Don't always remove patients from sessions

app/models/class_import.rb

@@ -56,10 +56,12 @@ def postprocess_rows!
 
     unknown_patients = session.patients - patients
 
-    unknown_patients.each do |unknown_patient|
-      unknown_patient.update!(school: nil)
-    end
-
-    session.patients.delete(unknown_patients)
+    session
+      .patient_sessions
+      .where(patient: unknown_patients)
+      .find_each do |patient_session|
+        patient_session.patient.update!(school: nil)
+        patient_session.destroy! if patient_session.added_to_session?
+      end
   end
 end

app/models/consent_form.rb

@@ -356,7 +356,11 @@ def match_with_patient!(patient)
       if school && school != patient.school
         patient.update!(school:)
 
-        patient.patient_sessions.find_by(session: scheduled_session)&.destroy!
+        existing_patient_session =
+          patient.patient_sessions.find_by(session: scheduled_session)
+        if existing_patient_session&.added_to_session?
+          existing_patient_session.destroy!
+        end
 
         upcoming_session =
           Session

app/models/immunisation_import.rb

@@ -148,9 +148,12 @@ def postprocess_rows!
         .includes(:vaccination_records)
         .select { _1.vaccinated?(programme) }
 
-    PatientSession.where(
-      session: team.sessions.upcoming,
-      patient: already_vaccinated_patients
-    ).delete_all
+    PatientSession
+      .where(
+        session: team.sessions.upcoming,
+        patient: already_vaccinated_patients
+      )
+      .select(&:added_to_session?)
+      .each(&:destroy!)
   end
 end

app/models/patient.rb

@@ -186,7 +186,7 @@ def update_from_pds!(pds_patient)
       self.date_of_death = pds_patient.date_of_death
 
       if date_of_death_changed?
-        upcoming_sessions.clear unless date_of_death.nil?
+        clear_upcoming_sessions unless date_of_death.nil?
         self.date_of_death_recorded_at = Time.current
       end
 
@@ -208,7 +208,7 @@ def invalidate!
     return if invalidated?
 
     ActiveRecord::Base.transaction do
-      upcoming_sessions.clear
+      clear_upcoming_sessions
       update!(invalidated_at: Time.current)
     end
   end
@@ -235,4 +235,11 @@ def destroy_childless_parents
       parent.destroy! if parent.parent_relationships.count.zero?
     end
   end
+
+  def clear_upcoming_sessions
+    patient_sessions
+      .where(session: upcoming_sessions)
+      .select(&:added_to_session?)
+      .each(&:destroy!)
+  end
 end

app/models/session.rb

@@ -131,6 +131,7 @@ def create_patient_sessions!
         required_programmes.all? { |programme| patient.vaccinated?(programme) }
       end
 
+    # First we remove patients from any other upcoming sessions.
     unvaccinated_patients.each do |patient|
       sessions_other_than_self = patient.upcoming_sessions.reject { _1 == self }
       next if sessions_other_than_self.empty?
@@ -142,6 +143,7 @@ def create_patient_sessions!
         .each(&:destroy!)
     end
 
+    # Next we can add the unvaccinated patients to this session.
     PatientSession.import!(
       %i[patient_id session_id],
       unvaccinated_patients.map { [_1.id, id] },

spec/models/class_import_spec.rb

@@ -360,6 +360,20 @@
         expect { record! }.to change { existing_patient.reload.school }.to(nil)
         expect(session.reload.patients).not_to include(existing_patient)
       end
+
+      context "when the patient has been seen" do
+        let(:existing_patient) do
+          create(:patient, :consent_given_triage_not_needed, session:)
+        end
+
+        it "doesn't remove the patient from the session" do
+          expect(session.patients).to include(existing_patient)
+          expect { record! }.to change { existing_patient.reload.school }.to(
+            nil
+          )
+          expect(session.reload.patients).to include(existing_patient)
+        end
+      end
     end
   end
 end