# Comparing datetimes for conditional

**URL:** <https://www.ruby-forum.com/t/comparing-datetimes-for-conditional/132081>\
**Category:** Rails\
**Created:** [March 16, 2008, 5:48pm UTC](https://www.ruby-forum.com/t/comparing-datetimes-for-conditional/132081 "2008-03-16T17:48:00Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![chunkyslink](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/f0a364/32.png) [@chunkyslink](https://www.ruby-forum.com/u/chunkyslink)\
**Post date:** [March 16, 2008, 5:48pm UTC](https://www.ruby-forum.com/t/comparing-datetimes-for-conditional/132081/1 "2008-03-16T17:48:00Z")

</div>

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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**Author:** ![chunkyslink](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/f0a364/32.png) [@chunkyslink](https://www.ruby-forum.com/u/chunkyslink)\
**Post date:** [March 16, 2008, 5:55pm UTC](https://www.ruby-forum.com/t/comparing-datetimes-for-conditional/132081/2 "2008-03-16T17:55:26Z")

</div>

On 16 Mar 2008, at 16:48, Neil C. 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

---

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**Author:** ![chunkyslink](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/f0a364/32.png) [@chunkyslink](https://www.ruby-forum.com/u/chunkyslink)\
**Post date:** [March 16, 2008, 6:23pm UTC](https://www.ruby-forum.com/t/comparing-datetimes-for-conditional/132081/3 "2008-03-16T18:23:21Z")

</div>

Frederick C. wrote:

> On 16 Mar 2008, at 16:48, Neil C. 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](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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**Author:** ![chunkyslink](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/f0a364/32.png) [@chunkyslink](https://www.ruby-forum.com/u/chunkyslink)\
**Post date:** [March 16, 2008, 6:49pm UTC](https://www.ruby-forum.com/t/comparing-datetimes-for-conditional/132081/4 "2008-03-16T18:49:42Z")

</div>

On 16 Mar 2008, at 17:23, Neil C. 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](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

---

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**Author:** ![chunkyslink](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/f0a364/32.png) [@chunkyslink](https://www.ruby-forum.com/u/chunkyslink)\
**Post date:** [March 16, 2008, 7:24pm UTC](https://www.ruby-forum.com/t/comparing-datetimes-for-conditional/132081/5 "2008-03-16T19:24:59Z")

</div>

Frederick C. wrote:

> On 16 Mar 2008, at 17:23, Neil C. 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](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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**Author:** ![chunkyslink](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/f0a364/32.png) [@chunkyslink](https://www.ruby-forum.com/u/chunkyslink)\
**Post date:** [March 16, 2008, 8:38pm UTC](https://www.ruby-forum.com/t/comparing-datetimes-for-conditional/132081/6 "2008-03-16T20:38:48Z")

</div>

Frederick C. wrote:

> On 16 Mar 2008, at 18:24, Neil C. 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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**Author:** ![chunkyslink](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/f0a364/32.png) [@chunkyslink](https://www.ruby-forum.com/u/chunkyslink)\
**Post date:** [March 16, 2008, 8:11pm UTC](https://www.ruby-forum.com/t/comparing-datetimes-for-conditional/132081/7 "2008-03-16T20:11:30Z")

</div>

On 16 Mar 2008, at 18:24, Neil C. 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