Help With Update method

Hi everybody,

I have some problem with rails update method.

My actual mysql query is like:

Update accounts set balance=balance+1000 where id=1

I tried with rails method update like :

Account.update(1,{:balance=>balance+1000})

But its not working, Can any one suggest the correct syntax for this.

I want to refer with previous value in that record and then want to
increment.

Regards,
Veeraa.

Veera S. wrote:

Hi everybody,

I have some problem with rails update method.

My actual mysql query is like:

Update accounts set balance=balance+1000 where id=1

I tried with rails method update like :

Account.update(1,{:balance=>balance+1000})

But its not working, Can any one suggest the correct syntax for this.

I want to refer with previous value in that record and then want to
increment.

Regards,
Veeraa.

Hi everybody,

Got the answer for this :
@account=Account.update(1,{:closing_balance=>@account.closing_balance+100})

Regards,
Veeraa.

On Dec 28, 2007, at 9:38 PM, Veera S. wrote:

Account.update(1,{:balance=>balance+1000})

But its not working, Can any one suggest the correct syntax for this.

I want to refer with previous value in that record and then want to
increment.

account = Account.find(1)
account.balance += 1000
account.save

Peace,
Phillip

On Dec 29, 2007, at 4:02 AM, Lionel B. wrote:

under any condition (at least with PostgreSQL).

Hi. Maybe it’s too early or maybe I’m just dense, but would you mind
explaining your statement? Maybe I don’t know enough about your
specific situation, but what I offered is the standard Rails way of
updating the attributes of an object. What are the business rules
around the update of this “account”?

Thanks,
Phillip

On Dec 29, 2007, at 4:02 AM, Lionel B. wrote:

Hm. For some reason, part of my reply was lost as it was applied to
ruby-forum ( I participate through email). Let me restate then:

Why do you say this is a source of race conditions? Are you using
this in a situation where many people could be updating the record at
the same time? If only one person could be updating, how can it be a
potential race? If this is in a multi-user environment, you didn’t
mention that earlier, so how was I to know? :slight_smile:

Peace,
Phillip

Phillip K. wrote the following on 29.12.2007 13:51 :

Account.update_all({ :id => 1 }, ‘balance = balance + 1000’) is safe
under any condition (at least with PostgreSQL).

Hi. Maybe it’s too early or maybe I’m just dense, but would you mind
explaining your statement? Maybe I don’t know enough about your
specific situation, but what I offered is the standard Rails way of
updating the attributes of an object. What are the business rules
around the update of this “account”?

I don’t know the business rules of this particular case, but usually
simultaneous account balance manipulations are expected to behave in the
way they would have if they’d been serialized. So clearly you’d want to
avoid the race condition I’m referring to in my previous post.

I merely pointed out that your example can’t enforce this by itself an
presented a solution. In your case you could use a transaction and lock
the account to be sure using account.lock or Account.find(1, :lock =>
true), it may makes sense in some situations but is slower and probably
more error-prone than the update_all solution.

Lionel

Phillip K. wrote:

account = Account.find(1)
account.balance += 1000
account.save

That’s a source of race conditions.

Account.update_all({ :id => 1 }, ‘balance = balance + 1000’) is safe
under any condition (at least with PostgreSQL).

Lionel

On Dec 29, 2007, at 7:17 AM, Lionel B. wrote:

I’m not the original poster so I’m not using this code. I was simply
pointing a potential source of problems that could potentially be of
concerns to people using the kind of code you presented.

Lionel

Ah, so you’re not. Okay, then it is too early. Maybe I should drink
more coffee and do some jumping jacks or something before I get post
happy.

Thanks for clearing that up, Lionel. You’re absolutely right. Used
in the wrong situation, the code that I provided would not be
reliable. Used in the right situation, however, it is perfectly
acceptable.

Peace,
Phillip

Phillip K. wrote the following on 29.12.2007 13:59 :

Why do you say this is a source of race conditions? Are you using
this in a situation where many people could be updating the record at
the same time? If only one person could be updating, how can it be a
potential race? If this is in a multi-user environment, you didn’t
mention that earlier, so how was I to know? :slight_smile:

I’m not the original poster so I’m not using this code. I was simply
pointing a potential source of problems that could potentially be of
concerns to people using the kind of code you presented.

