Mongoize is not called on update_all, when $set operator is used

[dem]

During our upgrade of mongoid to 8.x version hit into issue that that custom type field classes stops working in update_all, when
$set operator is used:

class LatLng
  attr_accessor :lat, :lng

  def self.demongoize(object)
    LatLng.new(object[1], object[0])
  end

  def initialize(lat, lng)
    @lat, @lng = lat, lng
  end

  def mongoize
    [ lng, lat ]
  end
end

class Bar
  include Mongoid::Document
  field :lat_lng, type: LatLng
end

Bar.create!

Bar.update_all('$set' => { lat_lng: LatLng.new(52.30, 13.25) })

BSON::Error::UnserializableClass: Value does not define its BSON serialized type: #<LatLng:0x000057678d11ddb0>

This problem is not highlighted by the tests due to mongoize does not work, but BSON still able serialize strings and send it to database:

context.update_all("$set" => { member_count: "1" })

And in test case reloaded results from database - string value transformed to Integer by demongoize. Tests always stays green, however problem exists:

          it "updates the first matching document" do
            expect(depeche_mode.reload.member_count).to eq(1)
          end

It is easy to find with couple of lines console output:

          it "updates the first matching document" do
            depeche_mode.reload
            pp depeche_mode.member_count
            pp depeche_mode.member_count.class
            pp depeche_mode.as_document[:member_count]
            pp depeche_mode.as_document[:member_count].class
            expect(depeche_mode.reload.member_count).to eq(1)
          end

Output of rspec test will be

bundle exec rspec ./spec/mongoid/contextual/mongo_spec.rb:3665
[mongoid] Warning: No private key present, creating unsigned gem.
Run options: include {:locations=>{"./spec/mongoid/contextual/mongo_spec.rb"=>[3665]}}                                                                                                               
W, [2024-05-10T13:16:15.670196 #31184]  WARN -- : Using BSON::Decimal128 as the field type is not supported. In BSON <= 4, the BSON::Decimal128 type will work as expected for both storing and querying, but will return a BigDecimal on query in BSON 5+. To use literal BSON::Decimal128 fields with BSON 5, set Mongoid.allow_bson5_decimal128 to true.
1
Integer
"1"
String
 1/1 |==================================================================================== 100 ====================================================================================>| Time: 00:00:00 

Finished in 0.18469 seconds (files took 0.65765 seconds to load)
1 example, 0 failures

Issue happens due to misuse of the mongoize_for third param. It should be key (i.e. field key), not operator

mongoize_for(operator, klass, operator, value)

In this case in mongoize_for field will be nil and it return original value without demongoization:

field = klass.fields[key.to_s]
return value unless field

This issue exists at least at 8.1-stable branch, but code is different there. I will provide separate PR to fix this issue in 8.1-stable branch.

8.1-stable fix is actually important for us (https://www.onepagecrm.com), we are aiming to make upgrade to 8.1.x, but do not go further for a minute.

This PR also fix a problem describer by midnight-wonderer in discussion #5812
He also right about line with misuse of first attribute:

(atomic_updates['$set'] ||= {})[key] = mongoize_for(key, klass, key, value)

I've included this small suggested fix in this PR.

[jamis]

Excellent catch, and elegant solution. 👍