.Model", models.CASCADE, default=42), 42),
+            (
+                models.ForeignKey(
+                    "tests.Bar", models.CASCADE, default=lambda: Bar(id=42)
+                ),
+                42,
+            ),
+            (
+                models.ForeignKey(
+                    "tests.bar",
+                    models.CASCADE,
+                    to_field="name",
+                    default=lambda: Bar(id=123, name="bar"),
+                ),
+                "bar",
+            ),
         ]
         for field in fields:
+            if isinstance(field, tuple):
+                field, expected_db_default = field
+            else:
+                expected_db_default = None
             with self.subTest(field=field):
-                self._test_field_addition(field)
+                self._test_field_addition(field, expected_db_default)
 
     def test_many_to_many_addition(self):
         from_model = ModelState("tests", "Model", [])

tests/test_migrations/null_field_removal/0001_initial.py

@@ -19,4 +19,20 @@ class Migration(migrations.Migration):
                 ("bar", models.IntegerField()),
             ],
         ),
+        migrations.CreateModel(
+            "Bar",
+            fields=[
+                (
+                    "id",
+                    models.AutoField(
+                        auto_created=True,
+                        primary_key=True,
+                        serialize=False,
+                        verbose_name="ID",
+                    ),
+                ),
+                ("name", models.CharField(unique=True)),
+            ],
+            options={"managed": False},
+        ),
     ]