Prevent changes_to_save from mutating attributes

When an array of hashes is added to a `HashWithIndifferentAccess`, the
hashes are replaced with HWIAs by mutating the array in place.

If an attribute's value is an array of hashes, `changes_to_save` will
convert it to an array of HWIAs as a side-effect of adding it to the
changes hash.

Using `merge!` instead of `[]=` fixes the problem, as `merge!` copies
any array values in the provided hash instead of mutating them.

1 files changed, 1 insertions, 1 deletions

diff --git a/activemodel/lib/active_model/attribute_mutation_tracker.rb b/activemodel/lib/active_model/attribute_mutation_tracker.rb
index 493be5bb88..1759f004a6 100644
--- a/activemodel/lib/active_model/attribute_mutation_tracker.rb
+++ b/activemodel/lib/active_model/attribute_mutation_tracker.rb
@@ -23,7 +23,7 @@ module ActiveModel
       attr_names.each_with_object({}.with_indifferent_access) do |attr_name, result|
         change = change_to_attribute(attr_name)
         if change
-          result[attr_name] = change
+          result.merge!(attr_name => change)
         end
       end
     end