Add fast path for patient lookup when patient_id is present

Previously, patient lookup always called existing_patients which performs
expensive queries to match on name, DOB, and postcode, even when we
already had a patient_id from a previous match.

This significantly improves performance on the review screen where the
patient is looked up to identify if it is a cross-team school move.

Author: murugapl

app/controllers/class_imports_controller.rb

@@ -68,7 +68,8 @@ def show
           end
         end
 
-      @nhs_discrepancies = @class_import.changesets.nhs_number_discrepancies
+      @nhs_discrepancies =
+        @class_import.changesets.includes(:patient).nhs_number_discrepancies
 
       @cancelled = @class_import.changesets.from_file.cancelled
     end

app/controllers/cohort_imports_controller.rb

@@ -67,7 +67,8 @@ def show
           end
         end
 
-      @nhs_discrepancies = @cohort_import.changesets.nhs_number_discrepancies
+      @nhs_discrepancies =
+        @cohort_import.changesets.includes(:patient).nhs_number_discrepancies
 
       @cancelled = @cohort_import.changesets.from_file.cancelled
     end

app/jobs/commit_import_job.rb

@@ -31,8 +31,13 @@ def perform(import_global_id)
     imported_school_move_ids = []
 
     ActiveRecord::Base.transaction do
-      import
-        .changesets
+      changesets =
+        PatientChangeset.includes(:patient).where(id: import.changesets.ids)
+
+      # Reset patient_ids to avoid stale associations
+      changesets.update_all(patient_id: nil)
+
+      changesets
         .from_file
         .includes(:school)
         .find_in_batches(batch_size: 100) do |changesets|

app/jobs/commit_patient_changesets_job.rb

@@ -19,7 +19,8 @@ class CommitPatientChangesetsJob
   queue_as :imports
 
   def perform(patient_changeset_ids)
-    changesets = PatientChangeset.where(id: patient_changeset_ids)
+    changesets =
+      PatientChangeset.includes(:patient).where(id: patient_changeset_ids)
     import = changesets.first.import
     imported_school_move_ids = []
 
@@ -30,6 +31,9 @@ def perform(patient_changeset_ids)
         .index_with { |col| import.public_send(col) || 0 }
 
     ActiveRecord::Base.transaction do
+      # Reset patient_ids to avoid stale associations
+      changesets.update_all(patient_id: nil)
+
       to_process = changesets.select { review_consistent?(it) }
 
       if to_process.any?

app/jobs/review_class_import_school_move_job.rb

@@ -13,8 +13,10 @@ def perform(import_id)
       return
     end
 
+    patients_in_import_changesets = import.changesets.from_file.ready_for_review
+
     patients_in_import =
-      import.changesets.from_file.ready_for_review.map(&:patient) +
+      Patient.joins(:changesets).merge(patients_in_import_changesets) +
         import.patients
 
     patients_in_future_review = import.changesets.needs_re_review.map(&:patient)

app/models/class_import.rb

@@ -56,22 +56,26 @@ def type_label
 
   def postprocess_rows!
     # Remove patients already in the sessions but not in the class list.
+    patients_in_import_changesets =
+      changesets.from_file.where.not(status: %i[cancelled processed])
+
     patients_in_import =
-      (changesets.from_file - changesets.cancelled - changesets.processed).map(
-        &:patient
-      )
+      Patient.joins(:changesets).merge(patients_in_import_changesets)
 
     unknown_patients = patients_to_create_moves_for(patients_in_import)
 
     if Flipper.enabled?(:import_review_screen)
       valid_changesets =
         changesets.not_from_file.committing.where(
-          patient_id: unknown_patients.pluck(:id)
+          patient_id: unknown_patients.ids
         )
-      valid_ids = valid_changesets.pluck(:id)
-      valid_patients = Patient.where(id: valid_changesets.pluck(:patient_id))
+      valid_patients = Patient.joins(:changesets).merge(valid_changesets)
 
-      changesets.not_from_file.committing.where.not(id: valid_ids).destroy_all
+      changesets
+        .not_from_file
+        .committing
+        .where.not(id: valid_changesets.ids)
+        .destroy_all
 
       missed_patients = unknown_patients - valid_patients
       if missed_patients.any? && (processed? || partially_processed?)

app/models/patient_changeset.rb

@@ -188,6 +188,7 @@ def processed!
   end
 
   def patient
+    return super if patient_id.present?
     @patient ||=
       if (existing_patient = existing_patients.first)
         prepare_patient_changes(existing_patient)
@@ -443,6 +444,7 @@ def changeset_type
 
   def calculate_review_data!
     clear_review_data!
+    reset_patient_id! if patient_id.present?
 
     update_column(:record_type, changeset_type)
     return if new_patient?
@@ -466,4 +468,8 @@ def clear_review_data!
     data["review"] = { patient: {}, school_move: {} }
     save!
   end
+
+  def reset_patient_id!
+    update_column(:patient_id, nil)
+  end
 end