Refs kiwitcms#1774. Play around with restoring non-nullable BugSystem…

… fields

in case the migration is applied backwards

NOTES:

- Can't use obj.serialize() b/c apps.get_model() retruns a fake
  class, not the real one. Fields are there but methods aren't.
- Use Serializer(model=...).serialize_model() instead
- ^^^ doesn't fail on previous migrations b/c the respective tables
  are empty during test! Will fail as soon as we have some records
  inside of them

- Make `validate_reg_exp` and `url_reg_exp` fields nullable before
  removing them so that in the reverse direction the values could be
  restored and finally the fields set to NOT NULL once again!

tcms/testcases/migrations/0011_trim_bugsystem_fields.py

@@ -1,37 +1,33 @@
-# Generated by Django 2.2.4 on 2019-08-16 12:17
+import glob
+import json
+
 from django.db import migrations, models
 
+from tcms.rpc.serializer import Serializer
+
 
 def forwards_store_data(apps, schema_editor):
     bug_system_model = apps.get_model('testcases', 'BugSystem')
-    file_name = '/tmp/kiwitcms-testcases-migration-0011-\
-BugSystem'  # nosec:B108:hardcoded_tmp_directory
 
-    with open(file_name, 'w') as temp_file:
-        for bug_system in bug_system_model.objects.all():
-            temp_file.write('{},{},{},{}\n'.format(
-                bug_system.name,
-                bug_system.description,
-                bug_system.url_reg_exp,
-                bug_system.validate_reg_exp
-            ))
+    for bug_system in bug_system_model.objects.all():
+        file_name = '/tmp/kiwitcms-testcases-migration\
+-0011-BugSystem-%d' % bug_system.pk  # nosec:B108:hardcoded_tmp_directory
+
+        with open(file_name, 'w') as outfile:
+            json.dump(Serializer(model=bug_system).serialize_model(), outfile)
 
 
 def backwards_restore_data(apps, schema_editor):
     bug_system_model = apps.get_model('testcases', 'BugSystem')
-    file_name = '/tmp/kiwitcms-testcases-migration-0011-\
-BugSystem'  # nosec:B108:hardcoded_tmp_directory
 
-    with open(file_name, 'r') as temp_file:
-        for line in temp_file:
-            try:
-                bug_system = bug_system_model.objects.get(name=line[0])
-                bug_system.description = line[1]
-                bug_system.url_reg_exp = line[2]
-                bug_system.validate_reg_exp = line[3]
-                bug_system.save()
-            except bug_system_model.DoesNotExist:
-                print('BugSystem {} not found'.format(line[0]))
+    for file_name in glob.glob(
+            '/tmp/kiwitcms-testcases-migration-0011-\
+BugSystem-*'  # nosec:B108:hardcoded_tmp_directory
+    ):
+        with open(file_name, 'r') as infile:
+            data = json.load(infile)
+            bug_system = bug_system_model(**data)
+            bug_system.save()
 
 
 class Migration(migrations.Migration):
@@ -41,7 +37,25 @@ class Migration(migrations.Migration):
     ]
 
     operations = [
+        # make these 2 fields null=True so they can be restored in the reverse migration
+        # once these 2 operations are reversed the fields should become once again NOT NULL
+        migrations.AlterField(model_name='bugsystem',
+                              name='validate_reg_exp',
+                              field=models.CharField(max_length=128,
+                                                     help_text='A valid JavaScript regular '
+                                                               'expression such as ^\\d$',
+                                                     verbose_name='RegExp for ID validation',
+                                                     null=True)),
+        migrations.AlterField(model_name='bugsystem',
+                              name='url_reg_exp',
+                              field=models.CharField(max_length=8192,
+                                                     help_text='A valid Python format string such '
+                                                               'as http://bugs.example.com/%s',
+                                                     verbose_name='URL format string',
+                                                     null=True)),
+
         migrations.RunPython(forwards_store_data, backwards_restore_data),
+
         migrations.RemoveField(
             model_name='bugsystem',
             name='description',
@@ -54,20 +68,4 @@ class Migration(migrations.Migration):
             model_name='bugsystem',
             name='validate_reg_exp',
         ),
-        migrations.AlterField(model_name='bugsystem',
-                              name='validate_reg_exp',
-                              preserve_default=False,
-                              field=models.CharField(max_length=128,
-                                                     help_text='A valid JavaScript regular '
-                                                               'expression such as ^\\d$',
-                                                     verbose_name='RegExp for ID validation',
-                                                     default='')),
-        migrations.AlterField(model_name='bugsystem',
-                              name='url_reg_exp',
-                              preserve_default=False,
-                              field=models.CharField(max_length=8192,
-                                                     help_text='A valid Python format string such '
-                                                               'as http://bugs.example.com/%s',
-                                                     verbose_name='URL format string',
-                                                     default='')),
     ]