Question

I’ve found some good information on STI but have not seen examples for what I want to do. This is the best example I can think of. I want to be able to track two entities but they don’t need their own tables. I need only distinguish type (STI can do this) but I also need to know if and how one entity is related to another. I’ll use books as an example. Some books are just books but other physical books are collections of multiple books.

table: books

id | title | author_id | book_id
1, 'Fellowship of the Ring, 34, (NULL)      # is a physical book
2, 'The Two Towers', 34, (NULL)             # is a physical book
3, 'American Gods', 1, (NULL)               # is a physical book
4, 'Complete Lord Of the Rings', 34, (NULL)
5, 'Fellowship of the Ring', 34, 4          # is collected within book id 4
6, 'The Two Towers', 34, 4                  # is also collected within book id 4
etc.

So I want to be able to query books, for all books and understand how and if they are related to each other by ‘book_id’

Is this possible in Rails? If so, how is it best implemented? Can I say ‘has_many :books’ in the books model? Are their gotchas or concerns, etc.?

Thank you in advance.

Answer 1

something like this might work for your situation?

class Book < ActiveRecord::Base
  # all attributes of a book
end

class SeriesBook < Book
  has_many :books, :class_name => "PhysicalBook", :foreign_key => "parent_id"
end

class PhysicalBook < Book
  belongs_to :series, :class_name => "SeriesBook", :foreign_key => "parent_id"
end

then when querying

# searches both Series and Physical
Book.where("title LIKE ?", "#{params[:title]}%") 
# searches only Series
SeriesBook.where("title LIKE ?", "#{params[:title]}%") 

you might find that you really want your models to be different ?
Series and Book and not use STI? it would make the queries across both more complex but might make the rest of the application easier to understand

UPDATE: added belongs_to to the PhysicalBook, class name on the has_many association

# How would I query a specific collection to see which and how many books it has within it?
series = SeriesBook.find(0000)
series.books.length # => how many 
series.books.each do |book|
  puts book.title
end

# How would I look at a book and see if it was in a collection and, 
# if so, the specific ID of the specific collection to which it is associated
book = PhysicalBook.find(0000)
puts book.series.title
puts book.series.id 

The book database table ends up looking something like

id # primary key
title
# other book columns ....
type # STI
parent_id # nullable, used by physical book when part of a series, points to books.id on the series book

ALSO: read this – http://rhnh.net/2010/07/02/3-reasons-why-you-should-not-use-single-table-inheritance

you might not want STI ? data model would look similar to above but without the STI, i.e. Series / Book

use of foreign_key in has_many and belongs_to can be confusing: read up on that here – http://guides.rubyonrails.org/association_basics.html