Fix Active Storage behavior on record dup

Prior to this commit, a `dup`ed record and its originating record would
share the same `Attached` proxy objects.  Those proxy objects, in turn,
would point to the same `Attachment` associations, causing changes made
through the proxy interface to affect both records.

activestorage/lib/active_storage/attached/model.rb

@@ -46,7 +46,8 @@ module ActiveStorage
         generated_association_methods.class_eval <<-CODE, __FILE__, __LINE__ + 1
           # frozen_string_literal: true
           def #{name}
-            @active_storage_attached_#{name} ||= ActiveStorage::Attached::One.new("#{name}", self)
+            @active_storage_attached ||= {}
+            @active_storage_attached[:#{name}] ||= ActiveStorage::Attached::One.new("#{name}", self)
           end
 
           def #{name}=(attachable)
@@ -116,7 +117,8 @@ module ActiveStorage
         generated_association_methods.class_eval <<-CODE, __FILE__, __LINE__ + 1
           # frozen_string_literal: true
           def #{name}
-            @active_storage_attached_#{name} ||= ActiveStorage::Attached::Many.new("#{name}", self)
+            @active_storage_attached ||= {}
+            @active_storage_attached[:#{name}] ||= ActiveStorage::Attached::Many.new("#{name}", self)
           end
 
           def #{name}=(attachables)
@@ -183,6 +185,12 @@ module ActiveStorage
       super || attachment_changes.any?
     end
 
+    def initialize_dup(*) #:nodoc:
+      super
+      @active_storage_attached = nil
+      @attachment_changes = nil
+    end
+
     def reload(*) #:nodoc:
       super.tap { @attachment_changes = nil }
     end

activestorage/test/models/attached/many_test.rb

@@ -552,6 +552,20 @@ class ActiveStorage::ManyAttachedTest < ActiveSupport::TestCase
     end
   end
 
+  test "duped record does not share attachments" do
+    @user.highlights.attach [ create_blob(filename: "funky.jpg") ]
+
+    assert_not_equal @user.highlights.first, @user.dup.highlights.first
+  end
+
+  test "duped record does not share attachment changes" do
+    @user.highlights.attach [ create_blob(filename: "funky.jpg") ]
+    assert_not_predicate @user, :changed_for_autosave?
+
+    @user.dup.highlights.attach [ create_blob(filename: "town.mp4") ]
+    assert_not_predicate @user, :changed_for_autosave?
+  end
+
   test "clearing change on reload" do
     @user.highlights = [ create_blob(filename: "funky.jpg"), create_blob(filename: "town.jpg") ]
     assert @user.highlights.attached?

activestorage/test/models/attached/one_test.rb

@@ -529,6 +529,20 @@ class ActiveStorage::OneAttachedTest < ActiveSupport::TestCase
     end
   end
 
+  test "duped record does not share attachments" do
+    @user.avatar.attach create_blob(filename: "funky.jpg")
+
+    assert_not_equal @user.avatar.attachment, @user.dup.avatar.attachment
+  end
+
+  test "duped record does not share attachment changes" do
+    @user.avatar.attach create_blob(filename: "funky.jpg")
+    assert_not_predicate @user, :changed_for_autosave?
+
+    @user.dup.avatar.attach create_blob(filename: "town.jpg")
+    assert_not_predicate @user, :changed_for_autosave?
+  end
+
   test "clearing change on reload" do
     @user.avatar = create_blob(filename: "funky.jpg")
     assert @user.avatar.attached?