Synchronize lazy relationships

[stokarenko]

BatchLoader behaves like a methods delegator but still can't delegate pure ruby operators:

class Level0Serializer4 < BaseTestSerializer
  lazy_has_one :level1

  attribute :conditional_level1 do
    # lazy_level1 is the instance of `BatchLoader` so the result will be always `exists`
    lazy_level1 ? 'exists' : 'missing'
  end

  attribute :safe_navigated_level1 do
    # lazy_level1 is the instance of `BatchLoader` so it will be always considered as
    # `not nil` by safe navigation operator. Then `id` method will be sent and
    # this time it will land to delegation object with a chance to raise
    # `NoMethodError: undefined method `id' for nil:NilClass`
    lazy_level1&.id
  end
end

Fortunately BatchLoader class provides public __sync method which instantly lands to delegation object keeping alive the laziness evaluation. This method is used by batch-loader gem itself for graphql integration. Lets do the same within our lazy_<relationship> instance methods.

[Bajena]

Great spot! I tested it and seems to work as a charm.

One thing I spotted is that we'll be also able to remove the following "hack" (calling itself to invoke the sync) from relationship_wrapper_methods.rb. I tested it and tests seem to pass when itself call gets removed, so if you'd like to add this change here feel free to do it.

If possible please rebase this PR to master before we 🚢 :)

[Bajena]

@stokarenko before we ship this please have a look at my previous comment :)

[stokarenko]

@Bajena Done, please take a look )

[Bajena]

Great stuff! I'll release a new version today 🎉