Question

If i have two tables Books, CDs with corresponding models.

I want to display to the user a list of books and CDs. I also want to be able to sort this list on common attributes (release date, genre, price, etc.). I also have basic filtering on the common attributes.

The list will be large so I will be using pagination in manage the load.

items = []
items << CD.all(:limit => 20, :page => params[:page], :order => "genre ASC")
items << Book.all(:limit => 20, :page => params[:page], :order => "genre ASC")
re_sort(items,"genre ASC")

Right now I am doing two queries concatenating them and then sorting them. This is very inefficient. Also this breaks down when I use paging and filtering. If I am on page 2 of how do I know what page of each table individual table I am really on? There is no way to determine this information without getting all items from each table.

I have though that if I create a new Class called items that has a one to one relationship with either a Book or CD and do something like

Item.all(:limit => 20, :page => params[:page], :include => [:books, :cds], :order => "genre ASC")

However this gives back an ambiguous error. So can only be refined as

Item.all(:limit => 20, :page => params[:page], :include => [:books, :cds], :order => "books.genre ASC")

And does not interleave the books and CDs in a way that I want.

Any suggestions.

Answer 1

You say you can’t change how books and CDs are stored in the database, but could you add an items view? Do you have any control over the database at all?

CREATE VIEW items
  (id, type, title, genre, created_at, updated_at)
  AS
  SELECT b.id, 'Book', b.title, b.genre, b.created_at, b.updated_at
    FROM books b
  UNION
  SELECT c.id, 'CD', c.title, c.genre, c.created_at, c.updated_at
    FROM cds c;