From ea33ffbb11b14989fa2ca0eae69c18a4cea2266d Mon Sep 17 00:00:00 2001
From: Dylan Knutson <tcdknutson@gmail.com>
Date: Fri, 18 Jul 2025 17:49:26 +0000
Subject: [PATCH] changed, changed_attributes

---
 lib/has_aux_table.rb                     | 19 +++++++++++
 spec/active_record/has_aux_table_spec.rb | 41 ++++++++++++++++++++++++
 2 files changed, 60 insertions(+)

diff --git a/lib/has_aux_table.rb b/lib/has_aux_table.rb
index d73085e..8dfc956 100644
--- a/lib/has_aux_table.rb
+++ b/lib/has_aux_table.rb
@@ -51,6 +51,7 @@ module HasAuxTable
       setup_attribute_types_hook!(config)
       setup_load_schema_hook!(config)
       setup_initialize_hook!(config)
+      setup_changed_hook!(config)
       setup_save_hook!(config)
       setup_reload_hook!(config)
       setup_attributes_hook!(config)
@@ -266,6 +267,24 @@ module HasAuxTable
       end
     end
 
+    sig { params(config: AuxTableConfig).void }
+    def setup_changed_hook!(config)
+      changed_method = self.instance_method(:changed?)
+      self.define_method(:changed?) do
+        T.bind(self, ActiveRecord::Base)
+        changed_method.bind(self).call || config.aux_model_for(self).changed?
+      end
+
+      changed_attributes_method = self.instance_method(:changed_attributes)
+      self.define_method(:changed_attributes) do
+        T.bind(self, ActiveRecord::Base)
+        changed_attributes_method
+          .bind(self)
+          .call
+          .merge(config.aux_model_for(self).changed_attributes)
+      end
+    end
+
     sig { params(config: AuxTableConfig).void }
     def setup_save_hook!(config)
       %i[save save!].each do |method_name|
diff --git a/spec/active_record/has_aux_table_spec.rb b/spec/active_record/has_aux_table_spec.rb
index 865e525..c30bbfb 100644
--- a/spec/active_record/has_aux_table_spec.rb
+++ b/spec/active_record/has_aux_table_spec.rb
@@ -99,6 +99,47 @@ RSpec.describe HasAuxTable do
     expect(lot.vehicles.first.name).to eq("vehicle1")
   end
 
+  describe "#changed?" do
+    it "returns true if the main record changes" do
+      car = Car.create!(name: "Honda Civic")
+      expect(car.changed?).to be_falsey
+      car.name = "Toyota Camry"
+      expect(car.changed?).to be_truthy
+    end
+
+    it "returns true if the aux record changes" do
+      car = Car.create!(name: "Honda Civic")
+      expect(car.changed?).to be_falsey
+      car.fuel_type = "hybrid"
+      expect(car.changed?).to be_truthy
+    end
+  end
+
+  describe "#changed_attributes" do
+    # changed_attributes returns a hash with the original values of the attribute
+
+    it "returns the changed attributes of the main record" do
+      car = Car.create!(name: "Honda Civic")
+      expect(car.changed_attributes).to eq({})
+      car.name = "Toyota Camry"
+      expect(car.changed_attributes).to eq({ "name" => "Honda Civic" })
+    end
+
+    it "returns the changed attributes of the aux record when original is nil" do
+      car = Car.create!(name: "Honda Civic")
+      expect(car.changed_attributes).to eq({})
+      car.fuel_type = "hybrid"
+      expect(car.changed_attributes).to eq({ "fuel_type" => nil })
+    end
+
+    it "returns the changed attributes of the aux record when original is not nil" do
+      car = Car.create!(name: "Honda Civic", fuel_type: "gasoline")
+      expect(car.changed_attributes).to eq({})
+      car.fuel_type = "hybrid"
+      expect(car.changed_attributes).to eq({ "fuel_type" => "gasoline" })
+    end
+  end
+
   describe "database integration" do
     it "provides automatic attribute accessors for auxiliary table columns" do
       vehicle = Car.create!(name: "Honda Civic")