On 3/20/2010 7:41 PM, Space Ship Traveller wrote:
> Hello,
>
> I am writing a Rack session class. I want to detect changes to the session so that I don't write the data to the session store if the underlying hash has not changed. Right now I'm doing the following but it doesn't seem right:
>
> 			def call(env)
> 				load_session(env)
> 				status, headers, body = @app.call(env)
> 				commit_session(env, status, headers, body)
> 			end
>
> load_session loads env["rack.session"] from the cookie, and also saves env["rack.session"].hash into env["myframework.session.hash"]
>
> commit_session checks env["rack.session"].hash against the saved one (env["myframework.session.hash"]) and if it is different, it writes the changes to disk.
>
> The reason to do this is that writing data is expensive in this particular case, since it happens for many kinds of requests.
>
> However my understanding is that while x.hash != y.hash implies that x and y are different, x.hash == y.hash does not imply that x and y are the same. Can someone comment on this?
>
> Kind regards,
> Samuel
>   
From http://ruby-doc.org/core/classes/Hash.html#M002847

hsh == other_hash    => true or false

Equality͵wo hashes are equal if they each
<http://ruby-doc.org/core/classes/Hash.html#M002861> contain the same
number of keys <http://ruby-doc.org/core/classes/Hash.html#M002866> and
if each <http://ruby-doc.org/core/classes/Hash.html#M002861> key-value
pair is equal to (according to Object
<http://ruby-doc.org/core/classes/Object.html>#==) the corresponding
elements in the other hash.

   h1 = { "a" => 1, "c" => 2 }
   h2 = { 7 => 35, "c" => 2, "a" => 1 }
   h3 = { "a" => 1, "c" => 2, 7 => 35 }
   h4 = { "a" => 1, "d" => 2, "f" => 35 }
   h1 == h2   #=> false
   h2 == h3   #=> true
   h3 == h4   #=> false

How much more equal do you need?