example.rb

u = User.first
p = Page.first

# Only like this does it work
p.user = u
u.pages << p

# this does not work (does not set the user_id in the pages table)
 > p.user(u).save
   (0.1ms)  begin transaction
  SQL (6.1ms)  INSERT INTO "pages" ("created_at", "name", "updated_at", "user_id") VALUES (?, ?, ?, ?)  [["created_at", Sat, 10 Nov 2012 17:08:39 UTC +00:00], ["name", "testpages"], ["updated_at", Sat, 10 Nov 2012 17:08:39 UTC +00:00], ["user_id", nil]]
   (16.3ms)  commit transaction
 => true

# nor does this
 > u.pages << p
   (0.1ms)  begin transaction
   (0.1ms)  commit transaction
 => false

page.rb

class Page < ActiveRecord::Base

  validates_presence_of :user

  attr_accessible :name, :body, :change_comment, :user, :user_id
  attr_accessor :body, :change_comment, :user

  FORMAT = :markdown

  belongs_to :user

  before_create :create_page
  before_update :update_page
  before_destroy :delete_page

  def content
  	page.formatted_data
  end

  def raw_content
  	page.raw_data
  end

  def author
    page.version.author.name.gsub(/<>/, '')
  end

  def date
    page.version.authored_date
  end
  
  def preview(data)
    wiki.preview_page('Preview', data, FORMAT).formatted_data
  end

  private

  def wiki
  	@@gollum ||= Gollum::Wiki.new user.wiki_path
  end

  def page
  	wiki.page(self.name)
  end

  def commit
    {:message => change_comment || "", :name => user.fullname, :author => "#{user.fullname} <#{user.email}>"}
  end

  def create_page
    wiki.write_page(name, FORMAT, body || '', commit)
  end
  
  def update_page
    wiki.update_page(page, name, FORMAT, body || self.raw_content, commit)
  end
  
  def delete_page
    #wiki.delete_page(page, COMMIT)
  end
end

user.rb

class User < ActiveRecord::Base
  # Include default devise modules. Others available are:
  # :token_authenticatable, :confirmable,
  # :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable

  after_create :create_user_wiki
  #before_destory :delete_user_wiki
  after_create :create_start_page  

  # Setup accessible (or protected) attributes for your model
  attr_accessible :email, :password, :password_confirmation, :remember_me, :username, :fullname, :wiki_path
  # attr_accessible :title, :body

  has_many :pages

  def has_wiki?
    File.directory? wiki_path
  end

  def wiki_path
    wiki_path = Rails.root.join("wikis", "#{username}.git").to_s
  end

  private

  def create_user_wiki
    if !File.directory? wiki_path
      Dir.mkdir wiki_path
      Grit::Repo.init_bare wiki_path
    end
  end

  def create_start_page
    page = Page.new name: "start", body: "o hai thar", change_comment: "created page", user: self
    page.save
    pages << page
  end
end