Allow nested collections

core/admin.py

@@ -4,6 +4,7 @@ from .models import ApiKey, Attachment, Collection, CollectionSection, Item, Kan
 
 class CollectionSectionInline(admin.TabularInline):
     model = CollectionSection
+    fk_name = 'collection'
     extra = 0
 
 

core/migrations/0019_remove_collectionsection_unique_collection_item_and_more.py

@@ -0,0 +1,40 @@
+# Generated by Django 5.2.17 on 2026-08-30 11:23
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0018_remove_collection_tags_collection_filter_tags_and_more'),
+    ]
+
+    operations = [
+        migrations.RemoveConstraint(
+            model_name='collectionsection',
+            name='unique_collection_item',
+        ),
+        migrations.AddField(
+            model_name='collectionsection',
+            name='sub_collection',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.PROTECT, related_name='containing_sections', to='core.collection'),
+        ),
+        migrations.AlterField(
+            model_name='collectionsection',
+            name='item',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.PROTECT, related_name='collection_sections', to='core.item'),
+        ),
+        migrations.AddConstraint(
+            model_name='collectionsection',
+            constraint=models.CheckConstraint(condition=models.Q(models.Q(('item__isnull', False), ('sub_collection__isnull', True)), models.Q(('item__isnull', True), ('sub_collection__isnull', False)), _connector='OR'), name='collection_section_exactly_one_target'),
+        ),
+        migrations.AddConstraint(
+            model_name='collectionsection',
+            constraint=models.UniqueConstraint(condition=models.Q(('item__isnull', False)), fields=('collection', 'item'), name='unique_collection_item'),
+        ),
+        migrations.AddConstraint(
+            model_name='collectionsection',
+            constraint=models.UniqueConstraint(condition=models.Q(('sub_collection__isnull', False)), fields=('collection', 'sub_collection'), name='unique_collection_sub_collection'),
+        ),
+    ]

core/models.py

@@ -222,7 +222,8 @@ class Collection(models.Model):
 
 class CollectionSection(models.Model):
     collection = models.ForeignKey(Collection, on_delete=models.CASCADE, related_name='sections')
-    item = models.ForeignKey(Item, on_delete=models.PROTECT, related_name='collection_sections')
+    item = models.ForeignKey(Item, on_delete=models.PROTECT, blank=True, null=True, related_name='collection_sections')
+    sub_collection = models.ForeignKey(Collection, on_delete=models.PROTECT, blank=True, null=True, related_name='containing_sections')
     title = models.CharField(max_length=200, blank=True)
     position = models.PositiveIntegerField(default=0)
     created_at = models.DateTimeField(auto_now_add=True)
@@ -230,12 +231,17 @@ class CollectionSection(models.Model):
     class Meta:
         ordering = ['position', 'id']
         constraints = [
-            models.UniqueConstraint(fields=['collection', 'item'], name='unique_collection_item'),
+            models.CheckConstraint(
+                check=(models.Q(item__isnull=False, sub_collection__isnull=True) | models.Q(item__isnull=True, sub_collection__isnull=False)),
+                name='collection_section_exactly_one_target',
+            ),
+            models.UniqueConstraint(fields=['collection', 'item'], condition=models.Q(item__isnull=False), name='unique_collection_item'),
+            models.UniqueConstraint(fields=['collection', 'sub_collection'], condition=models.Q(sub_collection__isnull=False), name='unique_collection_sub_collection'),
             models.UniqueConstraint(fields=['collection', 'position'], name='unique_collection_position'),
         ]
 
     def __str__(self):
-        return f'{self.collection}: {self.title or self.item}'
+        return f'{self.collection}: {self.title or self.item or self.sub_collection}'
 
 
 class UserPreference(models.Model):

core/templates/core/_collection_section_read.html

@@ -0,0 +1,14 @@
+{% load markdown_extras %}
+<section id="section-{{ section.id }}" class="collection-flow-section{% if nested %} ms-4 mt-4{% endif %}">
+  {% if section.item %}
+    {% if section.title %}{% if nested %}<h3>{{ section.title }}</h3>{% else %}<h2>{{ section.title }}</h2>{% endif %}{% endif %}
+    {% if nested %}{{ section.item|collection_sub_item_markdown }}{% else %}{{ section.item|collection_item_markdown }}{% endif %}
+    {% if section.item.comment %}<div class="text-muted border-start ps-3 mt-3">{{ section.item.comment|markdown }}</div>{% endif %}
+  {% elif section.sub_collection %}
+    {% if nested %}<h3>{{ section.title|default:section.sub_collection.title }}</h3>{% else %}<h2>{{ section.title|default:section.sub_collection.title }}</h2>{% endif %}
+    {% if section.sub_collection.description %}<div class="content lead mt-3">{{ section.sub_collection.description|markdown }}</div>{% endif %}
+    {% for child_section in section.sub_collection.sections.all %}
+      {% include 'core/_collection_section_read.html' with section=child_section nested=True %}
+    {% endfor %}
+  {% endif %}
+</section>

