DEV: Update change status on solve implementation

Update tests to verify that the change status on solve feature is working as expected.
Change the implementation to loop throught the topic assignments and update the status.

plugin.rb

@@ -591,19 +591,47 @@ after_initialize do
   if defined?(DiscourseAssign)
     on(:accepted_solution) do |post|
       next if SiteSetting.assignment_status_on_solve.blank?
-      assigned_user = User.find_by(id: post.topic.assignment.assigned_to_id)
-      Assigner.new(post.topic, assigned_user).assign(
-        assigned_user,
-        status: SiteSetting.assignment_status_on_solve,
-      )
+      assignements = Assignment.where(topic: post.topic)
+      assignements.each do |assignment|
+        assigned_user = User.find_by(id: assignment.assigned_to_id)
+        target_id = assignment.target_id
+
+        target =
+          case assignment.target_type
+          when "Post"
+            Post.find_by(id: target_id)
+          when "Topic"
+            Topic.find_by(id: target_id)
+          else
+            post.topic
+          end
+
+        Assigner.new(target, assigned_user).assign(
+          assigned_user,
+          status: SiteSetting.assignment_status_on_solve,
+        )
+      end
     end
     on(:unaccepted_solution) do |post|
       next if SiteSetting.assignment_status_on_unsolve.blank?
-      assigned_user = User.find_by(id: post.topic.assignment.assigned_to_id)
-      Assigner.new(post.topic, assigned_user).assign(
-        assigned_user,
-        status: SiteSetting.assignment_status_on_unsolve,
-      )
+      assignements = Assignment.where(topic: post.topic)
+      assignements.each do |assignment|
+        assigned_user = User.find_by(id: assignment.assigned_to_id)
+        target_id = assignment.target_id
+        target =
+          case assignment.target_type
+          when "Post"
+            Post.find_by(id: target_id)
+          when "Topic"
+            Topic.find_by(id: target_id)
+          else
+            post.topic
+          end
+        Assigner.new(target, assigned_user).assign(
+          assigned_user,
+          status: SiteSetting.assignment_status_on_unsolve,
+        )
+      end
     end
   end
 end

spec/integration/solved_spec.rb

@@ -466,7 +466,7 @@ RSpec.describe "Managing Posts solved status" do
 
         DiscourseSolved.accept_answer!(p1, user)
 
-        expect(p1.topic.assignment.status).to eq("Done")
+        expect(p1.reload.topic.assignment.reload.status).to eq("Done")
 
         DiscourseSolved.unaccept_answer!(p1)
 
@@ -491,14 +491,16 @@ RSpec.describe "Managing Posts solved status" do
         expect(result[:success]).to eq(true)
 
         post_response = Fabricate(:post, topic: topic_question, user: user_3)
+        Assigner.new(post_response, user_3).assign(user_3)
 
         DiscourseSolved.accept_answer!(post_response, user_1)
 
         expect(topic_question.assignment.assigned_to_id).to eq(user_2.id)
-
+        expect(post_response.assignment.assigned_to_id).to eq(user_3.id)
         DiscourseSolved.unaccept_answer!(post_response)
 
         expect(topic_question.assignment.assigned_to_id).to eq(user_2.id)
+        expect(post_response.assignment.assigned_to_id).to eq(user_3.id)
       end
 
       describe "assigned topic reminder"