FEATURE: add "assigned:<name>" filter (in the /filter page)

The "assigned:<name>" filter was already available in the advanced
search but it wasn't in the /filter page.

This commit adds that filter allowing anyone to filter topics assigned
to either a specific user (using their username) or to a specific group
(using its name).

plugin.rb

@@ -887,39 +887,49 @@ after_initialize do
     Assignment.active_for_group(group).destroy_all
   end
 
-  register_search_advanced_filter(/in:assigned/) do |posts|
+  add_filter_custom_filter("assigned") do |scope, filter_values|
-    posts.where(<<~SQL) if @guardian.can_assign?
+    user_or_group_name = filter_values.compact.first
-        topics.id IN (
+
-          SELECT a.topic_id FROM assignments a WHERE a.active
+    return scope if user_or_group_name.blank?
-        )
+    
+    if user_id = User.find_by_username(user_or_group_name)&.id
+      scope.where(<<~SQL, user_id)
+        topics.id IN (SELECT a.topic_id FROM assignments a WHERE a.assigned_to_id = ? AND a.assigned_to_type = 'User' AND a.active)
       SQL
+    elsif group_id = Group.find_by(name: user_or_group_name)&.id
+      scope.where(<<~SQL, group_id)
+        topics.id IN (SELECT a.topic_id FROM assignments a WHERE a.assigned_to_id = ? AND a.assigned_to_type = 'Group' AND a.active)
+      SQL
+    else
+      scope
+    end
+  end
+
+  register_search_advanced_filter(/in:assigned/) do |posts|
+    return if !@guardian.can_assign?
+
+    posts.where("topics.id IN (SELECT a.topic_id FROM assignments a WHERE a.active)")
   end
 
   register_search_advanced_filter(/in:unassigned/) do |posts|
-    posts.where(<<~SQL) if @guardian.can_assign?
+    return if !@guardian.can_assign?
-        topics.id NOT IN (
+    
-          SELECT a.topic_id FROM assignments a WHERE a.active
+    posts.where("topics.id NOT IN (SELECT a.topic_id FROM assignments a WHERE a.active)")
-        )
-      SQL
   end
 
   register_search_advanced_filter(/assigned:(.+)$/) do |posts, match|
-    if @guardian.can_assign?
+    return if !@guardian.can_assign? || match.blank?
+
     if user_id = User.find_by_username(match)&.id
       posts.where(<<~SQL, user_id)
-          topics.id IN (
+        topics.id IN (SELECT a.topic_id FROM assignments a WHERE a.assigned_to_id = ? AND a.assigned_to_type = 'User' AND a.active)
-            SELECT a.topic_id FROM assignments a WHERE a.assigned_to_id = ? AND a.assigned_to_type = 'User' AND a.active
-          )
       SQL
-      elsif group_id = Group.find_by_name(match)&.id
+    elsif group_id = Group.find_by(name: match)&.id
       posts.where(<<~SQL, group_id)
-          topics.id IN (
+        topics.id IN (SELECT a.topic_id FROM assignments a WHERE a.assigned_to_id = ? AND a.assigned_to_type = 'Group' AND a.active)
-            SELECT a.topic_id FROM assignments a WHERE a.assigned_to_id = ? AND a.assigned_to_type = 'Group' AND a.active
-          )
       SQL
     end
   end
-  end
 
   if defined?(DiscourseAutomation)
     add_automation_scriptable("random_assign") do

spec/requests/list_controller_spec.rb

@@ -355,4 +355,42 @@ describe ListController do
       expect(JSON.parse(response.body)["topic_list"]["topics"].map { |t| t["id"] }).to eq([pm.id])
     end
   end
+
+  describe "#filter" do
+    include_context "with group that is allowed to assign"
+
+    fab!(:group) { Fabricate(:group, assignable_level: Group::ALIAS_LEVELS[:mods_and_admins]) }
+
+    fab!(:topic_1) { Fabricate(:topic) }
+    fab!(:topic_2) { Fabricate(:topic) }
+    fab!(:topic_3) { Fabricate(:topic) }
+
+    fab!(:post_1) { Fabricate(:post, topic: topic_1) }
+    fab!(:post_2) { Fabricate(:post, topic: topic_2) }
+    fab!(:post_3) { Fabricate(:post, topic: topic_3) }
+
+    before do
+      sign_in(admin)
+    end
+
+    it "filters topics by assigned user" do
+      add_to_assign_allowed_group(user)
+
+      Assigner.new(topic_1, admin).assign(user)
+
+      get "/filter", params: { q: "assigned:#{user.username_lower}", format: :json }
+
+      expect(response.status).to eq(200)
+      expect(response.parsed_body.dig("topic_list", "topics").map { _1["id"] }).to contain_exactly(topic_1.id)
+    end
+
+    it "filters topics by assigned group" do
+      Assigner.new(topic_2, admin).assign(group)
+
+      get "/filter", params: { q: "assigned:#{group.name}", format: :json }
+
+      expect(response.status).to eq(200)
+      expect(response.parsed_body.dig("topic_list", "topics").map { _1["id"] }).to contain_exactly(topic_2.id)
+    end
+  end
 end