# How do I create an array to represent an empty record?

**URL:** <https://www.ruby-forum.com/t/how-do-i-create-an-array-to-represent-an-empty-record/77612>\
**Category:** Rails\
**Created:** [November 29, 2006, 5:46am UTC](https://www.ruby-forum.com/t/how-do-i-create-an-array-to-represent-an-empty-record/77612 "2006-11-29T05:46:03Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![shauna](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/6f9a4e/32.png) [@shauna](https://www.ruby-forum.com/u/shauna)\
**Post date:** [November 29, 2006, 5:46am UTC](https://www.ruby-forum.com/t/how-do-i-create-an-array-to-represent-an-empty-record/77612/1 "2006-11-29T05:46:03Z")

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Here’s something that’s a snap to do in desktop database management  
systems (like Paradox or Access) but I haven’t figured out how to do it  
in RoR, and can’t seem to find any examples by googling, etc.

My overall goal is to create a temporary space in which to create and  
process a single new record (representating a payment transaction), and  
when I’m done with it post it to the payments table. Some of the field  
values will be collected from the user and validated, then others will  
be calculated based on the user-supplied values and other factors. I  
don’t want to edit the payments table until I’m ready to post the  
completed record.

In the desktop software, this would be done by creating an array based  
on the payment record’s column names, using that to gather and validate  
the data, then copying the array values into a new record. A single  
command can copytoarray or copyfromarray a record.

I tried modelling this on the Cart example in Agile Web D. with  
Rails (which stores virtual lineitem records in the shopping cart). I  
was able to put a single empty record into @pmttrans, but couldn’t  
figure out how to address a field in that record in order to read or  
write a specific data value. The Cart example creates an @items array  
within @cart. Since I don’t need multiple items, how can I just create a  
simple hash of column names and values for the single record?

Or is there an even simpler way to approach this that I’m missing?

Thanks!  
Shauna

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<div class="post-metadata">

**Author:** ![shauna](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/6f9a4e/32.png) [@shauna](https://www.ruby-forum.com/u/shauna)\
**Post date:** [November 29, 2006, 6:26am UTC](https://www.ruby-forum.com/t/how-do-i-create-an-array-to-represent-an-empty-record/77612/2 "2006-11-29T06:26:05Z")

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do you mean like this (for a pointless example)?:

payment = Payment.new

payment.total = params[:total]  
payment.tax = payment.total \* 0.9

if payment.save  
# Data is now save to database table  
end

The point being that the table is only modified when you call the save  
method when creating a new activeRecord object.

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<div class="post-metadata">

**Author:** ![shauna](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/6f9a4e/32.png) [@shauna](https://www.ruby-forum.com/u/shauna)\
**Post date:** [November 29, 2006, 6:56am UTC](https://www.ruby-forum.com/t/how-do-i-create-an-array-to-represent-an-empty-record/77612/3 "2006-11-29T06:56:28Z")

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Thanks. That’s the way I first tried it, but I’m concerned about two  
things. First, a transaction number is generated based on existing  
records in payment. What if another user tries to post a record to the  
same loan while the first user is taking their time with the first post?  
Also, is the payment table actually open when you execute Payment.new?  
I’d like to open it only if and when I’m ready to post.

Or are these questions not relevant to a web-based app, and I need not  
worry about it?

Shauna

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<div class="post-metadata">

**Author:** ![shauna](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/6f9a4e/32.png) [@shauna](https://www.ruby-forum.com/u/shauna)\
**Post date:** [November 29, 2006, 7:33am UTC](https://www.ruby-forum.com/t/how-do-i-create-an-array-to-represent-an-empty-record/77612/4 "2006-11-29T07:33:30Z")

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Well, I’ve been trying to mimic the existing desktop app, where the  
number is presented first and then the user enters the date, selects the  
transaction code from a drop-down list, and fills in a few relevant  
fields which vary based on the transaction code they selected. It’s not  
always a simple consecutive number, because within a pay period there  
can be multiple decimalled transactions. But I’m just guessing that the  
user needs to verify this number before proceeding… I’ll ask the  
original developer. (Really I’d like to poll the existing users 🙂 )

Thanks for the help.

Shauna

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<div class="post-metadata">

**Author:** ![shauna](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/6f9a4e/32.png) [@shauna](https://www.ruby-forum.com/u/shauna)\
**Post date:** [November 29, 2006, 7:10am UTC](https://www.ruby-forum.com/t/how-do-i-create-an-array-to-represent-an-empty-record/77612/5 "2006-11-29T07:10:13Z")

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The table is not touched until you call save (and then only if the  
object validates)…

So you generate a transaction number before receiving all the  
information from the user? Is this necessary? Could you give them the  
transaction number after it has been accepted?

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<div class="post-metadata">

**Author:** ![shauna](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/6f9a4e/32.png) [@shauna](https://www.ruby-forum.com/u/shauna)\
**Post date:** [November 30, 2006, 9:35am UTC](https://www.ruby-forum.com/t/how-do-i-create-an-array-to-represent-an-empty-record/77612/6 "2006-11-30T09:35:15Z")

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Hi unknown (Guest),

I know whereof you speak but the transaction number in this case is not  
a unique identifier in the payments table, it’s just unique within a  
particular loan. Although it’s a long shot that two different users  
might possibly be entering transactions for the same loan, I just wanted  
to be sure when and where I’d have to intervene if it did occur. The  
numbers do need to be sequential for human reasons (so that the order in  
which the transactions for a particular loan were entered is visually  
obvious when data is retrieved for reports etc.).

Shauna

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<div class="post-metadata">

**Author:** ![shauna](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/6f9a4e/32.png) [@shauna](https://www.ruby-forum.com/u/shauna)\
**Post date:** [November 29, 2006, 2:08pm UTC](https://www.ruby-forum.com/t/how-do-i-create-an-array-to-represent-an-empty-record/77612/7 "2006-11-29T14:08:59Z")

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This is one of the classical breaking points of object-relational  
mapping - the transaction boundary. It occurs in single user and  
mutli-user paradigms equally.

If, for auditing purposes, there is a requirement that transaction  
numbers be sequential, _and_ you don’t need a transaction number until  
the transaction is comitted, then generate your transaction ID at save  
time within the boundaries of the database transaction.

If, for auditing purposes, there is a requirement that transaction  
numbers be sequential, _and_ you need transaction numbers early, then  
make sure that you record thetransactionearly, and be prepared to  
record the transaction as being canceled by user or aborted due to  
technical failure.

If there is no need for keeping the transaction number sequential (not  
likely), then youcan throw away unused numbers. Numbers are cheap, if  
they have enough digits.