Fetch "previous" document with mongoId

Question

I have Images stored in mongoDB. They are sorted by "created_at".

Now, when I load a single item with Image.find("documentID") I want to access the document just before and the one just after this one, based on the created_at order.

For that, I have two methods and a default_scope in image.rb:

default_scope asc(:created_at)     

def previous_image
  self.class.last(:conditions => {:created_at.lt => created_at})
end

def next_image
  self.class.first(:conditions => {:created_at.gt => created_at})
end

Is there a native way in mongoDB to access the document before and after the current document?

Is there a better way to create such named scopes with mongoID in rails to access previous and next items, also considering performance (not querying three times for one image)?

Edit: The "previous_image" should use last, not first. That solves the bug of "skipping" items. The questions about the proper way to achive this remain, though.

Answer 1

You may try something like this:

def previous_image mongoid
  self.class.find(:conditions => {:_id => {$lt: mongoid}}).sort({:_id=>-1}).limit(1)
end

def next_image mongoid
  self.class.find(:conditions => {:_id => {$gt: mongoid}}).sort({:_id=>1}).limit(1)
end

This approach utilizes sequentiality of Mongo ids.

Answer 2

You could try something like the following, based on jhavdra's response, but rewritten for Mongoid syntax.

def previous_image
  self.class.where(:_id.lt => self._id).order_by([[:_id, :desc]]).limit(1).first
end

def next_image
  self.class.where(:_id.gt => self._id).order_by([[:_id, :asc]]).limit(1).first
end