core/templates/core/collection_detail.html

@@ -22,7 +22,7 @@
   {% if sections or tag_items %}
     <nav class="collection-toc mb-5" aria-label="{% trans 'Contents' %}">
       <div class="small fw-semibold text-uppercase text-muted mb-2">{% trans "Contents" %}</div>
-      <ol class="mb-0">{% if collection.mode == 'tags' %}{% for item in tag_items %}<li><a href="#item-{{ item.id }}">{{ item.content|markdown_title|truncatechars:90 }}</a></li>{% endfor %}{% else %}{% for section in sections %}<li><a href="#section-{{ section.id }}">{{ section.title|default:section.item.content|markdown_title|truncatechars:90 }}</a></li>{% endfor %}{% endif %}</ol>
+      <ol class="mb-0">{% if collection.mode == 'tags' %}{% for item in tag_items %}<li><a href="#item-{{ item.id }}">{{ item.content|markdown_title|truncatechars:90 }}</a></li>{% endfor %}{% else %}{% for section in sections %}<li><a href="#section-{{ section.id }}">{% if section.item %}{{ section.title|default:section.item.content|markdown_title|truncatechars:90 }}{% else %}{{ section.title|default:section.sub_collection.title|truncatechars:90 }}{% endif %}</a></li>{% endfor %}{% endif %}</ol>
     </nav>
   {% endif %}
 
@@ -30,7 +30,7 @@
     {% if collection.mode == 'tags' %}
       {% for item in tag_items %}<section id="item-{{ item.id }}" class="collection-flow-section">{{ item|collection_item_markdown }}{% if item.comment %}<div class="text-muted border-start ps-3 mt-3">{{ item.comment|markdown }}</div>{% endif %}</section>{% empty %}<div class="text-center text-muted py-5">{% trans "No notes match the selected tags yet." %}</div>{% endfor %}
     {% else %}
-      {% for section in sections %}<section id="section-{{ section.id }}" class="collection-flow-section">{% if section.title %}<h2>{{ section.title }}</h2>{% endif %}{{ section.item|collection_item_markdown }}{% if section.item.comment %}<div class="text-muted border-start ps-3 mt-3">{{ section.item.comment|markdown }}</div>{% endif %}</section>{% empty %}<div class="text-center text-muted py-5">{% trans "This collection has no sections yet." %}</div>{% endfor %}
+      {% for section in sections %}{% include 'core/_collection_section_read.html' with section=section %}{% empty %}<div class="text-center text-muted py-5">{% trans "This collection has no sections yet." %}</div>{% endfor %}
     {% endif %}
   </div>
 </article>

core/templates/core/collection_manage.html

@@ -18,7 +18,7 @@
   {% if collection.description %}<div class="content lead mb-4">{{ collection.description|markdown }}</div>{% endif %}
 
   {% if sections %}
-    <div class="card item-card mb-4"><div class="card-body"><div class="fw-semibold mb-2">{% trans "Contents" %}</div><ol class="mb-0">{% for section in sections %}<li><a href="#section-{{ section.id }}">{{ section.title|default:section.item.content|markdown_title|truncatechars:90 }}</a></li>{% endfor %}</ol></div></div>
+    <div class="card item-card mb-4"><div class="card-body"><div class="fw-semibold mb-2">{% trans "Contents" %}</div><ol class="mb-0">{% for section in sections %}<li><a href="#section-{{ section.id }}">{% if section.item %}{{ section.title|default:section.item.content|markdown_title|truncatechars:90 }}{% else %}{{ section.title|default:section.sub_collection.title|truncatechars:90 }}{% endif %}</a></li>{% endfor %}</ol></div></div>
   {% endif %}
 
