MONGOID-5684 Ensure field_was returns the same type as field for hash-typed fields

lib/mongoid/extensions.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require 'mongoid/extensions/array'
+require 'mongoid/extensions/bson_document'
 require 'mongoid/extensions/big_decimal'
 require 'mongoid/extensions/binary'
 require 'mongoid/extensions/boolean'

lib/mongoid/extensions/bson_document.rb

@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+module Mongoid
+  module Extensions
+    # Adds behavior to BSON::Document.
+    module BsonDocument
+      # Make a deep copy of this document, preserving the BSON::Document type.
+      #
+      # Hash#__deep_copy__ returns a plain Hash, which causes field_was to
+      # return a different type than the field getter when the stored attribute
+      # is a BSON::Document.
+      #
+      # @example Make a deep copy of the document.
+      #   doc.__deep_copy__
+      #
+      # @return [ BSON::Document ] The copied document.
+      def __deep_copy__
+        self.class.new.tap do |copy|
+          each_pair do |key, value|
+            copy.store(key, value.__deep_copy__)
+          end
+        end
+      end
+    end
+  end
+end
+
+BSON::Document.include Mongoid::Extensions::BsonDocument

spec/mongoid/changeable_spec.rb

@@ -674,6 +674,24 @@
         expect(person.send(:attribute_was, 'title')).to eq('Grand Poobah')
       end
     end
+
+    context 'when the attribute is a Hash field stored as a BSON::Document' do
+      let(:person) do
+        Person.new(map: BSON::Document.new('foo' => true)).tap(&:move_changes)
+      end
+
+      before do
+        person.map['bar'] = true
+      end
+
+      it 'returns a BSON::Document, consistent with the field getter' do
+        expect(person.map_was).to be_a(BSON::Document)
+      end
+
+      it 'returns the correct previous value' do
+        expect(person.map_was).to eq('foo' => true)
+      end
+    end
   end
 
   describe '#attribute_previously_was' do