Problem with counter_cache implementation

You should be using User.reset_counters to do this. Additionally, I would recommend using find_each instead of each because it will iterate the collection in batches instead of all at once.

self.up
  add_column :users, :posts_count, :integer, :default => 0

  User.reset_column_information
  User.find_each do |u|
    User.reset_counters u.id, :posts
  end
end