Make use of inclusion & update spec

app/wizards/placements/add_placement/steps/subject.rb

@@ -8,10 +8,13 @@ class Placements::AddPlacement::Steps::Subject
 
   validates :school, presence: true
   validates :phase, presence: true
-  validates :subject_id, presence: true
+  validates :subject_id, presence: true, inclusion: { in: ->(step) { step.subjects_for_selection.ids } }, if: ->(step) { step.phase.present? }
 
   def subjects_for_selection
-    phase == "Primary" ? Subject.parent_subjects.primary : Subject.parent_subjects.secondary
+    {
+      "Primary" => Subject.parent_subjects.primary,
+      "Secondary" => Subject.parent_subjects.secondary,
+    }.fetch phase
   end
 
   def wizard_attributes

spec/wizards/placements/add_placement/steps/subject_spec.rb

@@ -8,7 +8,27 @@
   describe "validations" do
     it { is_expected.to validate_presence_of(:school) }
     it { is_expected.to validate_presence_of(:phase) }
-    it { is_expected.to validate_presence_of(:subject_id) }
+
+    describe "subject_id" do
+      it "is not required if phase is not present" do
+        step = described_class.new(phase: nil)
+
+        expect(step).not_to validate_presence_of(:subject_id)
+      end
+
+      it "is required if phase is present" do
+        step = described_class.new(phase: "Primary")
+
+        expect(step).to validate_presence_of(:subject_id)
+      end
+
+      it "is invalid if the subject is not in the selection" do
+        step = described_class.new(phase: "Primary", subject_id: 1)
+
+        expect(step).not_to be_valid
+        expect(step.errors[:subject_id]).to include "is not included in the list"
+      end
+    end
   end
 
   describe "#subjects_for_selection" do