Rails - Jun 2010 - Time-dependent values/constants in Models: An issue or not?

Hi there,

I remember reading in the comments of a Rails blog something along these
lines:

"Since a Model is loaded only once in production development (at server
start), certain time-dependent ''constants'' that you have
placed in a
method of a Model will always return the same value for those
''constants'', i.e. the value the ''constant''
had at server start."

Hypothetic example for the MySQL ''constant''
"UTC_TIMESTAMP()" in a Model
''User'':

  ====  def books_created_right_now
    self.books.find(:all, :conditions => "books.updated_at
UTC_TIMESTAMP()")
  end
  ====
Is the above statement utter BS or is there actually some issue similar
to the example or statement?

Thanks a lot!
Tom
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On Jun 26, 2010, at 6:00 AM, Tom Ha wrote:
> Hi there,
> 
> I remember reading in the comments of a Rails blog something along these
> lines:
> 
> "Since a Model is loaded only once in production development (at
server
> start), certain time-dependent ''constants'' that you have
placed in a
> method of a Model will always return the same value for those
> ''constants'', i.e. the value the
''constant'' had at server start."
> 
> Hypothetic example for the MySQL ''constant''
"UTC_TIMESTAMP()" in a Model
> ''User'':
> 
>  ====>  def books_created_right_now
>    self.books.find(:all, :conditions => "books.updated_at >
UTC_TIMESTAMP()")
>  end
>  ====> 
> Is the above statement utter BS or is there actually some issue similar
> to the example or statement?
The above example is perfectly okay.  However, the below isn''t:

named_scope :books_created_right_now, :conditions => ["updated_at =
?", Time.now]

The reason the above fails is that Time.now is evaluated *once* when the class
is loaded.  To get around that you can use a lambda for the named scope.

named_scope :books_created_right_now, lambda { {:conditions =>
["updated_at = ?", Time.now]} }

Or, for example you wouldn''t want to do this:

class Book << AR
 NOW = Time.now

 # use NOW somewhere in a method thinking it''s still going to be
''now''
end

I believe that is what the documents are referring to.

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Thanks a lot for your explanation (I believe a lot of folks are not 
necessarily aware of this "detail"...).

One question back, though:

Is the issue rather...

a) with Rails'' "Time.now" VS MySQL''s
"UTC_TIMESTAMP()" (where the former
would *never* return the "true now" when used *anywhere within a
Class*
and the latter would always be returned as the "true now"), or is the 
issue...

b) with the *context* of the "time constant" (equally in both cases), 
where the constant would always return the "true now" if it is used:
- *within a method* of a Class, or
- within a named_scope, together with a lambda,
- but NOT *outside of a method* within a Class?


You also say:
> The reason the above fails is that Time.now is evaluated *once* when the 
> class is loaded.
-> Are a Class'' methods not also evaluated only *once*?
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On Jun 28, 2010, at 11:42 AM, Tom Ha wrote:
> Thanks a lot for your explanation (I believe a lot of folks are not 
> necessarily aware of this "detail"...).
> 
> One question back, though:
> 
> Is the issue rather...
> 
> a) with Rails'' "Time.now" VS MySQL''s
"UTC_TIMESTAMP()" (where the former
> would *never* return the "true now" when used *anywhere within a
Class*
> and the latter would always be returned as the "true now"), or is
the
> issue...
> 
> b) with the *context* of the "time constant" (equally in both
cases),
> where the constant would always return the "true now" if it is
used:
> - *within a method* of a Class, or
> - within a named_scope, together with a lambda,
> - but NOT *outside of a method* within a Class?
> 
> 
> You also say:
>> The reason the above fails is that Time.now is evaluated *once* when
the
>> class is loaded.
> 
> -> Are a Class'' methods not also evaluated only *once*?
No.  The danger is that you might assign the result of Time.now to a class
variable or constant.  Or use it in a spot that *is* evaluated only once (like
my first named_scope example) and not realize it.

-------------------------------------------------------------
class Foo

  NOW = Time.now

  def self.klass_meth
    puts Time.now.to_s + '' '' + NOW.to_s
  end

  def inst_meth
    puts Time.now.to_s + '' '' + NOW.to_s
  end

end

Foo.klass_meth
sleep 1
Foo.klass_meth
sleep 1

f = Foo.new
sleep 1
f.inst_meth
sleep 1
f.inst_meth
-------------------------------------------------------------

Mon Jun 28 12:25:36 -0700 2010 Mon Jun 28 12:25:36 -0700 2010
Mon Jun 28 12:25:37 -0700 2010 Mon Jun 28 12:25:36 -0700 2010
Mon Jun 28 12:25:39 -0700 2010 Mon Jun 28 12:25:36 -0700 2010
Mon Jun 28 12:25:40 -0700 2010 Mon Jun 28 12:25:36 -0700 2010

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Got it - thanks a bunch !
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