will paginate miscounting oddness with named scope

Question

I recently broke up my query into 4 named scopes to make it easier to re-sort and will paginate which otherwise always worked fine now has problems calculating the number of pages.

  named_scope :loc, lambda { |id| { :conditions => ['location_id = ?', id ] } }
  named_scope :datem, lambda { |*args| { :joins => :scannables, :conditions => [ "scannables.bookdate BETWEEN ? and ?", (args[0].to_date || 3.days.from_now), (args[0].to_date+(args[1] || 3)) ], :group => 'scannables.hostel_id', :having => 'SUM(scannables.available) > ' + ((args[1] || 3).to_i-1).to_s } }
  named_scope :order_by_external_desc, :include => :external_ratings, :order => 'SUM(scannables.available) DESC, external_ratings.rating DESC'
  named_scope :order_by_external_asc, :include => :external_ratings, :order => 'SUM(scannables.available) DESC, external_ratings.rating ASC'

Used like so with paginate thrown on the end...

@location = Place.loc(params[:id]).datem(user_cart.getDate,user_cart.getDays).order_by_external_desc.paginate(:per_page => 15, :page => params[:page])

Paginate will for example show that there are 6 pages of 15 each but when you get to page 4, pages 5-6 disappear... and if you try jumping to 5 or 6 directly they don't exist.

Looking at it, I realized to problem will paginate is having is that

c=@location = Place.loc(params[:id]).datem(user_cart.getDate,user_cart.getDays).order_by_external_desc.size

c = 78

however

c=@location = Place.loc(params[:id]).datem(user_cart.getDate,user_cart.getDays).order_by_external_desc

c.size = 56

The sql getting generated for the former is about 8 lines shorter than that of the latter and neglects my sql HAVING clause which causes it to return more results...

Any ideas on how to fix this?

Answer 1

When using a group_by/having - as you do in your datem named_scope - ActiveRecord gets the count wrong unfortunately. I don't think there is much you can do about that. I am also not certain whether this is necessarily a bug in ActiveRecord - or just a thing that can't actually be calculated correctly when using ActiveRecord by the nature of the SQL generated.

Either way - you will need to work around this 'bug'.

The way to do this is to tackle the count in a separate SQL statement (the one that gives you the correct results), and adding it to the paginate results like so:

total_entries = Place.count(sql) # or the combinations of named_scopes etc with a .size or whichever one gives you the correct count

Place.scopes.paginate(:per_page => 15, :page => params[:page], :total_entries => total_entries) # where scopes are all of your named scopes as you have in your examples