diff --git a/lib/discourse_tagging.rb b/lib/discourse_tagging.rb
index 1b26986707c..ffd8b143e54 100644
--- a/lib/discourse_tagging.rb
+++ b/lib/discourse_tagging.rb
@@ -4,6 +4,13 @@ module DiscourseTagging
   TAGS_FILTER_REGEXP = /[\/\?#\[\]@!\$&'\(\)\*\+,;=\.%\\`^\s|\{\}"<>]+/ # /?#[]@!$&'()*+,;=.%\`^|{}"<>
   TAGS_STAFF_CACHE_KEY = "staff_tag_names"
 
+  TAG_GROUP_TAG_IDS_SQL = <<-SQL
+      SELECT tag_id
+        FROM tag_group_memberships tgm
+  INNER JOIN tag_groups tg
+          ON tgm.tag_group_id = tg.id
+  SQL
+
   def self.tag_topic_by_names(topic, guardian, tag_names_arg, append: false)
     if guardian.can_tag?(topic)
       tag_names = DiscourseTagging.tags_for_saving(tag_names_arg, guardian) || []
@@ -55,6 +62,19 @@ module DiscourseTagging
           end
         end
 
+        # add missing mandatory parent tags
+        parent_tags = TagGroup.includes(:parent_tag).where("tag_groups.id IN (
+          SELECT tg.id
+            FROM tag_groups tg
+      INNER JOIN tag_group_memberships tgm
+              ON tgm.tag_group_id = tg.id
+           WHERE tg.parent_tag_id IS NOT NULL
+             AND tgm.tag_id IN (?))", tags.map(&:id)).map(&:parent_tag)
+
+        parent_tags.reject { |t| tag_names.include?(t.name) }.each do |tag|
+          tags << tag
+        end
+
         # validate minimum required tags for a category
         if !guardian.is_staff? && category && category.minimum_required_tags > 0 && tags.length < category.minimum_required_tags
           topic.errors[:base] << I18n.t("tags.minimum_required_tags", count: category.minimum_required_tags)
@@ -147,28 +167,19 @@ module DiscourseTagging
         all_staff_tags = DiscourseTagging.staff_tag_names
         query = query.where('tags.name NOT IN (?)', all_staff_tags) if all_staff_tags.present?
       end
+    end
 
+    if opts[:for_input]
       # exclude tag groups that have a parent tag which is missing from selected_tags
 
-      select_sql = <<-SQL
-      SELECT tag_id
-            FROM tag_group_memberships tgm
-      INNER JOIN tag_groups tg
-              ON tgm.tag_group_id = tg.id
-      SQL
-
       if selected_tag_ids.empty?
-        sql = "tags.id NOT IN (#{select_sql} WHERE tg.parent_tag_id IS NOT NULL)"
+        sql = "tags.id NOT IN (#{TAG_GROUP_TAG_IDS_SQL} WHERE tg.parent_tag_id IS NOT NULL)"
         query = query.where(sql)
       else
-        # One tag per group restriction
-        exclude_group_ids = TagGroup.where(one_per_topic: true)
-          .joins(:tag_group_memberships)
-          .where('tag_group_memberships.tag_id in (?)', selected_tag_ids)
-          .pluck(:id)
+        exclude_group_ids = one_per_topic_group_ids(selected_tag_ids)
 
         if exclude_group_ids.empty?
-          sql = "tags.id NOT IN (#{select_sql} WHERE tg.parent_tag_id NOT IN (?))"
+          sql = "tags.id NOT IN (#{TAG_GROUP_TAG_IDS_SQL} WHERE tg.parent_tag_id NOT IN (?))"
           query = query.where(sql, selected_tag_ids)
         else
           # It's possible that the selected tags violate some one-tag-per-group restrictions,
@@ -177,10 +188,22 @@ module DiscourseTagging
             .where(tag_id: selected_tag_ids)
             .where(tag_group_id: exclude_group_ids)
             .map(&:tag_id)
-          sql = "(tags.id NOT IN (#{select_sql} WHERE (tg.parent_tag_id NOT IN (?) OR tg.id in (?))) OR tags.id IN (?))"
+          sql = "(tags.id NOT IN (#{TAG_GROUP_TAG_IDS_SQL} WHERE (tg.parent_tag_id NOT IN (?) OR tg.id in (?))) OR tags.id IN (?))"
           query = query.where(sql, selected_tag_ids, exclude_group_ids, limit_tag_ids)
         end
       end
+    elsif opts[:for_topic] && !selected_tag_ids.empty?
+      # One tag per group restriction
+      exclude_group_ids = one_per_topic_group_ids(selected_tag_ids)
+
+      unless exclude_group_ids.empty?
+        limit_tag_ids = TagGroupMembership.select('distinct on (tag_group_id) tag_id')
+          .where(tag_id: selected_tag_ids)
+          .where(tag_group_id: exclude_group_ids)
+          .map(&:tag_id)
+        sql = "(tags.id NOT IN (#{TAG_GROUP_TAG_IDS_SQL} WHERE (tg.id in (?))) OR tags.id IN (?))"
+        query = query.where(sql, exclude_group_ids, limit_tag_ids)
+      end
     end
 
     if guardian.nil? || guardian.is_staff?
@@ -190,6 +213,13 @@ module DiscourseTagging
     end
   end
 
