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Neil Cauldwell

2008-Mar-16 16:48 UTC

Comparing datetimes for conditional

I just hacked together my first bit of ruby code for pulling in rss feed
items into a database. It uses a conditional which checks the last
item''s pubDate against the ''last_checked_at''
attribute for the parent
model, ''Source'', of the item model. However, I''m
getting funny results
when trying to compare times;

if news_item[:pubDate] > @source.last_checked_at do

I''m betting the problem is due to how the datetimes are formatted.

For example, would Ruby consider "Sun Mar 16 16:22:00 +0000 2008" to
be
greater than "Sat Mar 15 16:22:00 +0000 2008"? Or do I need to make
sure
that the date formats are in year / month / day / time format?
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Frederick Cheung

2008-Mar-16 16:54 UTC

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Re: Comparing datetimes for conditional

On 16 Mar 2008, at 16:48, Neil Cauldwell wrote:
>
> I just hacked together my first bit of ruby code for pulling in rss  
> feed
> items into a database. It uses a conditional which checks the last
> item''s pubDate against the ''last_checked_at''
attribute for the parent
> model, ''Source'', of the item model. However, I''m
getting funny results
> when trying to compare times;
>
> if news_item[:pubDate] > @source.last_checked_at do
>
Are these actual instances of Date or Time or are they just strings  
containing a representation of a time ?
You really don''t want to be comparing the strings themselves. (also  
I''m slightly curious about the ''do'' and the end of
that line - it''s
probably not doing what you think it is.

Fred> I''m betting the problem is due to how the datetimes are formatted.
>
> For example, would Ruby consider "Sun Mar 16 16:22:00 +0000 2008"
to
> be
> greater than "Sat Mar 15 16:22:00 +0000 2008"? Or do I need to
make
> sure
> that the date formats are in year / month / day / time format?
> -- 
> Posted via http://www.ruby-forum.com/.
>
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Neil Cauldwell

2008-Mar-16 17:23 UTC

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Re: Comparing datetimes for conditional

Frederick Cheung wrote:> On 16 Mar 2008, at 16:48, Neil Cauldwell wrote:
> 
>>
>> I just hacked together my first bit of ruby code for pulling in rss  
>> feed
>> items into a database. It uses a conditional which checks the last
>> item''s pubDate against the ''last_checked_at''
attribute for the parent
>> model, ''Source'', of the item model. However,
I''m getting funny results
>> when trying to compare times;
>>
>> if news_item[:pubDate] > @source.last_checked_at do
>>
> 
> Are these actual instances of Date or Time or are they just strings
> containing a representation of a time ?
> You really don''t want to be comparing the strings themselves.
(also
> I''m slightly curious about the ''do'' and the end
of that line - it''s
> probably not doing what you think it is.
> 
> Fred
Thanks for the quick response. I think I''m comparing Time objects 
because I had to call Time.parse on the feed pubDate to get Active 
Controller to accept the comparison, otherwise it was throwing an error 
saying it couldn''t compare a String to a Time.

Do you know how Ruby would compare two Time objects from the example in 
the first post? And how would you go about comparing two Time objects in 
a conditional?

As for the ''do'', I made a more detailed post about the RSS
parsing and
controller action over on Railsforum, 
http://railsforum.com/viewtopic.php?id=16479. I didn''t think it was 
necessary to include all that info in this thread, but now we''re on the
topic, rather than me pasting the same stuff all over the Interwebs, 
maybe you could check out the last post in the thread over there and 
I''ll save the Rails community from a potential cross-threading!


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Frederick Cheung

2008-Mar-16 17:48 UTC

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Re: Comparing datetimes for conditional

On 16 Mar 2008, at 17:23, Neil Cauldwell wrote:
>>
>
> Thanks for the quick response. I think I''m comparing Time objects
> because I had to call Time.parse on the feed pubDate to get Active
> Controller to accept the comparison, otherwise it was throwing an  
> error
> saying it couldn''t compare a String to a Time.
>
> Do you know how Ruby would compare two Time objects from the example  
> in
> the first post? And how would you go about comparing two Time  
> objects in
> a conditional?
>You would just do a > b
> As for the ''do'', I made a more detailed post about the
RSS parsing and
> controller action over on Railsforum,
> http://railsforum.com/viewtopic.php?id=16479. I didn''t think it
was
> necessary to include all that info in this thread, but now we''re
on
> the
> topic, rather than me pasting the same stuff all over the Interwebs,
> maybe you could check out the last post in the thread over there and
> I''ll save the Rails community from a potential cross-threading!
>

so what you''ve got is:

if news_item[:pubDate] > @source.last_checked_at do
   # some stuff
end
end

(your original indentation rather hides the fact that there are 2 ends).

What this does is call the last_checked_at method on @source, passing  
the contents of the do-end block as  the block to that method (whereas  
i suspect you think it''s the body of the if statement). that block  
never gets evaluated because last_checked_at doesn''t yield to it. If  
the condition is true then the actual body of the if statement gets  
executed (ie the bit between the 2 ends) and so nothing happens.

