FIX: dont allow assigning user to topic when post assigned (#259)

When post is already assigned to user/group then don't allow assigning a topic to that user/group.

Similarly, right now when topic is already assigned we are not allowing to assign a post that that user.

lib/assigner.rb

@@ -181,7 +181,6 @@ class ::Assigner
   end
 
   def forbidden_reasons(assign_to:, type:)
-    forbidden_reason =
     case
     when assign_to.is_a?(User) && !can_assignee_see_target?(assign_to)
       topic.private_message? ? :forbidden_assignee_not_pm_participant : :forbidden_assignee_cant_see_topic
@@ -191,6 +190,8 @@ class ::Assigner
       assign_to.is_a?(User) ? :forbidden_assign_to : :forbidden_group_assign_to
     when topic.assignment&.assigned_to_id == assign_to.id && topic.assignment&.assigned_to_type == type
       assign_to.is_a?(User) ? :already_assigned : :group_already_assigned
+    when @target.is_a?(Topic) && Assignment.where(topic_id: topic.id, target_type: "Post").any? { |assignment| assignment.assigned_to_id == assign_to.id && assignment.assigned_to_type == type }
+      assign_to.is_a?(User) ? :already_assigned : :group_already_assigned
     when Assignment.where(topic: topic).count >= ASSIGNMENTS_PER_TOPIC_LIMIT
       :too_many_assigns_for_topic
     when !can_assign_to?(assign_to)

spec/integration/assign_spec.rb

@@ -146,6 +146,32 @@ describe 'integration tests' do
     end
   end
 
+  context 'already assigned' do
+    fab!(:post) { Fabricate(:post) }
+    fab!(:post_2) { Fabricate(:post, topic: post.topic) }
+    let(:topic) { post.topic }
+    fab!(:user) { Fabricate(:user) }
+
+    include_context 'A group that is allowed to assign'
+
+    it 'does not allow to assign topic if post is already assigned' do
+      add_to_assign_allowed_group(user)
+
+      assigner = Assigner.new(post, user)
+      response = assigner.assign(user)
+      expect(response[:success]).to be true
+
+      assigner = Assigner.new(post_2, user)
+      response = assigner.assign(user)
+      expect(response[:success]).to be true
+
+      assigner = Assigner.new(topic, user)
+      response = assigner.assign(user)
+      expect(response[:success]).to be false
+      expect(response[:reason]).to eq(:already_assigned)
+    end
+  end
+
   context 'move post' do
     fab!(:old_topic) { Fabricate(:topic) }
     fab!(:post) { Fabricate(:post, topic: old_topic) }