Mongoid Rails 4 sort by asc or desc order created_at

def index
   @books = Book.order(:created_at => 'desc')
end

It works for me, instead of order_by use order(depends on rails version)

You can try this

def index
  @books = Book.order_by(created_at: :desc)
end

it works fine.

You can use both "order" and "order_by", and they are equivalent. All these are equivalent:

Book.order_by(created_at: :desc)
Book.order_by(created_at: -1)
Book.order(created_at: :desc)
Book.order(created_at: -1)

This is the source code from mongoid 5.1.3 "lib/mongoid/criteria/queryable/optional.rb":

# Adds sorting criterion to the options.
#
# @example Add sorting options via a hash with integer directions.
#   optional.order_by(name: 1, dob: -1)
#
# @example Add sorting options via a hash with symbol directions.
#   optional.order_by(name: :asc, dob: :desc)
#
# @example Add sorting options via a hash with string directions.
#   optional.order_by(name: "asc", dob: "desc")
#
# @example Add sorting options via an array with integer directions.
#   optional.order_by([[ name, 1 ], [ dob, -1 ]])
#
# @example Add sorting options via an array with symbol directions.
#   optional.order_by([[ name, :asc ], [ dob, :desc ]])
#
# @example Add sorting options via an array with string directions.
#   optional.order_by([[ name, "asc" ], [ dob, "desc" ]])
#
# @example Add sorting options with keys.
#   optional.order_by(:name.asc, :dob.desc)
#
# @example Add sorting options via a string.
#   optional.order_by("name ASC, dob DESC")
#
# @param [ Array, Hash, String ] spec The sorting specification.
#
# @return [ Optional ] The cloned optional.
#
# @since 1.0.0
def order_by(*spec)
  option(spec) do |options, query|
    spec.compact.each do |criterion|
      criterion.__sort_option__.each_pair do |field, direction|
        add_sort_option(options, field, direction)
      end
      query.pipeline.push("$sort" => options[:sort]) if aggregating?
    end
  end
end
alias :order :order_by

Book.where(author: "Stephen King").sort({"created_at": 1})--> Ascending Order

Book.where(author: "Stephen King").sort({"created_at": -1}) --> Descending Order