Strange Problem With Unwanted, Transient Caching

Andrew Stewart
Wed, 07 Mar 2007 13:27:04 -0000

Hello,

I hope somebody can explain to me what''s going on here because
I''m
baffled!

In a controller''s action I want to create a new order for a
customer.  Because I post back to the same action (not RESTful I
know, but that''s for another day) I use code like this:

   def edit
     @order = Order.find_by_id(params[:id]) || Order.new
     ...
   end

I noticed that when params[:id] is nil, the finder still retrieves an
order -- the order whose id matches the :customer_id which is in my
params hash.  Weird.

I then noticed that if I ran the finder a second time, it correctly
returned nil.  Weirder.

And I found the same behaviour if I replaced the find_by_id with
find_by_sql using the SQL statement it generated in the log.  A weird
clue?

Putting all this another way, this code generates the output below:

   def edit
     puts "params: #{params.inspect}"
     puts "params[:id].nil?: #{params[:id].nil?}"
     o = Order.find_by_id(id)
     puts "order: #{o.nil? ? ''nil'' : o.inspect}"
     o = Order.find_by_id(id)
     puts "order: #{o.nil? ? ''nil'' : o.inspect}"

     ...
   end

This prints out:

   params: {"action"=>"edit",
"controller"=>"orders",
"customer_id"=>"1375"}
   params[:id].nil?: true
   order: #<Order:0x35f9a5c @attributes={"id"=>"1375",
"customer_id"=>"502", "required_at"=>nil,
"created_at"=>"2004-01-22
12:00:00"}>
   order: nil

So my questions are:

- Why does this finder/query pull out an order the first time instead
of nil?
- Why is the order it wrongly pulls out the one whose id matches the
customer_id?
- Why does the query magically work the second time?
- Have I lost the plot? :)

The fact that find_by_sql exhibits the same weird behaviour leads me
to suspect the database (MySQL).  But if that were caching, surely it
would return the same result both times?

Thanks and regards,
Andy Stewart