   {% if pending_item %}
@@ -39,11 +39,11 @@
     {% for section in sections %}
       <section id="section-{{ section.id }}" class="card item-card"><div class="card-body">
         <div class="d-flex justify-content-between align-items-start gap-3 mb-3">
-          <h2 class="h3 mb-0">{{ section.title|default:section.item.content|markdown_title|truncatechars:100 }}</h2>
+          <h2 class="h3 mb-0">{% if section.item %}{{ section.title|default:section.item.content|markdown_title|truncatechars:100 }}{% else %}{{ section.title|default:section.sub_collection.title|truncatechars:100 }}{% endif %}</h2>
-          <div class="d-flex gap-2 align-items-center"><a class="btn btn-sm btn-outline-secondary" href="{% url 'item_editor' section.item.id %}?next={% url 'collection_manage' collection.id %}">{% trans "Edit note" %}</a><a class="small text-muted" href="{{ section.item.get_absolute_url }}">#{{ section.item.id }}</a></div>
+          <div class="d-flex gap-2 align-items-center">{% if section.item %}<a class="btn btn-sm btn-outline-secondary" href="{% url 'item_editor' section.item.id %}?next={% url 'collection_manage' collection.id %}">{% trans "Edit note" %}</a><a class="small text-muted" href="{{ section.item.get_absolute_url }}">#{{ section.item.id }}</a>{% else %}<a class="btn btn-sm btn-outline-secondary" href="{% url 'collection_manage' section.sub_collection.id %}">{% trans "Edit collection" %}</a><a class="small text-muted" href="{{ section.sub_collection.get_absolute_url }}">#{{ section.sub_collection.id }}</a>{% endif %}</div>
         </div>
-        <div class="content">{{ section.item|item_markdown }}</div>
+        {% if section.item %}<div class="content">{{ section.item|item_markdown }}</div>
-        {% if section.item.comment %}<div class="content text-muted border-start ps-3 mt-3">{{ section.item.comment|markdown }}</div>{% endif %}
+        {% if section.item.comment %}<div class="content text-muted border-start ps-3 mt-3">{{ section.item.comment|markdown }}</div>{% endif %}{% else %}<div class="text-muted">{% trans "Nested collection" %}: {{ section.sub_collection.title }}</div>{% endif %}
         {% if can_edit %}<div class="border-top mt-4 pt-3 d-flex align-items-center gap-2">
           <form method="post" class="d-flex gap-2 flex-grow-1">{% csrf_token %}<input type="hidden" name="action" value="title"><input type="hidden" name="section_id" value="{{ section.id }}"><input class="form-control form-control-sm" name="title" value="{{ section.title }}" placeholder="{% trans 'Optional section title' %}"><button class="btn btn-sm btn-outline-secondary">{% trans "Save title" %}</button></form>
           <form method="post">{% csrf_token %}<input type="hidden" name="action" value="up"><input type="hidden" name="section_id" value="{{ section.id }}"><button class="btn btn-sm btn-outline-secondary" title="{% trans 'Move up' %}">↑</button></form>
@@ -56,11 +56,16 @@
 
   {% if can_edit %}
     <div class="card item-card"><div class="card-body">
-      <h2 class="h4">{% trans "Add existing note" %}</h2>
+      <h2 class="h4">{% trans "Add existing note or collection" %}</h2>
-      <form method="get" class="input-group mb-3"><input class="form-control" name="q" value="{{ q }}" placeholder="{% trans 'Search notes' %}"><button class="btn btn-outline-secondary">{% trans "Search" %}</button></form>
+      <form method="get" class="input-group mb-3"><input class="form-control" name="q" value="{{ q }}" placeholder="{% trans 'Search notes and collections' %}"><button class="btn btn-outline-secondary">{% trans "Search" %}</button></form>
-      <div class="vstack gap-2">
+      <h3 class="h6">{% trans "Notes" %}</h3>
+      <div class="vstack gap-2 mb-4">
         {% for item in candidates %}<div class="border rounded p-3 d-flex justify-content-between align-items-start gap-3"><div><div>{{ item.content|truncatechars:180 }}</div><div class="small text-muted">#{{ item.id }} · {{ item.get_visibility_display }}{% if item.team %} · {{ item.team.name }}{% endif %}</div></div><form method="post">{% csrf_token %}<input type="hidden" name="action" value="add"><input type="hidden" name="item_id" value="{{ item.id }}"><button class="btn btn-sm btn-dark">{% trans "Add" %}</button></form></div>{% empty %}<div class="text-muted">{% trans "No matching notes." %}</div>{% endfor %}
       </div>
+      <h3 class="h6">{% trans "Collections" %}</h3>
+      <div class="vstack gap-2">
+        {% for candidate in collection_candidates %}<div class="border rounded p-3 d-flex justify-content-between align-items-start gap-3"><div><div>{{ candidate.title }}</div><div class="small text-muted">#{{ candidate.id }} · {{ candidate.get_visibility_display }}{% if candidate.team %} · {{ candidate.team.name }}{% endif %}</div></div><form method="post">{% csrf_token %}<input type="hidden" name="action" value="add_collection"><input type="hidden" name="collection_id" value="{{ candidate.id }}"><button class="btn btn-sm btn-dark">{% trans "Add" %}</button></form></div>{% empty %}<div class="text-muted">{% trans "No matching collections." %}</div>{% endfor %}
+      </div>
     </div></div>
   {% endif %}
 </div>

