Rails - Mar 2010 - One-time script to update 1000s of values in a single column in a table: nil error?

Hi, I have a model called Stock with a value called "strength." I
wrote a simple method in the Stock.rb model file to update the value
of "strength" for each record in the stocks table. I created a
controller called fix_controller.rb. I don''t have access to the live
system, so the idea is that an admin will go to http://url/fix and a
script will run to check and potentially update the "strength" value
for every record in the stocks table.

fix_controller.rb
^^^^^^^^^^^^^^^
class FixController < ApplicationController

  def index
    stocks = Stock.find(:all)

    stocks.each do |s|
        if s.strength.nil? || s.strength == ''''
          puts ''nothing to update''
        else
          s.update_units
        end
    end
  end
end

Stock.rb
^^^^^^^^^
def update_units
    if self.strength == '''' || self.strength.nil?
   puts ''nil - cannot update''
elsif self.strength != '''' && self.strength != nil
 self.strength = self.strength.gsub!('' ml'',
''ml'') if
self.strength.gsub!('' ml'', ''ml'') != nil
 self.strength = self.strength.gsub!('' mL'',
''ml'') if
self.strength.gsub!('' mL'', ''ml'') != nil
 self.strength = self.strength.gsub!('' g'',
''g'') if self.strength.gsub!
('' g'', ''g'') != nil
 self.strength = self.strength.gsub!('' oz'',
''oz'') if
self.strength.gsub!('' oz'', ''oz'') != nil
 self.strength = self.strength.gsub!('' mg'',
''mg'') if
self.strength.gsub!('' mg'', ''mg'') != nil
end

When I try to run this, I always get the error:

NoMethodError: private method `gsub'' called for nil:NilClass

I''m sure this is a simple error and/or I''m not going about
this the
correct way. I really only need to run this code once... Any idea why
I''m getting that error? I''m checking for nil...

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it looks like you are trying to strip a leading space from each
"strength" value if its not nil.

something like

stocks = Stock.find(:all, :conditions => ''(strength IS NOT NULL) AND
(strength != "")'')

would give you all the stocks values with non-blank values. So you can
skip a lot of your testing above.

You could then do something like

stocks.each do |stock|
   stock.strength = stock.strength.to_s.gsub(/^\s+/,'''')
   stock.save
end

the above is using a regular expression to match the leading spaces.
the to_s shouldn''t be necessary, because you should only be dealing
with non-nil strength values at this point, but it''s defensive coding.

rails has a string extension .blank? which returns true for empty or
nil or spaces values, e.g.

nil.blank?
[].blank?
"   ".blank?
''''.blank?

all return true


On Mar 17, 7:26 pm, "Clay H."
<cchea...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> Hi, I have a model called Stock with a value called "strength." I
> wrote a simple method in the Stock.rb model file to update the value
> of "strength" for each record in the stocks table. I created a
> controller called fix_controller.rb. I don''t have access to the
live
> system, so the idea is that an admin will go tohttp://url/fixand a
> script will run to check and potentially update the "strength"
value
> for every record in the stocks table.
>
> fix_controller.rb
> ^^^^^^^^^^^^^^^
> class FixController < ApplicationController
>
>   def index
>     stocks = Stock.find(:all)
>
>     stocks.each do |s|
>         if s.strength.nil? || s.strength == ''''
>           puts ''nothing to update''
>         else
>           s.update_units
>         end
>     end
>   end
> end
>
> Stock.rb
> ^^^^^^^^^
> def update_units
>     if self.strength == '''' || self.strength.nil?
>    puts ''nil - cannot update''
> elsif self.strength != '''' && self.strength != nil
>  self.strength = self.strength.gsub!('' ml'',
''ml'') if
> self.strength.gsub!('' ml'', ''ml'') != nil
>  self.strength = self.strength.gsub!('' mL'',
''ml'') if
> self.strength.gsub!('' mL'', ''ml'') != nil
>  self.strength = self.strength.gsub!('' g'',
''g'') if self.strength.gsub!
> ('' g'', ''g'') != nil
>  self.strength = self.strength.gsub!('' oz'',
''oz'') if
> self.strength.gsub!('' oz'', ''oz'') != nil
>  self.strength = self.strength.gsub!('' mg'',
''mg'') if
> self.strength.gsub!('' mg'', ''mg'') != nil
> end
>
> When I try to run this, I always get the error:
>
> NoMethodError: private method `gsub'' called for nil:NilClass
>
> I''m sure this is a simple error and/or I''m not going
about this the
> correct way. I really only need to run this code once... Any idea why
> I''m getting that error? I''m checking for nil...
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On 18 March 2010 02:26, Clay H.
<ccheaton-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> Stock.rb
> ^^^^^^^^^
>  self.strength = self.strength.gsub!('' ml'',
''ml'') if
The "gsub bang" method returns nil if no changes are made:
it''s a
method for changing string variables in place, not really for
assigning the return value (unless you''re using the return value to
check whether any replacements were made.

Change to:
>  self.strength = self.strength.gsub('' ml'',
''ml'') if...
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On 18 March 2010 02:26, Clay H.
<ccheaton-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> Hi, I have a model called Stock with a value called "strength." I
> wrote a simple method in the Stock.rb model file to update the value
> of "strength" for each record in the stocks table. I created a
> controller called fix_controller.rb. I don''t have access to the
live
> system, so the idea is that an admin will go to http://url/fix and a
> script will run to check and potentially update the "strength"
value
> for every record in the stocks table.
>...
>
> I''m sure this is a simple error and/or I''m not going
about this the
> correct way. I really only need to run this code once... Any idea why
> I''m getting that error? I''m checking for nil...
As you only need to run this once it might be better as a database migration

If you leave it as an action it should really be a POST.  Otherwise
google may come along and ''fix'' it for you while scanning your
site.

Colin

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What exactly does the "...in place" part of that mean?

On Mar 18, 4:42 am, Michael Pavling
<pavl...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> On 18 March 2010 02:26, Clay H.
<cchea...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>
> > Stock.rb
> > ^^^^^^^^^
> >  self.strength = self.strength.gsub!('' ml'',
''ml'') if
>
> The "gsub bang" method returns nil if no changes are made:
it''s a
> method for changing string variables in place, not really for
> assigning the return value (unless you''re using the return value
to
> check whether any replacements were made.
>
> Change to:
>
>
>
> >  self.strength = self.strength.gsub('' ml'',
''ml'') if...
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On 18 March 2010 12:18, Clay H.
<ccheaton-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> What exactly does the "...in place" part of that mean?
>
> On Mar 18, 4:42 am, Michael Pavling
<pavl...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>> The "gsub bang" method returns nil if no changes are made:
it''s a
>> method for changing string variables in place, not really for
>> assigning the return value (unless you''re using the return
value to
>> check whether any replacements were made.
If you assign a value to a variable:

   my_variable = "100 mg"
you can alter the variable using the bang method without using an operator to
assign
so :
   my_variable = my_variable.gsub('' mg'',
''mg'')
is the same as:
   my_variable.gsub!('' mg'', ''mg'')
but beware of:
   my_variable = my_variable.gsub!('' mg'',
''mg'')
as if no substitution is made, the new value of my_variable will be
"nil" (as you discovered ;-)

http://ruby-doc.org/core/classes/String.html#M000817

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