[MCKIN-21879] Remove foreign keys from deprecated `progress-edx-platf…

…orm-extensions` (openedx#1874)

common/djangoapps/database_fixups/migrations/0002_remove_foreign_keys_from_progress_extensions.py

@@ -0,0 +1,64 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+"""
+The `progress-edx-platform-extensions` has been deprecated in favor of `edx-completion`.
+The requirement was removed in the commit linked as (1) below. However its migration (2) had not been reverted.
+That migration used `auth_user.id` as the foreign key in its models (3), but Django does not resolve this constraint
+between existing tables anymore, because the model has been removed.
+Therefore we need to drop the tables related to deprecated application in order to be able to remove users properly.
+
+Because of some performance concerns, deletion is implemented in (4).
+This migration drops only foreign keys from deprecated tables.
+If ran twice (for any reason), it will raise a custom error for better visibility that these keys do not exist.
+
+(1) https://github.com/edx-solutions/edx-platform/commit/59bf3efe71533de53b60bd979517e889d18a96bb
+(2) https://github.com/edx-solutions/progress-edx-platform-extensions/blob/master/progress/migrations/0001_initial.py
+(3) https://github.com/edx-solutions/progress-edx-platform-extensions/blob/master/progress/models.py
+(4) https://github.com/edx-solutions/edx-platform/pull/1862
+"""
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('database_fixups', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.RunSQL("""
+            -- Drop a procedure if it already exists - safety check.
+            DROP PROCEDURE IF EXISTS drop_foreign_key_from_table;
+
+            -- We are dropping constraints from 3 tables, so we create a temporary procedure to avoid code repetition.
+            CREATE PROCEDURE drop_foreign_key_from_table(given_table VARCHAR(64))
+            BEGIN
+                -- Find the ID of the foreign key (there is only one per table, otherwise it would fail).
+                SET @foreign_key = (
+                    SELECT CONSTRAINT_NAME FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS
+                    WHERE TABLE_NAME = given_table AND CONSTRAINT_TYPE = 'FOREIGN KEY'
+                );
+                IF @foreign_key IS NOT NULL THEN
+                    -- Prepare query (MySQL does not allow embedding queries in a standard way here).
+                    SET @statement = CONCAT('ALTER TABLE ', given_table, ' DROP FOREIGN KEY ', @foreign_key);
+                    PREPARE stmt FROM @statement;
+                    EXECUTE stmt;
+                    DEALLOCATE PREPARE stmt;
+                ELSE
+                    -- Raise custom error for having clearer logs in case of a failure.
+                    SET @error_message = CONCAT('Cannot find foreign key in ', given_table, ' table.');
+                    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = @error_message;
+                END IF;
+            END;
+
+            -- Call temporary procedure on relevant tables.
+            CALL drop_foreign_key_from_table('progress_coursemodulecompletion');
+            CALL drop_foreign_key_from_table('progress_studentprogress');
+            CALL drop_foreign_key_from_table('progress_studentprogresshistory');
+
+            -- Clean up.
+            DROP PROCEDURE IF EXISTS drop_foreign_key_from_table;
+        """)
+    ]