core/templatetags/markdown_extras.py

@@ -7,7 +7,7 @@ from django.utils.safestring import mark_safe
 register = template.Library()
 
 ALLOWED_TAGS = set(bleach.sanitizer.ALLOWED_TAGS) | {
-    'p', 'pre', 'code', 'blockquote', 'h1', 'h2', 'h3', 'h4', 'ul', 'ol', 'li',
+    'p', 'pre', 'code', 'blockquote', 'h1', 'h2', 'h3', 'h4', 'h5', 'h6', 'ul', 'ol', 'li',
     'strong', 'em', 'u', 'table', 'thead', 'tbody', 'tr', 'th', 'td', 'hr', 'br', 'img'
 }
 ALLOWED_ATTRS = {'a': ['href', 'title', 'rel'], 'code': ['class'], 'img': ['src', 'alt', 'title', 'class']}
@@ -71,7 +71,18 @@ def markdown_title(text):
     return ''
 
 
-def render_item_markdown(item, hide_tags=False):
+def offset_heading_tags(html, offset=0):
+    if not offset:
+        return html
+
+    def replace(match):
+        slash, level = match.groups()
+        return f'<{slash}h{min(6, int(level) + offset)}>'
+
+    return re.sub(r'<(/?)h([1-6])>', replace, str(html))
+
+
+def render_item_markdown(item, hide_tags=False, heading_offset=0):
     text = strip_tags_outside_code(item.content) if hide_tags else (item.content or '')
     attachments = {str(a.id): a for a in item.attachments.all()}
 
@@ -81,7 +92,7 @@ def render_item_markdown(item, hide_tags=False):
             return match.group(0)
         return f'![{attachment.file.name}]({attachment.file.url})'
 
-    return markdown(FILE_RE.sub(replace, text))
+    return mark_safe(offset_heading_tags(markdown(FILE_RE.sub(replace, text)), heading_offset))
 
 
 @register.filter
@@ -94,6 +105,11 @@ def collection_item_markdown(item):
     return render_item_markdown(item, hide_tags=True)
 
 
+@register.filter
+def collection_sub_item_markdown(item):
+    return render_item_markdown(item, hide_tags=True, heading_offset=1)
+
+
 @register.filter
 def is_image(file_name):
     return str(file_name).lower().split('?')[0].endswith(('.png', '.jpg', '.jpeg', '.gif', '.webp', '.bmp', '.svg'))

core/tests.py

@@ -341,6 +341,53 @@ class CollectionTests(TestCase):
         self.assertContains(response, '>Section</')
         self.assertNotContains(response, '>## Section</')
 
