class Array

Blog.find(:all).to_formatted_s(:db) # => "First Post,Second Post,Third Post"

output:
Adding in the :db argument as the format yields a prettier

Blog.find(:all).to_formatted_s # => "First PostSecond PostThird Post"

to_s on all elements and joining them:
Converts a collection of elements into a formatted string by calling

def to_formatted_s(format = :default)
  case format
    when :db
      if respond_to?(:empty?) && self.empty?
        "null"
      else
        collect { |element| element.id }.join(",")
      end
    else
      to_default_s
  end
end