Weird bug (or something I just don't get) on ActiveRecord relationships

app_models_wiki_page.rb

# WikiPage(id: integer, title: string, fetched_at: date, created_at: datetime, updated_at: datetime, ancestry: string, url: string, is_root: boolean, wiki_tree_id: integer)

# The whole code is at https://github.com/brenes/Wiki-Roots

class WikiPage < ActiveRecord::Base

  belongs_to :tree, :class_name => "WikiTree"

# ...

end

spec_models_wiki_page.rb

require "spec_helper.rb"

describe WikiPage do

  describe "when a page belongs to a tree" do

    it "should.. you know... belong" do

      page = WikiPage.create! :title => 'The page'
      tree = WikiTree.create! :name => 'The tree'
      page.wiki_tree_id = tree.id
      page.save!

      assert_equal WikiPage.first.tree, WikiTree.find(WikiPage.first.wiki_tree_id), "WTF!"
      # It fails as WikiPage.first.tree is nil !!! 
    end


  end
end