Rails - Feb 2010 - Checking column in table to see if value exists?

Hey all,

What if I want to check a column in a table to see if a value exists and
if it doesn''t, then update the existing record with the specified value
and if it does exist increment by 1 from the previous incrementation of
the value. And do it using the model (or controller if it''s unable to
be
done in the model). Something like this:

def test!
     update_attributes :student_number =>
     if student_number.array.include?(2001)
       Student.student_number.first(:order => "student_number
DESC").student_number + 1
     else
       2001
   end
  end

Can this be done?

Now this immediately returns a nil.array error, but what I''m more
concerned about is if it can search the table for a value and then if
exists increment by previous incrementation. Thanks for any response.
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On Feb 4, 2010, at 10:34 AM, John Merlino wrote:
> Hey all,
>
> What if I want to check a column in a table to see if a value exists  
> and
> if it doesn''t, then update the existing record with the specified
> value
> and if it does exist increment by 1 from the previous incrementation  
> of
> the value. And do it using the model (or controller if it''s unable
> to be
> done in the model). Something like this:
>
> def test!
>     update_attributes :student_number =>
>     if student_number.array.include?(2001)
>       Student.student_number.first(:order => "student_number
> DESC").student_number + 1
>     else
>       2001
>   end
>  end
>
> Can this be done?
>
> Now this immediately returns a nil.array error, but what I''m more
> concerned about is if it can search the table for a value and then if
> exists increment by previous incrementation. Thanks for any response.
Don''t do it this way.  You''re going to run into collisions. 
What
happens when two people run this method at the same time?

I''m not entirely sure what you''re trying to do, but you should
use a
database sequence for this to ensure you don''t get duplicates.

-philip

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Let me see if I got this right:
The model name is "student" and the attribute is called 
"student_number".
When a new student is created you want to make sure the student number 
has not been taken already, and if so, set it to the current maximum 
student number + 1.

I cant make much sense of the code you posted but you could put the code 
for it in a filter on the student model:

before_create(:check_student_number)

private

def check_student_number
  if(Student.find_by_student_number(self.student_number))
    self.student_number = Student.maximum(:student_number) + 1
  end
def
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John Merlino wrote:
> Hey all,
> 
> What if I want to check a column in a table to see if a value exists and
> if it doesn''t, then update the existing record with the specified
value
> and if it does exist increment by 1 from the previous incrementation of
> the value. And do it using the model (or controller if it''s unable
to be
> done in the model). Something like this:
> 
> def test!
>      update_attributes :student_number =>
>      if student_number.array.include?(2001)
>        Student.student_number.first(:order => "student_number
> DESC").student_number + 1
>      else
>        2001
>    end
>   end
> 
> Can this be done?
Yes, but you *really* don''t want to.  Just use an autoincrement field
in
the DB (perhaps the id field).

John, I''ve noticed that most of your posts here are on questions that 
could have been answered by a quick look at the Rails Guides or 
Programming Ruby.  May I suggest that you spend less time posting and 
more time reading documentation?

 Best,
-- 
Marnen Laibow-Koser
http://www.marnen.org
marnen-sbuyVjPbboAdnm+yROfE0A@public.gmane.org
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