Reduce size of lock_id field to 191 characters so that indexing will work with utf8mb4 character set

Author: jn123456789

db_mutex/migrations/0001_initial.py

@@ -14,7 +14,7 @@ class Migration(migrations.Migration):
             name='DBMutex',
             fields=[
                 ('id', models.AutoField(serialize=False, auto_created=True, verbose_name='ID', primary_key=True)),
-                ('lock_id', models.CharField(unique=True, max_length=255)),
+                ('lock_id', models.CharField(unique=True, max_length=191)),
                 ('creation_time', models.DateTimeField(auto_now_add=True)),
             ],
             options={

db_mutex/models.py

@@ -11,7 +11,7 @@ class DBMutex(models.Model):
     :type creation_time: datetime
     :param creation_time: The creation time of the mutex lock
     """
-    lock_id = models.CharField(max_length=255, unique=True)
+    lock_id = models.CharField(max_length=191, unique=True)
     creation_time = models.DateTimeField(auto_now_add=True)
 
     class Meta: