Fixed #31317 -- Avoided crash in CreateModel with unique_together and AlterUniqueTogether.

django/db/backends/base/schema.py

@@ -658,14 +658,33 @@ def _delete_composed_index(self, model, fields, constraint_kwargs, sql):
         }
         meta_index_names = {constraint.name for constraint in model._meta.indexes}
         columns = [model._meta.get_field(field).column for field in fields]
+
+        # Check if the constraint is still in deferred_sql. This happens when
+        # CreateModel with unique_together or index_together is followed by
+        # AlterUniqueTogether/AlterIndexTogether in the same migration.
+        is_unique_constraint = constraint_kwargs.get("unique") is True
+        table = model._meta.db_table
+        if is_unique_constraint:
+            for deferred in list(self.deferred_sql):
+                if (
+                    isinstance(deferred, Statement)
+                    and deferred.references_table(table)
+                    and all(
+                        deferred.references_column(table, column) for column in columns
+                    )
+                    and deferred.parts["columns"].columns == columns
+                ):
+                    self.deferred_sql.remove(deferred)
+                    return
+
         constraint_names = self._constraint_names(
             model,
             columns,
             exclude=meta_constraint_names | meta_index_names,
             **constraint_kwargs,
         )
         if (
-            constraint_kwargs.get("unique") is True
+            is_unique_constraint
             and constraint_names
             and self.connection.features.allows_multiple_constraints_on_same_fields
         ):

tests/migrations/test_operations.py

@@ -3823,6 +3823,62 @@ def test_alter_unique_together_remove(self):
             operation.describe(), "Alter unique_together for Pony (0 constraint(s))"
         )
 
+    def test_alter_unique_together_deferred(self):
+        """
+        AlterUniqueTogether handles deferred SQL constraints from previous
+        operations. Regression test for #31317.
+        """
+        app_label = "test_aluntod"
+        self.apply_operations(
+            app_label,
+            ProjectState(),
+            operations=[
+                migrations.CreateModel(
+                    "Pony",
+                    fields=[
+                        ("id", models.AutoField(primary_key=True)),
+                        ("pink", models.IntegerField(default=3)),
+                        ("weight", models.FloatField()),
+                    ],
+                    options={"unique_together": {("pink",)}},
+                ),
+                migrations.AlterUniqueTogether(
+                    name="Pony",
+                    unique_together={("pink", "weight")},
+                ),
+            ],
+        )
+
+        table_name = f"{app_label}_pony"
+        self.assertUniqueConstraintExists(table_name, ("pink", "weight"), value=True)
+        self.assertUniqueConstraintExists(table_name, ("pink",), value=False)
+
+    def test_alter_unique_together_deferred_overlapping_columns(self):
+        app_label = "test_aluntodoc"
+        self.apply_operations(
+            app_label,
+            ProjectState(),
+            operations=[
+                migrations.CreateModel(
+                    "Pony",
+                    fields=[
+                        ("id", models.AutoField(primary_key=True)),
+                        ("pink", models.IntegerField(default=3)),
+                        ("weight", models.FloatField()),
+                    ],
+                    options={"unique_together": [("pink", "weight"), ("pink",)]},
+                ),
+                migrations.AlterUniqueTogether(
+                    name="Pony",
+                    unique_together={("pink", "weight")},
+                ),
+            ],
+        )
+
+        table_name = f"{app_label}_pony"
+        self.assertUniqueConstraintExists(table_name, ("pink", "weight"), value=True)
+        self.assertUniqueConstraintExists(table_name, ("pink",), value=False)
+
     @skipUnlessDBFeature("allows_multiple_constraints_on_same_fields")
     def test_remove_unique_together_on_pk_field(self):
         app_label = "test_rutopkf"
@@ -4414,6 +4470,35 @@ def test_alter_index_together_remove_with_unique_together(self):
         self.assertIndexNotExists(table_name, ["pink", "weight"])
         self.assertUniqueConstraintExists(table_name, ["pink", "weight"])
 
+    def test_alter_index_together_deferred_overlapping_columns(self):
+        app_label = "test_alintodoc"
+        self.apply_operations(
+            app_label,
+            ProjectState(),
+            operations=[
+                migrations.CreateModel(
+                    "Pony",
+                    fields=[
+                        ("id", models.AutoField(primary_key=True)),
+                        ("pink", models.IntegerField(default=3)),
+                        ("weight", models.FloatField()),
+                    ],
+                    options={
+                        "unique_together": [("pink",)],
+                        "index_together": [("pink",)],
+                    },
+                ),
+                migrations.AlterIndexTogether(
+                    name="Pony",
+                    index_together=set(),
+                ),
+            ],
+        )
+
+        table_name = f"{app_label}_pony"
+        self.assertIndexNotExists(table_name, ["pink"])
+        self.assertUniqueConstraintExists(table_name, ("pink",), value=True)
+
     def test_add_constraint(self):
         project_state = self.set_up_test_model("test_addconstraint")
         gt_check = models.Q(pink__gt=2)