[1.11.x] Fixed #28305 -- Fixed "Cannot change column 'x': used in a foreign key constraint" crash on MySQL with a sequence of AlterField or RenameField operations.

Regression in 45ded053b1f4320284aa5dac63052f6d1baefea9.

Backport of c3e0adcad8d8ba94b33cabd137056166ed36dae0 from master

3 files changed, 32 insertions, 8 deletions

diff --git a/django/db/backends/base/schema.py b/django/db/backends/base/schema.py
index 56102ff27b..2c9e016d11 100644
--- a/django/db/backends/base/schema.py
+++ b/django/db/backends/base/schema.py
@@ -543,9 +543,15 @@ class BaseDatabaseSchemaEditor(object):
                 ))
             for constraint_name in constraint_names:
                 self.execute(self._delete_constraint_sql(self.sql_delete_unique, model, constraint_name))
-        # Drop incoming FK constraints if we're a primary key and things are going
-        # to change.
-        if old_field.primary_key and new_field.primary_key and old_type != new_type:
+        # Drop incoming FK constraints if the field is a primary key or unique,
+        # which might be a to_field target, and things are going to change.
+        drop_foreign_keys = (
+            (
+                (old_field.primary_key and new_field.primary_key) or
+                (old_field.unique and new_field.unique)
+            ) and old_type != new_type
+        )
+        if drop_foreign_keys:
             # '_meta.related_field' also contains M2M reverse fields, these
             # will be filtered out
             for _old_rel, new_rel in _related_non_m2m_objects(old_field, new_field):
@@ -772,7 +778,7 @@ class BaseDatabaseSchemaEditor(object):
                 new_field.db_constraint):
             self.execute(self._create_fk_sql(model, new_field, "_fk_%(to_table)s_%(to_column)s"))
         # Rebuild FKs that pointed to us if we previously had to drop them
-        if old_field.primary_key and new_field.primary_key and old_type != new_type:
+        if drop_foreign_keys:
             for rel in new_field.model._meta.related_objects:
                 if not rel.many_to_many and rel.field.db_constraint:
                     self.execute(self._create_fk_sql(rel.related_model, rel.field, "_fk"))
diff --git a/django/db/migrations/operations/fields.py b/django/db/migrations/operations/fields.py
index a458397fd4..0d5e690472 100644
--- a/django/db/migrations/operations/fields.py
+++ b/django/db/migrations/operations/fields.py
@@ -5,6 +5,7 @@ from django.db.models.fields import NOT_PROVIDED
 from django.utils.functional import cached_property
 
 from .base import Operation
+from .utils import is_referenced_by_foreign_key
 
 
 class FieldOperation(Operation):
@@ -201,8 +202,12 @@ class AlterField(FieldOperation):
         ]
         # TODO: investigate if old relational fields must be reloaded or if it's
         # sufficient if the new field is (#27737).
-        # Delay rendering of relationships if it's not a relational field
-        delay = not field.is_relation
+        # Delay rendering of relationships if it's not a relational field and
+        # not referenced by a foreign key.
+        delay = (
+            not field.is_relation and
+            not is_referenced_by_foreign_key(state, self.model_name_lower, self.field, self.name)
+        )
         state.reload_model(app_label, self.model_name_lower, delay=delay)
 
     def database_forwards(self, app_label, schema_editor, from_state, to_state):
@@ -275,8 +280,12 @@ class RenameField(FieldOperation):
         for index, (name, field) in enumerate(fields):
             if name == self.old_name:
                 fields[index] = (self.new_name, field)
-                # Delay rendering of relationships if it's not a relational field.
-                delay = not field.is_relation
+                # Delay rendering of relationships if it's not a relational
+                # field and not referenced by a foreign key.
+                delay = (
+                    not field.is_relation and
+                    not is_referenced_by_foreign_key(state, self.model_name_lower, field, self.name)
+                )
                 break
         else:
             raise FieldDoesNotExist(
diff --git a/django/db/migrations/operations/utils.py b/django/db/migrations/operations/utils.py
new file mode 100644
index 0000000000..af23ea9563
--- /dev/null
+++ b/django/db/migrations/operations/utils.py
@@ -0,0 +1,9 @@
+def is_referenced_by_foreign_key(state, model_name_lower, field, field_name):
+    for state_app_label, state_model in state.models:
+        for _, f in state.models[state_app_label, state_model].fields:
+            if (f.related_model and
+                    '%s.%s' % (state_app_label, model_name_lower) == f.related_model.lower() and
+                    hasattr(f, 'to_fields')):
+                if (f.to_fields[0] is None and field.primary_key) or field_name in f.to_fields:
+                    return True
+    return False