Use proc

app/models/placement.rb

@@ -22,7 +22,8 @@
 #  fk_rails_...  (subject_id => subjects.id)
 #
 class Placement < ApplicationRecord
-  after_save :assign_subject # TODO: Remove when data migrated
+  # TODO: Remove when data migrated
+  after_save :assign_subject, unless: proc { |placement| placement.subject.present? }
 
   has_many :placement_mentor_joins, dependent: :destroy
   has_many :mentors, through: :placement_mentor_joins, class_name: "Placements::Mentor"
@@ -58,9 +59,7 @@ def self.order_by_school_ids(school_ids)
 
   # TODO: Remove after data migrated
   def assign_subject
-    return if subjects.blank? ||
-      subject.present? ||
-      subject == subjects.last
+    return if subjects.blank?
 
     update!(subject: subjects.last)
   end

spec/factories/placements/placement_additional_subjects.rb

@@ -20,5 +20,7 @@
 #
 FactoryBot.define do
   factory :placement_additional_subject, class: "Placements::PlacementAdditionalSubject" do
+    association :subject
+    association :placement
   end
 end

spec/models/placement_spec.rb

@@ -110,8 +110,8 @@
       end
     end
 
-    context "when a subject has been associated with the placement" do
-      context "when the subject is not same as the already associated subject" do
+    context "when subjects have been associated with the placement" do
+      context "when subject is nil" do
         let(:subjects) { [create(:subject)] }
         let(:a_subject) { nil }
 
@@ -120,9 +120,9 @@
         end
       end
 
-      context "when the subject is same as the already associated subject" do
+      context "when subject is not nil" do
         let(:subjects) { [create(:subject)] }
-        let(:a_subject) { subjects.last }
+        let(:a_subject) { create(:subject) }
 
         it "keeps the original subject assigned to the placement" do
           expect(placement.subject).to eq(a_subject)