From 2c19390d7c69c3c930def43153567b3cd807a379 Mon Sep 17 00:00:00 2001
From: Jeremy Stretch <stretch@packetlife.net>
Date: Thu, 7 May 2020 17:20:32 -0400
Subject: [PATCH] Introduce CustomFieldManager (WIP)

---
 .../migrations/0042_customfield_manager.py    | 20 ++++++++++
 netbox/extras/models/customfields.py          | 38 +++++++++++++++++--
 2 files changed, 54 insertions(+), 4 deletions(-)
 create mode 100644 netbox/extras/migrations/0042_customfield_manager.py

diff --git a/netbox/extras/migrations/0042_customfield_manager.py b/netbox/extras/migrations/0042_customfield_manager.py
new file mode 100644
index 000000000..7d80b567a
--- /dev/null
+++ b/netbox/extras/migrations/0042_customfield_manager.py
@@ -0,0 +1,20 @@
+# Generated by Django 3.0.5 on 2020-05-07 21:06
+
+from django.db import migrations
+import extras.models.customfields
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('extras', '0041_tag_description'),
+    ]
+
+    operations = [
+        migrations.AlterModelManagers(
+            name='customfield',
+            managers=[
+                ('objects', extras.models.customfields.CustomFieldManager()),
+            ],
+        ),
+    ]
diff --git a/netbox/extras/models/customfields.py b/netbox/extras/models/customfields.py
index 47bccd98a..f00a52a5d 100644
--- a/netbox/extras/models/customfields.py
+++ b/netbox/extras/models/customfields.py
@@ -43,10 +43,7 @@ class CustomFieldModel(models.Model):
         """
         Return a dictionary of custom fields for a single object in the form {<field>: value}.
         """
-
-        # Find all custom fields applicable to this type of object
-        content_type = ContentType.objects.get_for_model(self)
-        fields = CustomField.objects.filter(obj_type=content_type)
+        fields = CustomField.objects.get_for_model(self)
 
         # If the object exists, populate its custom fields with values
         if hasattr(self, 'pk'):
@@ -57,6 +54,37 @@ class CustomFieldModel(models.Model):
             return OrderedDict([(field, None) for field in fields])
 
 
+class CustomFieldManager(models.Manager):
+    use_in_migrations = True
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+
+        # Initialize a cache for fetched CustomFields
+        self._cache = {}
+
+    def get_for_model(self, model):
+        """
+        Return all CustomFields assigned to the given model.
+        """
+        model = model._meta.concrete_model
+
+        # First try to return from cache
+        try:
+            return self._cache[model]
+        except KeyError:
+            pass
+
+        # Fetch from the database if the model's CustomFields have not been cached
+        content_type = ContentType.objects.get_for_model(model)
+        customfields = CustomField.objects.filter(obj_type=content_type)
+
+        # Cache the retrieved CustomFields
+        self._cache[model] = customfields
+
+        return customfields
+
+
 class CustomField(models.Model):
     obj_type = models.ManyToManyField(
         to=ContentType,
@@ -106,6 +134,8 @@ class CustomField(models.Model):
         help_text='Fields with higher weights appear lower in a form.'
     )
 
+    objects = CustomFieldManager()
+
     class Meta:
         ordering = ['weight', 'name']