FIX: reassign when assignment limit is fulfilled (#419)

Previously, when topic was already assigned 5 times, reassign was not possible. User had to first unassign topic and then assign to someone else.

app/models/assignment.rb

@@ -71,11 +71,13 @@ end
 #  target_type         :string           not null
 #  active              :boolean          default(TRUE)
 #  note                :string
+#  status              :text
 #
 # Indexes
 #
 #  index_assignments_on_active                               (active)
 #  index_assignments_on_assigned_to_id_and_assigned_to_type  (assigned_to_id,assigned_to_type)
 #  index_assignments_on_target_id_and_target_type            (target_id,target_type) UNIQUE
+#  index_assignments_on_topic_id                             (topic_id)
 #  unique_target_and_assigned                                (assigned_to_id,assigned_to_type,target_id,target_type) UNIQUE
 #

db/migrate/20230112033741_add_topic_id_index_to_assignments.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddTopicIdIndexToAssignments < ActiveRecord::Migration[7.0]
+  def change
+    add_index :assignments, :topic_id
+  end
+end

lib/assigner.rb

@@ -213,7 +213,8 @@ class ::Assigner
       assign_to.is_a?(User) ? :forbidden_assign_to : :forbidden_group_assign_to
     when already_assigned?(assign_to, type, note, status)
       assign_to.is_a?(User) ? :already_assigned : :group_already_assigned
-    when Assignment.where(topic: topic, active: true).count >= ASSIGNMENTS_PER_TOPIC_LIMIT
+    when Assignment.where(topic: topic, active: true).count >= ASSIGNMENTS_PER_TOPIC_LIMIT &&
+           !reassign?
       :too_many_assigns_for_topic
     when !can_assign_to?(assign_to)
       :too_many_assigns
@@ -532,6 +533,11 @@ class ::Assigner
     false
   end
 
+  def reassign?
+    return false if !@target.is_a?(Topic)
+    Assignment.exists?(topic_id: @target.id, target: @target, active: true)
+  end
+
   def no_assignee_change?(assignee)
     @target.assignment&.assigned_to_id == assignee.id
   end

spec/lib/assigner_spec.rb

@@ -189,8 +189,11 @@ RSpec.describe Assigner do
       it "doesn't assign if the topic has more than 5 assignments" do
         other_post = nil
 
+        status = described_class.new(topic, admin).assign(Fabricate(:moderator))
+        expect(status[:success]).to eq(true)
+
         # Assign many posts to reach the limit
-        1.upto(described_class::ASSIGNMENTS_PER_TOPIC_LIMIT) do
+        1.upto(described_class::ASSIGNMENTS_PER_TOPIC_LIMIT - 1) do
           other_post = Fabricate(:post, topic: topic)
           user = Fabricate(:moderator)
           status = described_class.new(other_post, admin).assign(user)
@@ -203,6 +206,10 @@ RSpec.describe Assigner do
         expect(status[:success]).to eq(false)
         expect(status[:reason]).to eq(:too_many_assigns_for_topic)
 
+        # Allows to reassign Topic
+        status = described_class.new(topic, admin).assign(Fabricate(:moderator))
+        expect(status[:success]).to eq(true)
+
         # Delete a post to mark the assignment as inactive
         PostDestroyer.new(admin, other_post).destroy