Update tag viewing to cover "ends with" as well

Previously, topic lists filtered down to a tag were using "descendant
of", so for example a listing based around the "ask" tag would show
topics that have the exact tag "ask" as well as any sub-tags, such as
"ask.survey". However, it wouldn't show a hypothetical "example.ask"
tag, where the "ask" was at the end.

This commit updates the method used to utilize lqueries to search for
tags that either start or end with the specified one (which includes
exact full matches as well).

tildes/tildes/lib/database.py

@@ -14,6 +14,7 @@ from sqlalchemy.engine.interfaces import Dialect
 from sqlalchemy.orm.session import Session
 from sqlalchemy.types import UserDefinedType
 from sqlalchemy_utils import LtreeType
+from sqlalchemy_utils.types.ltree import LQUERY
 
 
 # https://www.postgresql.org/docs/current/static/errcodes-appendix.html
@@ -128,8 +129,8 @@ class ArrayOfLtree(ARRAY):  # pylint: disable=too-many-ancestors
     class comparator_factory(ARRAY.comparator_factory):
         """Add custom comparison functions.
 
-        The ancestor_of and descendant_of functions are supported by LtreeType, so this
-        duplicates them here so they can be used on ArrayOfLtree too.
+        The ancestor_of, descendant_of, and lquery functions are supported by LtreeType,
+        so this duplicates them here so they can be used on ArrayOfLtree too.
         """
 
         def ancestor_of(self, other):  # type: ignore
@@ -139,3 +140,10 @@ class ArrayOfLtree(ARRAY):  # pylint: disable=too-many-ancestors
         def descendant_of(self, other):  # type: ignore
             """Return whether the array contains any descendant of `other`."""
             return self.op("<@")(other)
+
+        def lquery(self, other):  # type: ignore
+            """Return whether the array matches the lquery/lqueries in `other`."""
+            if isinstance(other, list):
+                return self.op("?")(cast(other, ARRAY(LQUERY)))
+            else:
+                return self.op("~")(other)

tildes/tildes/models/topic/topic_query.py

@@ -134,14 +134,15 @@ class TopicQuery(PaginatedQuery):
         return self.filter(Topic.created_time > start_time)
 
     def has_tag(self, tag: Ltree) -> "TopicQuery":
-        """Restrict the topics to ones with a specific tag (generative)."""
-        # casting tag to string really shouldn't be necessary, but some kind of strange
-        # interaction seems to be happening with the ArrayOfLtree class, this will need
-        # some investigation
-        tag = str(tag)
+        """Restrict the topics to ones with a specific tag (generative).
+
+        Note that this method searches for topics that have any tag that either starts
+        or ends with the specified tag, not only exact/full matches.
+        """
+        queries = [f"{tag}.*", f"*.{tag}"]
 
         # pylint: disable=protected-access
-        return self.filter(Topic._tags.descendant_of(tag))  # type: ignore
+        return self.filter(Topic._tags.lquery(queries))  # type: ignore
 
     def search(self, query: str) -> "TopicQuery":
         """Restrict the topics to ones that match a search query (generative)."""