FIX: List parent/child tags correctly for categories restricted to tag groups (#23708)

Co-authored-by: Sérgio Saquetim <saquetim@discourse.org>

lib/discourse_tagging.rb

@@ -379,13 +379,13 @@ module DiscourseTagging
 
   CATEGORY_RESTRICTIONS_SQL ||= <<~SQL
     category_restrictions AS (
-      SELECT t.id as tag_id, ct.id as ct_id, ct.category_id as category_id
+      SELECT t.id as tag_id, ct.id as ct_id, ct.category_id as category_id, NULL AS category_tag_group_id
       FROM tags t
       INNER JOIN category_tags ct ON t.id = ct.tag_id /*and_name_like*/
 
       UNION
 
-      SELECT t.id as tag_id, ctg.id as ctg_id, ctg.category_id as category_id
+      SELECT t.id as tag_id, ctg.id as ctg_id, ctg.category_id as category_id, ctg.tag_group_id AS category_tag_group_id
       FROM tags t
       INNER JOIN tag_group_memberships tgm ON tgm.tag_id = t.id /*and_name_like*/
       INNER JOIN category_tag_groups ctg ON tgm.tag_group_id = ctg.tag_group_id
@@ -458,7 +458,7 @@ module DiscourseTagging
       FROM tags t
       INNER JOIN tag_group_restrictions tgr ON tgr.tag_id = t.id
       #{outer_join ? "LEFT OUTER" : "INNER"}
-        JOIN category_restrictions cr ON t.id = cr.tag_id
+        JOIN category_restrictions cr ON t.id = cr.tag_id AND (tgr.tag_group_id = cr.category_tag_group_id OR cr.category_tag_group_id IS NULL)
       /*where*/
       /*order_by*/
       /*limit*/
@@ -572,7 +572,7 @@ module DiscourseTagging
          WHERE tg.one_per_topic
       SQL
 
-      if !one_tag_per_group_ids.empty?
+      if one_tag_per_group_ids.present?
         builder.where(
           "t.id NOT IN (SELECT DISTINCT tag_id FROM tag_group_restrictions WHERE tag_group_id IN (?)) OR id IN (:selected_tag_ids)",
           one_tag_per_group_ids,

spec/lib/discourse_tagging_spec.rb

@@ -1482,6 +1482,52 @@ RSpec.describe DiscourseTagging do
     end
   end
 
+  describe "Tag in multiple tag groups" do
+    fab!(:parent) { Fabricate(:tag) }
+    fab!(:child) { Fabricate(:tag) }
+    fab!(:no_show_tag) { Fabricate(:tag) }
+
+    fab!(:no_show_tag_group) do
+      Fabricate(:tag_group, permissions: { "everyone" => 1 }, tag_names: [no_show_tag.name])
+    end
+
+    fab!(:child_tag_group) do
+      Fabricate(
+        :tag_group,
+        permissions: {
+          "everyone" => 1,
+        },
+        tag_names: [child.name, no_show_tag.name],
+        parent_tag_id: parent.id,
+      )
+    end
+
+    fab!(:parent_tag_group) do
+      Fabricate(:tag_group, permissions: { "everyone" => 1 }, tag_names: [parent.name])
+    end
+
+    fab!(:category) do
+      Fabricate(:category, allowed_tag_groups: [parent_tag_group.name, child_tag_group.name])
+    end
+
+    # this test is to make sure that the parent tag is the only one returned when the child tag is also in a tag group
+    # allowed in the category
+    it "Will only return the parent tag" do
+      tags =
+        DiscourseTagging.filter_allowed_tags(
+          Guardian.new(user),
+          selected_tags: nil,
+          for_input: true,
+          category: category,
+          term: "",
+        ).map(&:name)
+
+      expect(tags).to include(parent.name)
+      expect(tags).not_to include(child.name)
+      expect(tags).not_to include(no_show_tag.name)
+    end
+  end
+
   describe "staff_tag_names" do
     fab!(:tag) { Fabricate(:tag) }