[#5199] Optimize /project/children endpoint

# Counting children

This was slow because we made a query for each project.
Now we get the children and grand children.
A simple dict with each project and a list of their children is then built in python.

**Side effect**

Paging doesn't work anymore as the queryset includes grand children.
Reimplementing it can be done another time if we really do have projects with enormous hierarchies.

# Parent retrieval

We already know who the parent is since we're getting its children --> no need to make another query.

# Locations

We weren't retrieving related objects, which we now do.
Additionally, we iterated over the list of countries twice when one iteration was enough.

akvo/rest/serializers/project.py

@@ -1,12 +1,12 @@
 # -*- coding: utf-8 -*-
-
+import functools
 # Akvo RSR is covered by the GNU Affero General Public License.
 # See more details in the license.txt file located at the root folder of the Akvo RSR module.
 # For additional details on the GNU license please see < http://www.gnu.org/licenses/agpl.html >.
 
 from datetime import timedelta
 import logging
-from typing import List
+from typing import List, Optional
 
 from django.conf import settings
 from django.utils.timezone import now
@@ -358,26 +358,44 @@ class ProjectMetadataSerializer(BaseRSRSerializer):
     children_count = serializers.SerializerMethodField()
 
     def get_children_count(self, obj):
-        return obj.children().count()
+        if parents_to_children := self.context.get("parents_to_children"):
+            return len(parents_to_children.get(obj.uuid, []))
+        else:
+            return obj.children().count()
 
     def get_locations(self, obj):
-        countries = {location.country for location in obj.locations.all() if location.country}
-        return [
-            {'country': c.name, 'iso_code': c.iso_code}
-            for c
-            in countries
-        ]
+        countries = set()
+        results = []
+        for location in obj.locations.all():
+            country = location.country
+            if not country or country in countries:
+                continue
+            countries.add(country)
+            results.append({
+                "country": country.name,
+                "iso_code": country.iso_code,
+            })
+        return results
 
     def get_parent(self, obj):
-        p = obj.parent()
-        user = self.context['request'].user
-        if not user.can_view_project(p):
-            return None
-        return (
-            {'id': p.id, 'title': p.title, 'is_lead': p.is_hierarchy_root()}
-            if p is not None
-            else None
-        )
+        if "parent" in self.context:
+            p = self.context.get("parent")
+        else:
+            p = obj.parent()
+            user = self.context['request'].user
+            if not user.can_view_project(p):
+                return None
+        return self._parent_to_dict(p)
+
+    @staticmethod
+    @functools.lru_cache
+    def _parent_to_dict(parent: Project) -> Optional[dict]:
+        if parent is not None:
+            return {
+                'id': parent.id,
+                'title': parent.title,
+                'is_lead': parent.is_hierarchy_root()
+            }
 
     def get_editable(self, obj):
         """Method used by the editable SerializerMethodField"""

akvo/rest/views/project.py

@@ -6,6 +6,8 @@
 """
 
 from datetime import timedelta
+from typing import Dict, List
+from uuid import UUID
 
 from django.conf import settings
 from django.db.models import Count, Q
@@ -104,22 +106,39 @@ def external_project(self, request, **kwargs):
     def children(self, request, **kwargs):
         project = self.get_object()
 
-        queryset = self._filter_queryset(project.children().select_related(
+        queryset = self._filter_queryset(project.descendants(max_depth=2, with_self=False).select_related(
             "primary_organisation",
+            "projecthierarchy",
+            "publishingstatus",
         ).prefetch_related(
-            "locations",
+            "locations__country",
             "recipient_countries",
             "sectors",
+            "projectrole_set",
         ))
-        page = self.paginate_queryset(queryset)
-
+        descendants = list(queryset)
         serializer_context = self.get_serializer_context()
 
-        if page is not None:
-            serializer = ProjectMetadataSerializer(page, many=True, context=serializer_context)
-            return self.get_paginated_response(serializer.data)
+        # Optimization to count the children
+        # Collect the children of each project in a list
+        # The alternative is a subquery, but that's extra slow due to lacking psql indices
+        parent_to_children: Dict[UUID, List[Project]] = dict()
+        children: List[Project] = []
+        parent_uuid = project.uuid
+        for descendant in descendants:
+            if not (curr_parent_uuid := descendant.get_parent_uuid()):
+                continue
+            parent_to_children.setdefault(curr_parent_uuid, []).append(descendant)
+
+            if curr_parent_uuid == parent_uuid:
+                children.append(descendant)
+
+        serializer_context["parents_to_children"] = parent_to_children
+
+        # optimization for ProjectMetadataSerializer.get_parent
+        serializer_context["parent"] = project
 
-        serializer = ProjectMetadataSerializer(queryset, many=True, context=serializer_context).data
+        serializer = ProjectMetadataSerializer(children, many=True, context=serializer_context)
         return Response(serializer.data)
 
     @action(

akvo/rsr/spa/app/modules/hierarchy/services.js

@@ -1,13 +1,9 @@
 import api from '../../utils/api'
 import { getProjectUuids } from '../../utils/misc'
 
-export const getChildrenApi = async (id, page = 1) => {
-  const response = await api.get(`/project/${id}/children/?format=json&page=${page}`)
-  const { results, next } = response.data
-  if (next) {
-    return results?.concat(await getChildrenApi(id, page + 1))
-  }
-  return results
+export const getChildrenApi = async (id) => {
+  const response = await api.get(`/project/${id}/children/?format=json`)
+  return response.data
 }
 
 export const getProgramApi = (id, successCallback, errorCallback) => {