rails memcached dalli Marshalling error for key

I have such action in my controller:

def my

  @user = Ads::User\.find current\_user\.id

  @postings = Rails\.cache\.fetch\("@user\.postings\.includes\(:category\)"\) do

    @postings = @user\.postings\.includes\(:category\)

  end

end

I'm trying to cache @postings and get such error:

This would try and cache the activerecord::relation object rather the
actual query results.
You need to force execution of the query, ie

Rails.cache.fetch("@user.postings.includes(:category)") do
  @user.postings.includes(:category).to_a
end

Fred