✨(backend) add "tree" action on document API endpoint

We want to display the tree structure to which a document belongs
on the left side panel of its detail view. For this, we need an
endpoint to retrieve the list view of the document's ancestors
opened.

By opened, we mean that when display the document, we also need to
display its siblings. When displaying the parent of the current
document, we also need to display the siblings of the parent...

src/backend/core/api/serializers.py

@@ -128,26 +128,13 @@ class TemplateAccessSerializer(BaseAccessSerializer):
         read_only_fields = ["id", "abilities"]
 
 
-class BaseResourceSerializer(serializers.ModelSerializer):
-    """Serialize documents."""
-
-    abilities = serializers.SerializerMethodField(read_only=True)
-    accesses = TemplateAccessSerializer(many=True, read_only=True)
-
-    def get_abilities(self, document) -> dict:
-        """Return abilities of the logged-in user on the instance."""
-        request = self.context.get("request")
-        if request:
-            return document.get_abilities(request.user)
-        return {}
-
-
-class ListDocumentSerializer(BaseResourceSerializer):
+class ListDocumentSerializer(serializers.ModelSerializer):
     """Serialize documents with limited fields for display in lists."""
 
     is_favorite = serializers.BooleanField(read_only=True)
     nb_accesses = serializers.IntegerField(read_only=True)
     user_roles = serializers.SerializerMethodField(read_only=True)
+    abilities = serializers.SerializerMethodField(read_only=True)
 
     class Meta:
         model = models.Document
@@ -185,6 +172,18 @@ class ListDocumentSerializer(BaseResourceSerializer):
             "user_roles",
         ]
 
+    def get_abilities(self, document) -> dict:
+        """Return abilities of the logged-in user on the instance."""
+        request = self.context.get("request")
+
+        if request:
+            paths_links_mapping = self.context.get("paths_links_mapping", None)
+            return document.get_abilities(
+                request.user, paths_links_mapping=paths_links_mapping
+            )
+
+        return {}
+
     def get_user_roles(self, document):
         """
         Return roles of the logged-in user for the current document,
@@ -359,7 +358,7 @@ class ServerCreateDocumentSerializer(serializers.Serializer):
         raise NotImplementedError("Update is not supported for this serializer.")
 
 
-class LinkDocumentSerializer(BaseResourceSerializer):
+class LinkDocumentSerializer(serializers.ModelSerializer):
     """
     Serialize link configuration for documents.
     We expose it separately from document in order to simplify and secure access control.
@@ -431,9 +430,12 @@ class FileUploadSerializer(serializers.Serializer):
         return attrs
 
 
-class TemplateSerializer(BaseResourceSerializer):
+class TemplateSerializer(serializers.ModelSerializer):
     """Serialize templates."""
 
+    abilities = serializers.SerializerMethodField(read_only=True)
+    accesses = TemplateAccessSerializer(many=True, read_only=True)
+
     class Meta:
         model = models.Template
         fields = [
@@ -447,6 +449,13 @@ class TemplateSerializer(BaseResourceSerializer):
         ]
         read_only_fields = ["id", "accesses", "abilities"]
 
+    def get_abilities(self, document) -> dict:
+        """Return abilities of the logged-in user on the instance."""
+        request = self.context.get("request")
+        if request:
+            return document.get_abilities(request.user)
+        return {}
+
 
 # pylint: disable=abstract-method
 class DocumentGenerationSerializer(serializers.Serializer):

src/backend/core/api/utils.py

@@ -11,6 +11,35 @@ import botocore
 from rest_framework.throttling import BaseThrottle
 
 
+def nest_tree(flat_list, steplen):
+    """
+    Convert a flat list of serialized documents into a nested tree making advantage
+    of the`path` field and its step length.
+    """
+    node_dict = {}
+    roots = []
+
+    # Sort the flat list by path to ensure parent nodes are processed first
+    flat_list.sort(key=lambda x: x["path"])
+
+    for node in flat_list:
+        node["children"] = []  # Initialize children list
+        node_dict[node["path"]] = node
+
+        # Determine parent path
+        parent_path = node["path"][:-steplen]
+
+        if parent_path in node_dict:
+            node_dict[parent_path]["children"].append(node)
+        else:
+            roots.append(node)  # Collect root nodes
+
+    if len(roots) > 1:
+        raise ValueError("More than one root element detected.")
+
+    return roots[0] if roots else None
+
+
 def filter_root_paths(paths, skip_sorting=False):
     """
     Filters root paths from a list of paths representing a tree structure.