+  def self.one_per_topic_group_ids(selected_tag_ids)
+    TagGroup.where(one_per_topic: true)
+      .joins(:tag_group_memberships)
+      .where('tag_group_memberships.tag_id in (?)', selected_tag_ids)
+      .pluck(:id)
+  end
+
   def self.filter_visible(query, guardian = nil)
     guardian&.is_staff? ? query : query.where("tags.id NOT IN (#{hidden_tags_query.select(:id).to_sql})")
   end
diff --git a/spec/components/discourse_tagging_spec.rb b/spec/components/discourse_tagging_spec.rb
index c9ee958a709..52b52fd95b3 100644
--- a/spec/components/discourse_tagging_spec.rb
+++ b/spec/components/discourse_tagging_spec.rb
@@ -158,6 +158,39 @@ describe DiscourseTagging do
         expect(topic.reload.tags).to eq([hidden_tag])
       end
     end
+
+    context 'tag group with parent tag' do
+      let(:topic) { Fabricate(:topic, user: user) }
+      let(:post) { Fabricate(:post, user: user, topic: topic, post_number: 1) }
+
+      before do
+        tag_group = Fabricate(:tag_group, parent_tag_id: tag1.id)
+        tag_group.tags = [tag3]
+      end
+
+      it "can tag with parent" do
+        valid = DiscourseTagging.tag_topic_by_names(topic, Guardian.new(user), [tag1.name])
+        expect(valid).to eq(true)
+        expect(topic.reload.tags.map(&:name)).to eq([tag1.name])
+      end
+
+      it "can tag with parent and a tag" do
+        valid = DiscourseTagging.tag_topic_by_names(topic, Guardian.new(user), [tag1.name, tag3.name])
+        expect(valid).to eq(true)
+        expect(topic.reload.tags.map(&:name)).to contain_exactly(*[tag1, tag3].map(&:name))
+      end
+
+      it "adds all parent tags that are missing" do
+        parent_tag = Fabricate(:tag, name: 'parent')
+        tag_group2 = Fabricate(:tag_group, parent_tag_id: parent_tag.id)
+        tag_group2.tags = [tag2]
+        valid = DiscourseTagging.tag_topic_by_names(topic, Guardian.new(user), [tag3.name, tag2.name])
+        expect(valid).to eq(true)
+        expect(topic.reload.tags.map(&:name)).to contain_exactly(
+          *[tag1, tag2, tag3, parent_tag].map(&:name)
+        )
+      end
+    end
   end
 
   describe '#tags_for_saving' do
diff --git a/spec/integration/category_tag_spec.rb b/spec/integration/category_tag_spec.rb
index 80f76705e64..e855786a6cf 100644
--- a/spec/integration/category_tag_spec.rb
+++ b/spec/integration/category_tag_spec.rb
@@ -196,7 +196,7 @@ describe "category tag restrictions" do
   end
 
   context "tag groups with parent tag" do
-    it "filter_allowed_tags returns results based on whether parent tag is present or not" do
+    it "for input field, filter_allowed_tags returns results based on whether parent tag is present or not" do
       tag_group = Fabricate(:tag_group, parent_tag_id: tag1.id)
       tag_group.tags = [tag3, tag4]
       expect(filter_allowed_tags(for_input: true)).to contain_exactly(tag1, tag2)
@@ -204,6 +204,14 @@ describe "category tag restrictions" do
       expect(filter_allowed_tags(for_input: true, selected_tags: [tag1.name, tag3.name])).to contain_exactly(tag2, tag4)
     end
 
+    it "for tagging a topic, filter_allowed_tags allows tags without parent tag" do
+      tag_group = Fabricate(:tag_group, parent_tag_id: tag1.id)
+      tag_group.tags = [tag3, tag4]
+      expect(filter_allowed_tags(for_topic: true)).to contain_exactly(tag1, tag2, tag3, tag4)
+      expect(filter_allowed_tags(for_topic: true, selected_tags: [tag1.name])).to contain_exactly(tag1, tag2, tag3, tag4)
+      expect(filter_allowed_tags(for_topic: true, selected_tags: [tag1.name, tag3.name])).to contain_exactly(tag1, tag2, tag3, tag4)
+    end
+
     context "and category restrictions" do
       let(:car_category)    { Fabricate(:category) }
       let(:other_category)  { Fabricate(:category) }