+    def test_collection_can_include_collection(self):
+        parent = Collection.objects.create(owner=self.user, title='Parent')
+        child = Collection.objects.create(owner=self.user, title='Child')
+        note = Item.objects.create(owner=self.user, kind=Item.Kind.NOTE, content='## Child note #internal')
+        CollectionSection.objects.create(collection=child, item=note, position=0)
+        manage_url = reverse('collection_manage', args=[parent.id])
+
+        response = self.client.post(manage_url, {'action': 'add_collection', 'collection_id': child.id})
+
+        self.assertRedirects(response, manage_url)
+        self.assertTrue(parent.sections.filter(sub_collection=child).exists())
+        response = self.client.get(parent.get_absolute_url())
+        self.assertContains(response, '<h2>Child</h2>', html=True)
+        self.assertContains(response, '<h3>Child note</h3>', html=True)
+        self.assertNotContains(response, '#internal')
+
+    def test_collection_loop_is_rejected(self):
+        parent = Collection.objects.create(owner=self.user, title='Parent')
+        child = Collection.objects.create(owner=self.user, title='Child')
+        CollectionSection.objects.create(collection=parent, sub_collection=child, position=0)
+        manage_url = reverse('collection_manage', args=[child.id])
+
+        response = self.client.post(manage_url, {'action': 'add_collection', 'collection_id': parent.id}, follow=True)
+
+        self.assertContains(response, 'collection loop')
+        self.assertFalse(child.sections.filter(sub_collection=parent).exists())
+
+    def test_public_collection_cannot_include_private_collection(self):
+        parent = Collection.objects.create(owner=self.user, title='Parent', visibility=Item.Visibility.PUBLIC)
+        child = Collection.objects.create(owner=self.user, title='Child', visibility=Item.Visibility.PRIVATE)
+        manage_url = reverse('collection_manage', args=[parent.id])
+
+        response = self.client.post(manage_url, {'action': 'add_collection', 'collection_id': child.id}, follow=True)
+
+        self.assertContains(response, 'incompatible visibility')
+        self.assertFalse(parent.sections.filter(sub_collection=child).exists())
+
+    def test_private_collection_can_include_public_collection(self):
+        parent = Collection.objects.create(owner=self.user, title='Parent', visibility=Item.Visibility.PRIVATE)
+        child = Collection.objects.create(owner=self.user, title='Child', visibility=Item.Visibility.PUBLIC)
+        manage_url = reverse('collection_manage', args=[parent.id])
+
+        response = self.client.post(manage_url, {'action': 'add_collection', 'collection_id': child.id})
+
+        self.assertRedirects(response, manage_url)
+        self.assertTrue(parent.sections.filter(sub_collection=child).exists())
+
     def test_public_collection_is_visible_without_login(self):
         collection = Collection.objects.create(owner=self.user, title='Public', visibility=Item.Visibility.PUBLIC)
         self.client.logout()

core/views.py

@@ -542,7 +542,7 @@ def team_accept_invite(request, token):
 
 
 def visible_collections(user):
-    qs = Collection.objects.select_related('owner', 'team').prefetch_related('filter_tags', 'sections__item')
+    qs = Collection.objects.select_related('owner', 'team').prefetch_related('filter_tags', 'sections__item', 'sections__sub_collection')
     if user.is_authenticated:
         return qs.filter(Q(owner=user) | Q(team_id__in=user_team_ids(user)) | Q(visibility=Item.Visibility.PUBLIC)).distinct()
     return qs.filter(visibility=Item.Visibility.PUBLIC)
@@ -574,6 +574,22 @@ def collection_item_is_compatible(collection, item):
     )
 
 
+def collection_subcollection_is_compatible(collection, sub_collection):
+    return collection_scope_allows_item(
+        collection.visibility, collection.team_id, collection.owner_id,
+        sub_collection.visibility, sub_collection.team_id, sub_collection.owner_id,
+    )
+
+
+def collection_contains_collection(collection, target):
+    if collection.pk == target.pk:
+        return True
+    for section in collection.sections.select_related('sub_collection'):
+        if section.sub_collection and collection_contains_collection(section.sub_collection, target):
+            return True
+    return False
+
+
 def collection_target_from_form(form, user):
     team = form.cleaned_data.get('team')
     if team:
@@ -639,11 +655,22 @@ def copy_item_for_collection(item, collection, owner):
     return adapt_item_for_collection(copied, collection)
 
 
-def add_collection_section(collection, item):
+def next_collection_section_position(collection):
     last_position = collection.sections.order_by('-position').values_list('position', flat=True).first()
+    return (last_position + 1) if last_position is not None else 0
+
+
+def add_collection_section(collection, item):
     return CollectionSection.objects.get_or_create(
         collection=collection, item=item,
-        defaults={'position': (last_position + 1) if last_position is not None else 0},
+        defaults={'position': next_collection_section_position(collection)},
+    )
+
+
+def add_collection_subcollection(collection, sub_collection):
+    return CollectionSection.objects.get_or_create(
+        collection=collection, sub_collection=sub_collection,
+        defaults={'position': next_collection_section_position(collection)},
     )
 
