Confusion with Rails belongs_to association

belongs_to.rb

#------model definition-----
class Book < ActiveRecord::Base
end

class Page < ActiveRecord::Base
  belongs_to :book
end
#------migration----------
class CreateBooks < ActiveRecord::Migration
  def change
    create_table :books do |t|
      t.string :name
      t.string :author_name

      t.timestamps
    end
  end
end

class CreatePages < ActiveRecord::Migration
  def change
    create_table :pages do |t|
      t.integer :page_number
      t.integer :book_id

      t.timestamps
    end
  end
end

#-------------------------------
# I used the `#book_build` method.
2.1.2 :058 > new_book = page.build_book(:name => 'Ruby science II')
 => #<Book id: nil, name: "Ruby science II", author_name: nil, created_at: nil, updated_at: nil> 
# as book is created in memory, but not saved in DB, thus book_id is nil. I got it.
2.1.2 :059 > page.book
 => #<Book id: nil, name: "Ruby science II", author_name: nil, created_at: nil, updated_at: nil> 
# Now I saved it to the DB.
2.1.2 :060 > new_book.save
   (0.1ms)  begin transaction
  SQL (0.2ms)  INSERT INTO "books" ("created_at", "name", "updated_at") VALUES (?, ?, ?)  [["created_at", "2014-06-12 09:58:22.539135"], ["name", "Ruby science II"], ["updated_at", "2014-06-12 09:58:22.539135"]]
   (0.9ms)  commit transaction
 => true 
# as the record is got saved to DB, thus id got value as 5
2.1.2 :061 > page.book
 => #<Book id: 5, name: "Ruby science II", author_name: nil, created_at: "2014-06-12 09:58:22", updated_at: "2014-06-12 09:58:22"> 

# Now my question is why the book_id is still nil in the page object ?
2.1.2 :062 > page
 => #<Page id: 2, page_number: nil, book_id: nil, created_at: "2014-06-12 09:51:52", updated_at: "2014-06-12 09:51:52"> 
2.1.2 :063 > Page.find(2)
  Page Load (0.2ms)  SELECT  "pages".* FROM "pages"  WHERE "pages"."id" = ? LIMIT 1  [["id", 2]]
 => #<Page id: 2, page_number: nil, book_id: nil, created_at: "2014-06-12 09:51:52", updated_at: "2014-06-12 09:51:52"> 
2.1.2 :064 > 

2.1.2 :065 > reload!
Reloading...
 => true 
2.1.2 :066 > Page.find(2)
  Page Load (0.2ms)  SELECT  "pages".* FROM "pages"  WHERE "pages"."id" = ? LIMIT 1  [["id", 2]]
 => #<Page id: 2, page_number: nil, book_id: nil, created_at: "2014-06-12 09:51:52", updated_at: "2014-06-12 09:51:52"> 
2.1.2 :067 > 

2.1.2 :068 > page.reload
  Page Load (0.1ms)  SELECT  "pages".* FROM "pages"  WHERE "pages"."id" = ? LIMIT 1  [["id", 2]]
 => #<Page id: 2, page_number: nil, book_id: nil, created_at: "2014-06-12 09:51:52", updated_at: "2014-06-12 09:51:52"> 
2.1.2 :069 > page.reload
  Page Load (0.1ms)  SELECT  "pages".* FROM "pages"  WHERE "pages"."id" = ? LIMIT 1  [["id", 2]]
 => #<Page id: 2, page_number: nil, book_id: nil, created_at: "2014-06-12 09:51:52", updated_at: "2014-06-12 09:51:52"> 
2.1.2 :070 > Page.find(2)
  Page Load (0.1ms)  SELECT  "pages".* FROM "pages"  WHERE "pages"."id" = ? LIMIT 1  [["id", 2]]
 => #<Page id: 2, page_number: nil, book_id: nil, created_at: "2014-06-12 09:51:52", updated_at: "2014-06-12 09:51:52"> 
2.1.2 :071 >