src/backend/core/api/viewsets.py

@@ -4,6 +4,7 @@
 import logging
 import re
 import uuid
+from collections import defaultdict
 from urllib.parse import urlparse
 
 from django.conf import settings
@@ -424,10 +425,11 @@ class DocumentViewSet(
     ]
     queryset = models.Document.objects.all()
     serializer_class = serializers.DocumentSerializer
+    ai_translate_serializer_class = serializers.AITranslateSerializer
+    children_serializer_class = serializers.ListDocumentSerializer
     list_serializer_class = serializers.ListDocumentSerializer
     trashbin_serializer_class = serializers.ListDocumentSerializer
-    children_serializer_class = serializers.ListDocumentSerializer
-    ai_translate_serializer_class = serializers.AITranslateSerializer
+    tree_serializer_class = serializers.ListDocumentSerializer
 
     def annotate_is_favorite(self, queryset):
         """
@@ -523,7 +525,7 @@ class DocumentViewSet(
             queryset = queryset.filter(path__in=root_paths)
 
             # Annotate the queryset with an attribute marking instances as highest ancestor
-            # in order to save some time while computing abilities in the instance
+            # in order to save some time while computing abilities on the instance
             queryset = queryset.annotate(
                 is_highest_ancestor_for_user=db.Value(
                     True, output_field=db.BooleanField()
@@ -767,6 +769,68 @@ class DocumentViewSet(
         queryset = self.annotate_user_roles(queryset)
         return self.get_response_for_queryset(queryset)
 
+    @drf.decorators.action(
+        detail=True,
+        methods=["get"],
+        ordering=["path"],
+    )
+    def tree(self, request, pk, *args, **kwargs):
+        """
+        List ancestors tree above the document.
+        What we need to display is the tree structure opened for the current document.
+        """
+        try:
+            current_document = self.queryset.only("depth", "path").get(pk=pk)
+        except models.Document.DoesNotExist as excpt:
+            raise drf.exceptions.NotFound from excpt
+
+        ancestors = (
+            (current_document.get_ancestors() | self.queryset.filter(pk=pk))
+            .filter(ancestors_deleted_at__isnull=True)
+            .order_by("path")
+        )
+
+        # Get the highest readable ancestor
+        highest_readable = ancestors.readable_per_se(request.user).only("depth").first()
+        if highest_readable is None:
+            raise (
+                drf.exceptions.PermissionDenied()
+                if request.user.is_authenticated
+                else drf.exceptions.NotAuthenticated()
+            )
+
+        ancestors_links_definitions = defaultdict(set)
+        children_clause = db.Q()
+        for ancestor in ancestors:
+            if ancestor.depth < highest_readable.depth:
+                continue
+
+            ancestors_links_definitions[ancestor.link_reach].add(ancestor.link_role)
+            children_clause |= db.Q(
+                path__startswith=ancestor.path, depth=ancestor.depth + 1
+            )
+
+        children = self.queryset.filter(children_clause, deleted_at__isnull=True)
+
+        queryset = ancestors.filter(depth__gte=highest_readable.depth) | children
+        queryset = queryset.order_by("path")
+        queryset = self.annotate_user_roles(queryset)
+        queryset = self.annotate_is_favorite(queryset)
+
+        # Pass ancestors' links definitions to the serializer as a context variable
+        # in order to allow saving time while computing abilities on the instance
+        serializer = self.get_serializer(
+            queryset,
+            many=True,
+            context={
+                "request": request,
+                "ancestors_links_definitions": ancestors_links_definitions,
+            },
+        )
+        return drf.response.Response(
+            utils.nest_tree(serializer.data, self.queryset.model.steplen)
+        )
+
     @drf.decorators.action(detail=True, methods=["get"], url_path="versions")
     def versions_list(self, request, *args, **kwargs):
         """