Implement task merging

todo/models.py

@@ -4,11 +4,45 @@ import textwrap
 
 from django.conf import settings
 from django.contrib.auth.models import Group
-from django.db import models
+from django.db import models, DEFAULT_DB_ALIAS
+from django.db.transaction import Atomic, get_connection
 from django.urls import reverse
 from django.utils import timezone
 
 
+class LockedAtomicTransaction(Atomic):
+    """
+    modified from https://stackoverflow.com/a/41831049
+    this is needed for safely merging
+
+    Does a atomic transaction, but also locks the entire table for any transactions, for the duration of this
+    transaction. Although this is the only way to avoid concurrency issues in certain situations, it should be used with
+    caution, since it has impacts on performance, for obvious reasons...
+    """
+    def __init__(self, *models, using=None, savepoint=None):
+        if using is None:
+            using = DEFAULT_DB_ALIAS
+        super().__init__(using, savepoint)
+        self.models = models
+
+    def __enter__(self):
+        super(LockedAtomicTransaction, self).__enter__()
+
+        # Make sure not to lock, when sqlite is used, or you'll run into problems while running tests!!!
+        if settings.DATABASES[self.using]['ENGINE'] != 'django.db.backends.sqlite3':
+            cursor = None
+            try:
+                cursor = get_connection(self.using).cursor()
+                for model in self.models:
+                    cursor.execute(
+                        'LOCK TABLE {table_name}'.format(
+                            table_name=model._meta.db_table)
+                    )
+            finally:
+                if cursor and not cursor.closed:
+                    cursor.close()
+
+
 class TaskList(models.Model):
     name = models.CharField(max_length=60)
     slug = models.SlugField(default="")
@@ -67,6 +101,14 @@ class Task(models.Model):
             self.completed_date = datetime.datetime.now()
         super(Task, self).save()
 
+    def merge_into(self, merge_target):
+        # lock the comments to avoid concurrent additions of comments after the
+        # update request. these comments would be irremediably lost because of
+        # the cascade clause
+        with LockedAtomicTransaction(Comment):
+            Comment.objects.filter(task=self).update(task=merge_target)
+            self.delete()
+
     class Meta:
         ordering = ["priority"]
 

todo/templates/todo/task_detail.html

@@ -73,6 +73,16 @@
             {{ task.task_list }}
           </a>
         </li>
+        <li class="list-group-item">
+        <h5>Merge task into</h5>
+        <form action="" method="post">
+          {% csrf_token %}
+          <div class="form-group">
+            <textarea class="form-control" name="merge-target" rows="3"></textarea>
+          </div>
+          <input class="btn btn-sm btn-primary" type="submit" name="merge_task_into" value="Merge task">
+        </form>
+        </li>
       </ul>
     </div>
   </div>
@@ -82,6 +92,7 @@
     {% include 'todo/include/task_edit.html' %}
   </div>
 
+
   <h5>Add comment</h5>
   <form action="" method="post">
     {% csrf_token %}

todo/utils.py

@@ -2,7 +2,6 @@ from django.conf import settings
 from django.contrib.sites.models import Site
 from django.core.mail import send_mail
 from django.template.loader import render_to_string
-
 from todo.models import Comment, Task
 
 

todo/views/task_detail.py

@@ -1,6 +1,6 @@
+import bleach
 import datetime
 
-import bleach
 from django.contrib import messages
 from django.contrib.auth.decorators import login_required, user_passes_test
 from django.core.exceptions import PermissionDenied
@@ -26,6 +26,12 @@ def task_detail(request, task_id: int) -> HttpResponse:
     if task.task_list.group not in request.user.groups.all() and not request.user.is_staff:
         raise PermissionDenied
 
+    if request.POST.get("merge_task_into"):
+        target_task_id = int(request.POST.get("merge-target"))
+        merge_target = Task.objects.get(pk=target_task_id)
+        task.merge_into(merge_target)
+        return redirect("todo:task_detail", task_id=merge_target.id)
+
     # Save submitted comments
     if request.POST.get("add_comment"):
         Comment.objects.create(