✨(backend) search users

We need to search users by their email.
For that we will use the trigram similarity algorithm
provided by PostgreSQL. To use it we have to
activate the pg_trgm extension in postgres db.
To query the email we will use the query param
`q`.
We have another query param `document_id`, it is
necessary to exclude the users that have already
access to the document.

src/backend/core/api/viewsets.py

@@ -111,8 +111,7 @@ class Pagination(pagination.PageNumberPagination):
 
 
 class UserViewSet(
-    mixins.UpdateModelMixin,
-    viewsets.GenericViewSet,
+    mixins.UpdateModelMixin, viewsets.GenericViewSet, mixins.ListModelMixin
 ):
     """User ViewSet"""
 
@@ -120,6 +119,26 @@ class UserViewSet(
     queryset = models.User.objects.all()
     serializer_class = serializers.UserSerializer
 
+    def get_queryset(self):
+        """
+        Limit listed users by querying the email field with a trigram similarity
+        search if a query is provided.
+        Limit listed users by excluding users already in the document if a document_id
+        is provided.
+        """
+        queryset = self.queryset
+
+        if self.action == "list":
+            # Exclude all users already in the given document
+            if document_id := self.request.GET.get("document_id", ""):
+                queryset = queryset.exclude(documentaccess__document_id=document_id)
+
+            # Filter users by email similarity
+            if query := self.request.GET.get("q", ""):
+                queryset = queryset.filter(email__trigram_word_similar=query)
+
+        return queryset
+
     @decorators.action(
         detail=False,
         methods=["get"],