Fred

>
> -- 
> Posted via http://www.ruby-forum.com/.
>
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Neil Cauldwell

2008-Mar-16 18:24 UTC

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Re: Comparing datetimes for conditional

Frederick Cheung wrote:> On 16 Mar 2008, at 17:23, Neil Cauldwell wrote:
> 
>> the first post? And how would you go about comparing two Time  
>> objects in
>> a conditional?
>>
> You would just do a > b
> 
>> As for the ''do'', I made a more detailed post about
the RSS parsing and
>> controller action over on Railsforum,
>> http://railsforum.com/viewtopic.php?id=16479. I didn''t think
it was
>> necessary to include all that info in this thread, but now
we''re on
>> the
>> topic, rather than me pasting the same stuff all over the Interwebs,
>> maybe you could check out the last post in the thread over there and
>> I''ll save the Rails community from a potential
cross-threading!
>>
> 
> 
> so what you''ve got is:
> 
> if news_item[:pubDate] > @source.last_checked_at do
>    # some stuff
> end
> end
> 
> (your original indentation rather hides the fact that there are 2 ends).
> 
> What this does is call the last_checked_at method on @source, passing
> the contents of the do-end block as  the block to that method (whereas
> i suspect you think it''s the body of the if statement). that block
> never gets evaluated because last_checked_at doesn''t yield to it.
If
> the condition is true then the actual body of the if statement gets
> executed (ie the bit between the 2 ends) and so nothing happens.
> 
> Fred
Okay, I understand that - but now I don''t know the right way to
approach
my problem. I''ve taken a stab at it, but how would you change the 
following action so that it only creates a news_item if the pubDate is 
later (greater) than the @source.last_checked_at(?);

   1. class SourceUpdatesController < ApplicationController
   2.
   3.   def index
   4.     @source = Source.find(:first, :order => "last_checked_at
ASC")
   5.
   6.     feed = RssParser.run("#{@source.rss_url}")
   7.     news_item = feed[:items][0]
   8.     news_item[:pubDate] = Time.parse(news_item[:pubDate].to_s)
   9.
  10.     # Create a new news_item
  11.     if (news_item[:pubDate] > @source.last_checked_at) == true
  12.       @news_item = NewsItem.new(:url => news_item[:link],
  13.                                 :title => news_item[:title],
  14.                                 :creator => news_item[:creator],
  15.                                 :published_at => 
news_item[:pubDate],
  16.                                 :description => 
news_item[:description],
  17.                                 :source_id => @source.id)
  18.       @news_item.save
  19.     end
  20.
  21.   @source.update_attribute(:last_checked_at, Time.now)
  22.   end
  23.
  24. end



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Frederick Cheung

2008-Mar-16 19:10 UTC

head link

Re: Comparing datetimes for conditional

On 16 Mar 2008, at 18:24, Neil Cauldwell wrote:
>
> Okay, I understand that - but now I don''t know the right way to  
> approach
> my problem. I''ve taken a stab at it, but how would you change the
> following action so that it only creates a news_item if the pubDate is
> later (greater) than the @source.last_checked_at(?);
>just
if news_item[:pubDate] > @source.last_checked_at
   #do stuff
end

Should be enough. If it''s not working, check that the time is getting  
parsed properly. Also check that the save isn''t failing.

Fred
>   1. class SourceUpdatesController < ApplicationController
>   2.
>   3.   def index
>   4.     @source = Source.find(:first, :order => "last_checked_at  
> ASC")
>   5.
>   6.     feed = RssParser.run("#{@source.rss_url}")
>   7.     news_item = feed[:items][0]
>   8.     news_item[:pubDate] = Time.parse(news_item[:pubDate].to_s)
>   9.
>  10.     # Create a new news_item
>  11.     if (news_item[:pubDate] > @source.last_checked_at) == true
>  12.       @news_item = NewsItem.new(:url => news_item[:link],
>  13.                                 :title => news_item[:title],
>  14.                                 :creator => news_item[:creator],
>  15.                                 :published_at =>
> news_item[:pubDate],
>  16.                                 :description =>
> news_item[:description],
>  17.                                 :source_id => @source.id)
>  18.       @news_item.save
>  19.     end
>  20.
>  21.   @source.update_attribute(:last_checked_at, Time.now)
>  22.   end
>  23.
>  24. end
>
>
>
> -- 
> Posted via http://www.ruby-forum.com/.
>
> >

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Neil Cauldwell

2008-Mar-16 19:38 UTC

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Re: Comparing datetimes for conditional

Frederick Cheung wrote:> On 16 Mar 2008, at 18:24, Neil Cauldwell wrote:
> 
>>
>> Okay, I understand that - but now I don''t know the right way
to
>> approach
>> my problem. I''ve taken a stab at it, but how would you change
the
>> following action so that it only creates a news_item if the pubDate is
>> later (greater) than the @source.last_checked_at(?);
>>
> just
> if news_item[:pubDate] > @source.last_checked_at
>    #do stuff
> end
> 
> Should be enough. If it''s not working, check that the time is
getting
> parsed properly. Also check that the save isn''t failing.
> 
> Fred
Thanks Fred. Obviously it was working with and without "== true", I
just
made the mistake of comparing the pubDate to the last_checked_at date 
without considering the possibility of the timezone from which the feed 
is published being ahead of that on my local machine. The action was 
duplicating news_items for one feed because of the time difference, and 
that''s why I thought it wasn''t working correctly.

There probably is a better way to tackle this, but for now I''m storing 
an additional ''last_item_published_at'' attribute for the
Source, and
using that in the conditional instead. I imagine the attribute will come 
in handy further down the line, anyway. Although, I now need to work out 
how to handle this attribute when a ''source'' is created. Maybe
I''ll do
an initial check on the feed pubDate in the ''create'' action.
Could I use
the same helpers that Rails uses to handle the defaults for created_at 
and updated_at but with an additional DateTime attribute? Or do you have 
a better suggestion?

p.s. How would you go about checking RSS feeds for updates? Would you 
use something wildly different to what I have?

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Rails - Mar 2008 - Comparing datetimes for conditional

Comparing datetimes for conditional

Re: Comparing datetimes for conditional

Re: Comparing datetimes for conditional

Re: Comparing datetimes for conditional

Re: Comparing datetimes for conditional

Re: Comparing datetimes for conditional

Re: Comparing datetimes for conditional