Fixes: #17976 DeviceType_Count on Manufacturer

[renatoalmeidaoliveira]

Fixes: #17976 DeviceType_Count on Manufacturer

• Changed get_prefetches_for_serializer to support nested serializers
• For each nested field select all RelatedObjectCountField fields and create a Prefetch with the correct annotation

[jeremystretch]

@renatoalmeidaoliveira it would be ideal to solve this by enabling recursion through nested serializers inside get_annotations_for_serializer() if possible. This is because these annotations employ the count_related() utility function, which IIRC works a bit differently from the standard Count(). Did you try going down this path at all?

[renatoalmeidaoliveira]

@jeremystretch that was my first try, i.e. using get_annotations_for_serializer I tryed that in two ways the one proposed by @arthanson in the issue and another version I made.
In both ways the annotated attribute falls inside the base model instead of the nested one and even using the nottation {nested_model}__count_devices.
It's possible to reprodute that behaivour inside nbshell, then I tryed to work on get_prefetches_for_serializer, making it return the fields using Count.
Now I think it's possible to use count_related inside the get_prefetches_for_serializer, gonna try to implement that way.

[renatoalmeidaoliveira]

About using count_related I was able to use it inside the get_prefetches_for_serializer.
Looking at the query at Django Debug we have the following differences:

Using count_related

SELECT "dcim_manufacturer"."id",
       "dcim_manufacturer"."created",
       "dcim_manufacturer"."last_updated",
       "dcim_manufacturer"."custom_field_data",
       "dcim_manufacturer"."name",
       "dcim_manufacturer"."slug",
       "dcim_manufacturer"."description",
       COALESCE((SELECT COUNT(*) AS "c" FROM "dcim_manufacturer" U0 INNER JOIN "dcim_devicetype" U1 ON (U0."id" = U1."manufacturer_id") WHERE U1."id" = ("dcim_manufacturer"."id") GROUP BY U1."id"), 0) AS "devicetype_count"
  FROM "dcim_manufacturer"
 WHERE "dcim_manufacturer"."id" IN (3)

Performance using count_related

Using Count

SELECT "dcim_manufacturer"."id",
       "dcim_manufacturer"."created",
       "dcim_manufacturer"."last_updated",
       "dcim_manufacturer"."custom_field_data",
       "dcim_manufacturer"."name",
       "dcim_manufacturer"."slug",
       "dcim_manufacturer"."description",
       COUNT("dcim_devicetype"."id") AS "devicetype_count"
  FROM "dcim_manufacturer"
  LEFT OUTER JOIN "dcim_devicetype"
    ON ("dcim_manufacturer"."id" = "dcim_devicetype"."manufacturer_id")
 WHERE "dcim_manufacturer"."id" IN (3)
 GROUP BY "dcim_manufacturer"."id"

Performance using Count

Summary

In my environment, using NetBox demo data, the performance using Count was a bit better, but both ways works.
Please let me know the preffered way.

[renatoalmeidaoliveira]

About changing get_annotations_for_serializer, I tryed with this approach:

def get_annotations_for_serializer(serializer_class, fields_to_include=None):
   """
   Return a mapping of field names to annotations to be applied to the queryset for a serializer.
   """
   annotations = {}
   # If specific fields are not specified, default to all
   if not fields_to_include:
       fields_to_include = serializer_class.Meta.fields

   model = serializer_class.Meta.model

   for field_name, field in serializer_class._declared_fields.items():
       if issubclass(type(field), Serializer):
           nested_fields = field.Meta.brief_fields if field.nested else []
           for subfield_name, subfield in field.get_fields().items():
               if subfield_name in nested_fields and type(subfield) is RelatedObjectCountField:
                   subfield_model = field.Meta.model
                   annotations[f'{field_name}__{subfield_name}'] = count_related(subfield_model, subfield.relation)
       if field_name in fields_to_include and type(field) is RelatedObjectCountField:
           related_field = getattr(model, field.relation).field
           annotations[field_name] = count_related(related_field.model, related_field.name)
   return annotations

Running with this with Debug it executes the following query:

SELECT "dcim_devicetype"."id",
       "dcim_devicetype"."created",
       "dcim_devicetype"."last_updated",
       "dcim_devicetype"."custom_field_data",
       "dcim_devicetype"."description",
       "dcim_devicetype"."comments",
       "dcim_devicetype"."weight",
       "dcim_devicetype"."weight_unit",
       "dcim_devicetype"."_abs_weight",
       "dcim_devicetype"."manufacturer_id",
       "dcim_devicetype"."model",
       "dcim_devicetype"."slug",
       "dcim_devicetype"."default_platform_id",
       "dcim_devicetype"."part_number",
       "dcim_devicetype"."u_height",
       "dcim_devicetype"."exclude_from_utilization",
       "dcim_devicetype"."is_full_depth",
       "dcim_devicetype"."subdevice_role",
       "dcim_devicetype"."airflow",
       "dcim_devicetype"."front_image",
       "dcim_devicetype"."rear_image",
       "dcim_devicetype"."console_port_template_count",
       "dcim_devicetype"."console_server_port_template_count",
       "dcim_devicetype"."power_port_template_count",
       "dcim_devicetype"."power_outlet_template_count",
       "dcim_devicetype"."interface_template_count",
       "dcim_devicetype"."front_port_template_count",
       "dcim_devicetype"."rear_port_template_count",
       "dcim_devicetype"."device_bay_template_count",
       "dcim_devicetype"."module_bay_template_count",
       "dcim_devicetype"."inventory_item_template_count",
       COALESCE((SELECT COUNT(*) AS "c" FROM "dcim_manufacturer" U0 INNER JOIN "dcim_devicetype" U1 ON (U0."id" = U1."manufacturer_id") WHERE U1."id" = ("dcim_devicetype"."id") GROUP BY U1."id"), 0) AS "manufacturer__devicetype_count",
       COALESCE((SELECT COUNT(*) AS "c" FROM "dcim_platform" U0 INNER JOIN "dcim_device" U1 ON (U0."id" = U1."platform_id") WHERE U1."id" = ("dcim_devicetype"."id") GROUP BY U1."id"), 0) AS "default_platform__device_count",
       COALESCE((SELECT COUNT(*) AS "c" FROM "dcim_platform" U0 INNER JOIN "virtualization_virtualmachine" U1 ON (U0."id" = U1."platform_id") WHERE U1."id" = ("dcim_devicetype"."id") GROUP BY U1."id"), 0) AS "default_platform__virtualmachine_count",
       COALESCE((SELECT COUNT(*) AS "c" FROM "dcim_device" U0 WHERE U0."device_type_id" = ("dcim_devicetype"."id") GROUP BY U0."device_type_id"), 0) AS "device_count"
  FROM "dcim_devicetype"
 WHERE "dcim_devicetype"."id" = 7
 LIMIT 21

And looking at the results inside Django debug, its returning the correct results but as an attribute of DeviceType :

So even the query returning the expected results the JSON output omits the result instead of adding it inside the Nested Object