 
@@ -657,17 +684,23 @@ def collection_detail_context(request, collection, pending_item=None):
     can_edit = user_can_edit_collection(request.user, collection)
     q = request.GET.get('q', '').strip()
     candidates = Item.objects.none()
+    collection_candidates = Collection.objects.none()
     if can_edit:
         candidates = editable_collection_notes(request.user).exclude(collection_sections__collection=collection)
+        collection_candidates = visible_collections(request.user).filter(mode=Collection.Mode.MANUAL).exclude(pk=collection.pk).exclude(containing_sections__collection=collection)
+        collection_candidates = [candidate for candidate in collection_candidates if collection_subcollection_is_compatible(collection, candidate) and not collection_contains_collection(candidate, collection)]
         if q:
             candidates = candidates.filter(Q(content__icontains=q) | Q(tags__name__icontains=q)).distinct()
+            collection_candidates = [candidate for candidate in collection_candidates if q.lower() in candidate.title.lower()]
         candidates = candidates[:30]
+        collection_candidates = collection_candidates[:30]
     return {
         'collection': collection,
-        'sections': collection.sections.select_related('item').prefetch_related('item__tags', 'item__attachments'),
+        'sections': collection.sections.select_related('item', 'sub_collection').prefetch_related('item__tags', 'item__attachments', 'sub_collection__sections__item', 'sub_collection__sections__sub_collection'),
         'can_edit': can_edit,
         'can_delete': user_can_delete_collection(request.user, collection),
         'candidates': candidates,
+        'collection_candidates': collection_candidates,
         'q': q,
         'pending_item': pending_item,
         'pending_can_convert': pending_item and item_can_change_for_collection(request.user, pending_item, collection),
@@ -718,10 +751,16 @@ def collection_edit(request, pk):
             visibility, team = collection_target_from_form(form, request.user)
             target_mode = form.cleaned_data['mode']
             has_manual_sections = collection.mode == Collection.Mode.MANUAL and collection.sections.exists()
-            incompatible = target_mode == Collection.Mode.MANUAL and any(not collection_scope_allows_item(
+            incompatible = target_mode == Collection.Mode.MANUAL and any(
-                visibility, team.id if team else None, collection.owner_id,
+                not collection_scope_allows_item(
-                section.item.visibility, section.item.team_id, section.item.owner_id,
+                    visibility, team.id if team else None, collection.owner_id,
-            ) for section in collection.sections.select_related('item'))
+                    section.item.visibility, section.item.team_id, section.item.owner_id,
+                ) if section.item else not collection_scope_allows_item(
+                    visibility, team.id if team else None, collection.owner_id,
+                    section.sub_collection.visibility, section.sub_collection.team_id, section.sub_collection.owner_id,
+                )
+                for section in collection.sections.select_related('item', 'sub_collection')
+            )
             if target_mode == Collection.Mode.TAGS and has_manual_sections:
                 form.add_error('mode', _('Remove all manual sections before changing to a tag collection.'))
             elif incompatible:
@@ -747,7 +786,7 @@ def collection_detail(request, pk):
     collection = get_object_or_404(visible_collections(request.user), pk=pk)
     return render(request, 'core/collection_detail.html', {
         'collection': collection,
-        'sections': collection.sections.select_related('item').prefetch_related('item__tags', 'item__attachments') if collection.mode == Collection.Mode.MANUAL else [],
+        'sections': collection.sections.select_related('item', 'sub_collection').prefetch_related('item__tags', 'item__attachments', 'sub_collection__sections__item', 'sub_collection__sections__sub_collection') if collection.mode == Collection.Mode.MANUAL else [],
         'tag_items': collection_tag_items(collection) if collection.mode == Collection.Mode.TAGS else [],
         'can_edit': user_can_edit_collection(request.user, collection),
     })
@@ -782,6 +821,15 @@ def collection_manage(request, pk):
                 return redirect(manage_url)
             add_collection_section(collection, item)
             return redirect(manage_url)
+        if action == 'add_collection':
+            sub_collection = get_object_or_404(visible_collections(request.user), pk=request.POST.get('collection_id'), mode=Collection.Mode.MANUAL)
+            if sub_collection.pk == collection.pk or collection_contains_collection(sub_collection, collection):
+                messages.error(request, _('This would create a collection loop.'))
+            elif not collection_subcollection_is_compatible(collection, sub_collection):
+                messages.error(request, _('This collection has an incompatible visibility.'))
+            else:
+                add_collection_subcollection(collection, sub_collection)
+            return redirect(manage_url)
         section = get_object_or_404(CollectionSection, pk=request.POST.get('section_id'), collection=collection)
         if action == 'remove':
             section.delete()