Introduce admin filter by changed field

src/auditlog/admin.py

@@ -6,7 +6,7 @@ from django.utils.functional import cached_property
 
 from .models import LogEntry
 from .mixins import LogEntryAdminMixin
-from .filters import ResourceTypeFilter
+from .filters import ResourceTypeFilter, FieldFilter
 
 
 class TimeLimitedPaginator(Paginator):
@@ -30,7 +30,7 @@ class TimeLimitedPaginator(Paginator):
 class LogEntryAdmin(admin.ModelAdmin, LogEntryAdminMixin):
     list_display = ['created', 'resource_url', 'action', 'msg_short', 'user_url']
     search_fields = ['timestamp', 'object_repr', 'changes', 'actor__first_name', 'actor__last_name']
-    list_filter = ['action', ResourceTypeFilter]
+    list_filter = ['action', ResourceTypeFilter, FieldFilter]
     readonly_fields = ['created', 'resource_url', 'action', 'user_url', 'msg']
     fieldsets = [
         (None, {'fields': ['created', 'user_url', 'resource_url']}),

src/auditlog/filters.py

@@ -1,7 +1,9 @@
 from django.contrib.admin import SimpleListFilter
 from django.contrib.contenttypes.models import ContentType
+from django.contrib.postgres.fields import JSONField
+from django.db import connection
 from django.db.models import Value
-from django.db.models.functions import Concat
+from django.db.models.functions import Concat, Cast
 
 from auditlog.registry import auditlog
 
@@ -27,3 +29,38 @@ class ResourceTypeFilter(SimpleListFilter):
         if self.value() is None:
             return queryset
         return queryset.filter(content_type_id=self.value())
+
+
+class FieldFilter(SimpleListFilter):
+    title = 'Field'
+    parameter_name = 'field'
+    parent = ResourceTypeFilter
+
+    def __init__(self, request, *args, **kwargs):
+        self.target_model = self._get_target_model(request)
+        super().__init__(request, *args, **kwargs)
+
+    def _get_target_model(self, request):
+        # the parameters consumed by previous filters aren't passed to subsequent filters,
+        # so we have to look into the request parameters explicitly
+        content_type_id = request.GET.get(self.parent.parameter_name)
+        if not content_type_id:
+            return None
+
+        return ContentType.objects.get(id=content_type_id).model_class()
+
+    def lookups(self, request, model_admin):
+        if connection.vendor != 'postgresql':
+            # filtering inside JSON is PostgreSQL-specific for now
+            return []
+        if not self.target_model:
+            return []
+        return [(field.name, field.name) for field in self.target_model._meta.fields]
+
+    def queryset(self, request, queryset):
+        if self.value() is None:
+            return queryset
+        return (
+            queryset.annotate(changes_json=Cast("changes", JSONField()))
+            .filter(**{'changes_json__{}__isnull'.format(self.value()): False})
+        )