Rails - Jan 2006 - type mismatch because of misled array index

Most times I look up a list of books I want to know a page reference for
each one, so the views that display booklists expect an array of
two-element arrays, constructed via a method in the model like so:
(class Category)
  def book
    @book = Array.new
    self.bookshelves.each { |x| @book << [x.book, x.pages] }
    return @book
  end

When I get the list of all books that a user has listed, I have no page
numbers in that table, nor do I want any.  I tried to fill the second
slot with nils or empty strings, since that what x.pages would be for a
book with no page reference anyway.
(class User)
  def book
    @books = []
    self.books.each { |b| @books << [b, nil] }
    return @books
  end

But when I call that method in my controller,

when ''user''
      @books = User.find(params[:id]).book

I get this error:

ActiveRecord::AssociationTypeMismatch in View#bookslist

Book expected, got NilClass
RAILS_ROOT: /Volumes/TRAVELDRIVE/sandcastle/script/../config/..

Application Trace | Framework Trace | Full Trace
#{RAILS_ROOT}/vendor/rails/activerecord/lib/active_record/associations/association_proxy.rb:115:in
`raise_on_type_mismatch''
#{RAILS_ROOT}/vendor/rails/activerecord/lib/active_record/associations/association_collection.rb:24:in
`<<''
#{RAILS_ROOT}/vendor/rails/activerecord/lib/active_record/associations/association_collection.rb:23:in
`each''
#{RAILS_ROOT}/vendor/rails/activerecord/lib/active_record/associations/association_collection.rb:23:in
`<<''
#{RAILS_ROOT}/vendor/rails/activerecord/lib/active_record/associations/association_collection.rb:22:in
`transaction''
#{RAILS_ROOT}/vendor/rails/activerecord/lib/active_record/transactions.rb:91:in
`transaction''
#{RAILS_ROOT}/vendor/rails/activerecord/lib/active_record/transactions.rb:118:in
`transaction''
#{RAILS_ROOT}/vendor/rails/activerecord/lib/active_record/associations/association_collection.rb:22:in
`<<''
#{RAILS_ROOT}/app/models/user.rb:106:in `book''
#{RAILS_ROOT}/app/models/user.rb:106:in `each''


I changed the nil in User.book to '''', and it said "Book
expected, got
String", so it''s clear that it is referring to the second argument
there.  But I don''t even see how it knows to ''expect''
a book from that
context.  If I change User.book to put a book in the second array slot,
like so:

    self.books.each { |b| @books << [b, b] }

then WEBrick starts churning up as much of my cpu as it can get
(60-70%), and mysqld also gets quite a workout (3% of cpu).

My suspicion is that I''ve once again run afoul of some reserved words,
because equivalent code seems to work elsewhere.

-Mike

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