How to define previous in Rails for a different model?

Question

Rails has in default .next . Is there .previous method in ruby, should i define it myself?

"#{photo.id.previous}"

I have it defined in my Photo model and it works for my photo views, but this is being used in the User views. Does it need to be defined in the User Model?

users_controller.rb

def show
    @user = User.find(params[:id])
    @photos = @user.photos.order('created_at desc').paginate(page: params[:page], per_page: 12)
end

show.html.erb (users)

<% @photos.in_groups_of(3, false).each do |group| %>
    <div class="row">
      <% group.each do |photo| %>
        <a data-toggle="modal" href=<%="#"+"#{photo.id}"%>>
          <div class="col-xs-4 insta">
            <%= image_tag(photo.picture.ad.url, class: "img-responsive") %>
          </div>
        </a>  
        <div class="modal" id=<%="#{photo.id}"%> tabindex="-1" role="dialog" aria-labelledby="myLargeModalLabel">
          <button type="button" class="close" data-dismiss="modal" aria-label="Close">
            <span aria-hidden="true">x</span>
          </button>
...
...
...

<a class="prev" data-dismiss="modal" data-toggle="modal" href=<%="#"+"#{photo.id.previous}"%>>
  "Previous"
</a>
<a class="next" data-dismiss="modal" data-toggle="modal" href=<%="#"+"#{photo.id.next}"%>>
   "Next"
</a>

user.rb

def previous 
  #.. ?
end

Answer 1

You can find this record like this:

class User
  def self.previous_by_id(id)
    where("id < ?", id).last
  end
end

User.previous_by_id(5)
#=> #<User:0x0000000741b818 id: 4 .....>

Or if you want to define it like instance method:

class User
  def previous
    self.class.where("id < ?", id).last
  end
end

Answer 2

Ruby already has this method. For eg:

1.pred
# => 0

Check out this for more detail

And Rails also supports this. For eg:

photo.id.pred

Answer 3

class User
  def previous
    self.class.where("id < ?", id).last
  end
end

class User
  def next
    self.class.where("id > ?", id).first
  end
end

href=<%="#"+"#{photo.previous.id}"%>>

href=<%="#"+"#{photo.previous.id}"%>>