Improve data migration logic and add test coverage

db/migrate/20220105085730_migrate_customers_data.rb

@@ -11,14 +11,47 @@ class MigrateCustomersData < ActiveRecord::Migration[6.1]
   end
 
   def migrate_customer_name_data!
-    Customer.where(first_name: "", last_name: "").where.not(name: [nil, ""]).find_each do |customer|
-      name_words = customer.name.split(' ')
-      next if name_words.empty?
+    customers_with_bill_addresses.find_each do |customer|
+      if bill_address_name_matches?(customer)
+        apply_name_from_bill_address!(customer)
+        next
+      end
 
-      customer.update(
-        first_name: name_words.first,
-        last_name: name_words[1..].join(' ')
-      )
+      split_customer_name!(customer)
+    end
+
+    customers_without_bill_addresses.find_each do |customer|
+      split_customer_name!(customer)
     end
   end
+
+  def customers_with_bill_addresses
+    Customer.joins(:bill_address).where(first_name: "", last_name: "").where.not(name: [nil, ""])
+  end
+
+  def customers_without_bill_addresses
+    Customer.where(bill_address_id: nil, first_name: "", last_name: "").where.not(name: [nil, ""])
+  end
+
+  def bill_address_name_matches?(customer)
+    customer.name.delete(" ") == (customer.bill_address.firstname + customer.bill_address.lastname).delete(" ")
+  end
+
+  def split_customer_name!(customer)
+    return if (name_parts = customer.name.split(' ')).empty?
+
+    customer.update_columns(
+      first_name: name_parts.first,
+      last_name: name_parts[1..].join(' '),
+      updated_at: Time.zone.now
+    )
+  end
+
+  def apply_name_from_bill_address!(customer)
+    customer.update_columns(
+      first_name: customer.bill_address.firstname,
+      last_name: customer.bill_address.lastname,
+      updated_at: Time.zone.now
+    )
+  end
 end

spec/migrations/split_customer_names_spec.rb

@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+require_relative '../../db/migrate/20220105085730_migrate_customers_data'
+
+describe MigrateCustomersData do
+  let!(:customer1) {
+    create(:customer, name: "Timmy Test", first_name: "", last_name: "", bill_address: nil)
+  }
+  let!(:customer2) {
+    create(:customer, name: "Frank Lee Ridiculous", first_name: "", last_name: "",
+                      bill_address: create(:address, first_name: "Frank Lee", last_name: "Ridiculous"))
+  }
+  let!(:customer3) {
+    create(:customer, name: "Shia Le Boeuf", first_name: "", last_name: "",
+                      bill_address: create(:address, first_name: "Shia", last_name: "Le Boeuf"))
+  }
+  let!(:customer4) {
+    create(:customer, name: "No Eyed Deer", first_name: "", last_name: "", bill_address: nil)
+  }
+  let!(:customer5) {
+    create(:customer, name: "   Space     Invader   ", first_name: "", last_name: "", bill_address: nil)
+  }
+  let!(:customer6) {
+    create(:customer, name: "How   Many Names  Do You   Need?", first_name: "", last_name: "", bill_address: nil)
+  }
+  let!(:customer7) {
+    create(:customer, name: "Customer Name", first_name: "", last_name: "",
+                      bill_address: create(:address, first_name: "Different", last_name: "AddressName"))
+  }
+
+  it "migrates customer names" do
+    subject.up
+
+    expect([customer1.reload.first_name, customer1.last_name]).to eq ["Timmy", "Test"]
+    expect([customer2.reload.first_name, customer2.last_name]).to eq ["Frank Lee", "Ridiculous"]
+    expect([customer3.reload.first_name, customer3.last_name]).to eq ["Shia", "Le Boeuf"]
+    expect([customer4.reload.first_name, customer4.last_name]).to eq ["No", "Eyed Deer"]
+    expect([customer5.reload.first_name, customer5.last_name]).to eq ["Space", "Invader"]
+    expect([customer6.reload.first_name, customer6.last_name]).to eq ["How", "Many Names Do You Need?"]
+    expect([customer7.reload.first_name, customer7.last_name]).to eq ["Customer", "Name"]
+  end
+end