Lionel

On Dec 29, 2:02 am, Lionel B. [email protected]
wrote:

Account.update_all({ :id => 1 }, ‘balance = balance + 1000’)

Not that I’m suggesting it, but Account.update_counters(1, :balance =>
1000) would also work.

///ark

Mark W. wrote:

On Dec 29, 2:02 am, Lionel B. [email protected]
wrote:

Account.update_all({ :id => 1 }, ‘balance = balance + 1000’)

Not that I’m suggesting it,

Why? Seems pretty safe and concise to me :slight_smile:

but Account.update_counters(1, :balance =>
1000) would also work.

I’ll have to remember this when switching to Rails 2 (I looked it up in
1.2.6’s rdoc and didn’t find it).

BTW, I think I was confused by the initial examples, the syntax in my
example is not right, it should probably be:

Account.update_all(‘balance = balance + 1000’, { :id => 1 })

and what I know works on Rails since 1.0 is:

Account.update_all(‘balance = balance + 1000’, ‘id = 1’)

Lionel

Wouldn’t it be super-cool if increment took a number argument as well?

That way you could do:

@account = Account.find(params[:id])
@account.increment!(“balance”,1000)

And it would do balance += 1000 for you.

On Dec 29, 2007, at 9:21 PM, Ryan B. wrote:

Wouldn’t it be super-cool if increment took a number argument as well?

That way you could do:

@account = Account.find(params[:id])
@account.increment!(“balance”,1000)

And it would do balance += 1000 for you.

Just a casual glance… Would like this work?

module ActiveRecord
class Base
def change_value(attribute, amount)
self[attribute] += amount
end

 def change_value!(attribute, amount)
   change_value(attribute, amount).update_attribute(attribute,

self[attribute])
end

 def increment(attribute, amount = 1)
   change_value(attribute, amount)
 end

 def increment!(attribute, amount = 1)
   change_value(attribute, amount)
 end

 def decrement(attribute, amount = 1)
   change_value(attribute, -amount)
 end

 def decrement!(attribute, amount = 1)
   change_value(attribute, -amount)
 end

end
end

By the looks of things I would say yes. Very nicely done Mr. Ross.
Perhaps
you could submit it as a proposed improvement on Rails?

On Dec 30, 2007 5:39 PM, s.ross [email protected] wrote:

def change_value!(attribute, amount)
end


Ryan B.
http://www.frozenplague.net
Feel free to add me to MSN and/or GTalk as this email.

Done! Thanks a lot for that.

On Dec 31, 2007 10:33 AM, s.ross [email protected] wrote:

By the looks of things I would say yes. Very nicely done Mr. Ross. Perhaps

That way you could do:

@account = Account.find(params[:id])
@account.increment!(“balance”,1000)

And it would do balance += 1000 for you.


Ryan B.
http://www.frozenplague.net
Feel free to add me to MSN and/or GTalk as this email.

Well, if you really like it, it’s ticket 10656
(http://dev.rubyonrails.org/attachment/ticket/10656/change_ar_increment_to_accept_by_argument.diff
). +1 it if you think it’s a good idea.

Happy New Year (early),

–steve

On Dec 30, 2007, at 4:27 PM, Ryan B. wrote:

Just reading through the code, I noticed the comments for the patch
are actually within the method. Shouldn’t they be above the method,
so they can be documented?

I think the diff reads that way. Note that the method signature
changed so the first def is in red (deleted), then the comment(s),
then the new def with the new method signature are in green (added).
Did I miss something?

Ah right, I see. I blame the heat here for messing with my head. My
apologies.

On Dec 31, 2007 11:03 AM, s.ross [email protected] wrote:

Did I miss something?


Ryan B.
http://www.frozenplague.net
Feel free to add me to MSN and/or GTalk as this email.

Just reading through the code, I noticed the comments for the patch are
actually within the method. Shouldn’t they be above the method, so they
can
be documented?

On Dec 31, 2007 10:38 AM, Ryan B. [email protected] wrote:

http://www.frozenplague.net
Feel free to add me to MSN and/or GTalk as this email.


Ryan B.
http://www.frozenplague.net
Feel free to add me to MSN and/or GTalk as this email.

On Dec 29, 11:09 pm, “s.ross” [email protected] wrote:

Just a casual glance… Would like this work?

module ActiveRecord
class Base
def change_value(attribute, amount)
self[attribute] += amount
end

This looks good (some of the calls to change_value should be calling
change_value!), but I think it’s too close to existing functionality
(update_counters).

///ark

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