Updating a DB entry without having to instantiate an object

Rails
1

Hi,

I noticed that in order to use update_attributes(), AR firsts needs to
make a SELECT query to the DB to fetch the data, then AR creates an
object from it, and then it issues the UPDATE statement.

What I am trying to do is update a field in the DB without having to do
this preliminary SELECT (thus saving a query on my InnoDB table) and the
subsequent ORM that will occur. Is it possible with AR? Or do I need to
use my own custom SQL query?

2

I don’t know if it’s possible, but I’d like to ask: Have you measured
the performance of your app and found it to be unsatisfactory and the
cause to be update_attributes? If no, then just let update_attributes
do it’s thing. If yes, then I’m sure you’ll receive some helpful
advice from those who know.

Regards,
Craig

3

Hi Craig, I know what you mean, but I prefer to have my app slightly
optimized when it seems it is not yet required. I am sure that the
claims about Rails being slow comes from people who rely too much on AR
magics to handle SQL, and they don’t notice that they are making a pure
mess with their DB server being hit too many times.

It is like talking about the n+1 problem, you don’t see its drawbacks
when the DB is small, but you absolutely must get rid of it from the
very beginning even if the current speed measured is satisfactory.

It’s by leaving small bits and pieces that will eventually pile up that
you get a sluggish application.

4

On May 17, 2008, at 2:44 PM, Fernando P. wrote:

subsequent ORM that will occur. Is it possible with AR? Or do I need
to
use my own custom SQL query?

Have you looked at update_all?

MyModel.update_all([‘hair = ?, eye = ?’, ‘brown’, ‘green’],
[‘full_name = ?’, ‘Fernando P.’])

The first argument is the setting part of the SQL and the second
argument is the where conditions. They are run through sanitize_sql.
It’s just one step away from

MyModel.connection.execute(“UPDATE my_models SET hair = ‘brown’, eye =
‘green’ WHERE full_name = ‘Fernando P.’”)

-Rob

Rob B. http://agileconsultingllc.com
[email protected]

5

Rob is right, update_all will take care of updating without
instantiating.

I don’t believe, though, that update_attributes itself triggers a
select. More likely you’re seeing the select from initially
retrieving the object on which you are calling update_attributes.
That is, if you’re looking at a typical update action from rails’
scaffolding:

def update

this line triggers the select

@my_object = MyObject.find(params[:id])

this line triggers the update

if @my_object.update_attributes(params[:my_object])
…
end

The advantage you get here is that you can have business logic
triggered from several different places: after_save callbacks,
ActiveRecord observers, etc. You may not need them now but at some
point they’re likely to provide you some extra help that you’ll miss
if you make a habit of bypassing update_attributes.

Just a note… in 2.1 there IS a nice performance enhancement related
to update_attributes. Currently (2.0.x) update_attributes always
updates all the attributes even if you only submit a subset of
attributes for updating. Rails 2.1 fixes this, only updating the
‘dirty’ attributes.

On May 17, 4:40 pm, Rob B. [email protected]

6

AndyV wrote:

Rob is right, update_all will take care of updating without
instantiating.
Exactly, thank you Rob.

this line triggers the update

if @my_object.update_attributes(params[:my_object])
…
end
In order to to the update this way, you first must do a SELECT.

I tried also MyModel.update(), and it also issues a SELECT before making
the update.

Using Rob’s MyModel.update_all() simply does the UPDATE without anything
else.

The advantage you get here is that you can have business logic
triggered from several different places: after_save callbacks,
ActiveRecord observers, etc. You may not need them now but at some
point they’re likely to provide you some extra help that you’ll miss
if you make a habit of bypassing update_attributes.
The models in which I need validation or callbacks use the regular
update_attributes, in this very particular case, there is no need for
initially fetching from DB.

Just a note… in 2.1 there IS a nice performance enhancement related
to update_attributes. Currently (2.0.x) update_attributes always
updates all the attributes even if you only submit a subset of
attributes for updating. Rails 2.1 fixes this, only updating the
‘dirty’ attributes.
At last they did it! That is excellent. Where did you find the changelog
about the new features of 2.1?

Best regards,

7

At last they did it! That is excellent. Where did you find the changelog
about the new features of 2.1?

I just follow the guys who are playing with the latest toys: