app-autoload.rb

require 'sinatra/base'
require 'sequel'
$LOAD_PATH << "."
autoload :Post, "post"

class MyApp < Sinatra::Base
  configure do
    DB = Sequel.connect("sqlite://sample.db")
  end
end

app.rb

require 'sinatra/base'
require 'sequel'
$LOAD_PATH << "."
require 'post'

class MyApp < Sinatra::Base
  configure do
    DB = Sequel.connect("sqlite://sample.db")
  end
end

gistfile1.txt

masa@vardamir> ruby app.rb
/Users/masa/.rvm/gems/ruby-2.0.0-p353/gems/sequel-4.5.0/lib/sequel/model/base.rb:226:in `db': No database associated with Sequel::Model: have you called Sequel.connect or Sequel::Model.db= ? (Sequel::Error)
        from /Users/masa/.rvm/gems/ruby-2.0.0-p353/gems/sequel-4.5.0/lib/sequel/model/base.rb:371:in `inherited'
        from /Users/masa/devel/hoge/post.rb:1:in `<top (required)>'
        from /Users/masa/.rvm/rubies/ruby-2.0.0-p353/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
        from /Users/masa/.rvm/rubies/ruby-2.0.0-p353/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
        from app.rb:4:in `<main>'

post.rb

  class Post < Sequel::Model
  end