Allow setting a Rails Engine to resolve file path

lib/cache_crispies/base.rb

@@ -64,6 +64,21 @@ class << self
       alias do_caching? do_caching
     end
 
+    # Get or set root path of Rails application or engine.
+    # It uses Rails by default, but can be overriden with your Rails Engine class.
+    # Calling the method with an argument will set the value, calling it without
+    # any arguments will get the value.
+    def self.engine(value = nil)
+      @engine ||= superclass.try(:engine)
+      @engine ||= Rails
+
+      # method called with no args so act as a getter
+      return @engine if value.nil?
+
+      # method called with args so act as a setter
+      @engine = value
+    end
+
     # Get or set a JSON key to use as a root key on a non-collection
     # serializable. By default it's the name of the class without the
     # "Serializer" part. But it can be overridden in a subclass to be anything.
@@ -190,7 +205,7 @@ def self.path
         parts = %w[app serializers]
         parts += to_s.deconstantize.split('::').map(&:underscore)
         parts << "#{to_s.demodulize.underscore}.rb"
-        Rails.root.join(*parts)
+        engine.root.join(*parts)
       end
     end
     private_class_method :path

spec/cache_crispies/base_spec.rb

@@ -39,6 +39,15 @@ def visible?
     end
   end
 
+  class MyEngine
+    def self.root
+    end
+  end
+
+  class CacheCrispiesEngineTestSerializer < CacheCrispiesTestSerializer
+    engine MyEngine
+  end
+
   let(:model) do
     OpenStruct.new(
       id: 42,
@@ -52,6 +61,7 @@ def visible?
   end
 
   let(:serializer) { CacheCrispiesTestSerializer }
+  let(:engine_serializer) { CacheCrispiesEngineTestSerializer }
   let(:instance) { serializer.new(model) }
   subject { instance }
 
@@ -161,6 +171,24 @@ def visible?
     end
   end
 
+  describe '.engine' do
+    it 'defaults to Rails' do
+      expect(serializer.engine).to eq Rails
+    end
+
+    context 'when called with an argument' do
+      after { serializer.remove_instance_variable :@engine }
+
+      it 'sets and returns a value' do
+        expect {
+          serializer.engine MyEngine
+        }.to change {
+          serializer.engine
+        }.from(Rails).to MyEngine
+      end
+    end
+  end
+
   describe '.cache_key_addons' do
     it 'returns an empty array by default' do
       expect(serializer.cache_key_addons).to eq []
@@ -203,6 +231,12 @@ def visible?
     let(:serializer_file_digest) {
       Digest::MD5.file(serializer_file_path).to_s
     }
+    let(:engine_serializer_file_path) {
+      File.expand_path('../fixtures/engine_test_serializer.rb', __dir__)
+    }
+    let(:engine_serializer_file_digest) {
+      Digest::MD5.file(engine_serializer_file_path).to_s
+    }
 
     before do
       allow(NutritionSerializer).to receive(:file_hash).and_return(
@@ -211,6 +245,9 @@ def visible?
       allow(Rails).to receive_message_chain(:root, :join).and_return(
         serializer_file_path
       )
+      allow(MyEngine).to receive_message_chain(:root, :join).and_return(
+        engine_serializer_file_path
+      )
     end
 
     it 'includes the file name' do
@@ -230,6 +267,12 @@ def visible?
         "#{serializer}-#{serializer_file_digest}+#{nested_serializer_digest}"
       )
     end
+
+    context 'when an engine is set' do
+      it "includes a digest of the serializer class file's contents" do
+        expect(engine_serializer.cache_key_base).to include engine_serializer_file_digest
+      end
+    end
   end
 
   describe '.attributes' do

spec/fixtures/engine_test_serializer.rb

@@ -0,0 +1,3 @@
+# Nothing has to actually be in this file.
+# It's just here so test can MD5 sum it's content.
+# This is an alternative serializer file.