# Collection.last does not seem to jive with collection.count after collection.build

**URL:** <https://www.ruby-forum.com/t/collection-last-does-not-seem-to-jive-with-collection-count-after-collection-build/189266>\
**Category:** Rails\
**Created:** [May 27, 2010, 6:49pm UTC](https://www.ruby-forum.com/t/collection-last-does-not-seem-to-jive-with-collection-count-after-collection-build/189266 "2010-05-27T18:49:27Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![dee](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/58956e/32.png) [@dee](https://www.ruby-forum.com/u/dee)\
**Post date:** [May 27, 2010, 6:49pm UTC](https://www.ruby-forum.com/t/collection-last-does-not-seem-to-jive-with-collection-count-after-collection-build/189266/1 "2010-05-27T18:49:27Z")

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I was just trying to write a “smart” copy method in a class\* and  
noticed that the last and count collection methods don’t work the same  
after build and before save. In other words, if in my code I do this:

```
user = User.new
user.save
purchase = user.purchases.build

```

Then I get the following:  
user.purchases.count =\> 0  
user.purchases.last =\> #Purchase:0x01234567

It’s never bothered me before now. As a human, I can ask the question  
how can the count be 0 if the collection has a last element? I would  
expect the last method to return nil in this case because I haven’t  
saved purchase yet.

I’m assuming the solution is to create my own “last” method, one that  
filters out records with a nil value for created\_at. Or another build  
method that calls last before build. But both solutions seem un-DRY,  
or worse un-DRRoR. But I thought maybe I am missing something.

\*In case you’re wondering, the method I’m writing copies certain  
values of the last “purchase” model created by that user into a new  
one, as a way of setting some default values. Maybe there is a more  
rails-ee way of doing that too.

Thanks in advance.

---

<div class="post-metadata">

**Author:** ![dee](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/58956e/32.png) [@dee](https://www.ruby-forum.com/u/dee)\
**Post date:** [May 27, 2010, 9:11pm UTC](https://www.ruby-forum.com/t/collection-last-does-not-seem-to-jive-with-collection-count-after-collection-build/189266/2 "2010-05-27T21:11:11Z")

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On May 27, 11:47 am, Dee [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> ```
> user.purchases.last => #<Purchase:0x01234567>
> 
> ```

That’s because .count is using SQL to look at the table, but you  
haven’t committed the new purchase to the db yet. .last knows to use  
the in-memory collection instead.

Instead of .count, use .size, which is aware of in-memory changes.

That said, consider using .clone, which will copy an object for you  
that you can start with.

Jeff  
[purpleworkshops.com](http://purpleworkshops.com)

---

<div class="post-metadata">

**Author:** ![dee](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/58956e/32.png) [@dee](https://www.ruby-forum.com/u/dee)\
**Post date:** [May 27, 2010, 10:35pm UTC](https://www.ruby-forum.com/t/collection-last-does-not-seem-to-jive-with-collection-count-after-collection-build/189266/3 "2010-05-27T22:35:31Z")

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Thanks, Jeff.

Thanks for the clarification of the difference between .size  
and .count. I was actually looking more for something that does .last  
like .count, rather than .count like .last. I got around that by doing  
and :order\_by =\> ‘created\_at’ before using .last, which puts the nils  
at the head, at least in MySQL and SQLite.

Big thanks for clone. It’s brilliant. I didn’t know it existed. It was  
beginning to look like a maintenance nightmare without it.