Fixes #21763: Replace M2M selection field with separate add/remove fields

netbox/utilities/forms/rendering.py

@@ -5,6 +5,7 @@ from functools import cached_property
 __all__ = (
     'FieldSet',
     'InlineFields',
+    'M2MAddRemoveFields',
     'ObjectAttribute',
     'TabbedGroups',
 )
@@ -73,6 +74,21 @@ class TabbedGroups:
         ]
 
 
+class M2MAddRemoveFields:
+    """
+    Represents an add/remove field pair for a many-to-many relationship. Rather than rendering
+    a single multi-select pre-populated with all current values (which can crash the browser for
+    large datasets), this renders two fields: one for adding new relations and one for removing
+    existing relations.
+
+    Parameters:
+        name: The name of the M2M field on the model (e.g. 'asns'). The form must define
+              corresponding 'add_{name}' and 'remove_{name}' fields.
+    """
+    def __init__(self, name):
+        self.name = name
+
+
 class ObjectAttribute:
     """
     Renders the value for a specific attribute on the form's instance. This may be used to

netbox/utilities/templatetags/form_helpers.py

@@ -1,6 +1,6 @@
 from django import template
 
-from utilities.forms.rendering import InlineFields, ObjectAttribute, TabbedGroups
+from utilities.forms.rendering import InlineFields, M2MAddRemoveFields, ObjectAttribute, TabbedGroups
 
 __all__ = (
     'getfield',
@@ -80,6 +80,13 @@ def render_fieldset(form, fieldset):
                 ('tabs', None, tabs)
             )
 
+        elif type(item) is M2MAddRemoveFields:
+            for field_name in (f'add_{item.name}', f'remove_{item.name}'):
+                if field_name in form.fields:
+                    rows.append(
+                        ('field', None, [form[field_name]])
+                    )
+
         elif type(item) is ObjectAttribute:
             value = getattr(form.instance, item.name)
             label = value._meta.verbose_name if hasattr(value, '_meta') else item.name