Get a collection using an array of ids, keeping the order

Rails
1

Say if i have an array of resource ids, of size n. Is there a way to
get all the corresponding resources, without doing n individual Find
calls?

eg, i could do this:

@resources = []
array.each{|id| @resources << Resource.find(id)}

But, this requires n Find calls which isn’t acceptable.

Any ideas anyone?

thanks
max

2

On 4 Dec 2008, at 12:24, Max W. wrote:

But, this requires n Find calls which isn’t acceptable.
Resource.find @resources

Fred

3

On 4 Dec 2008, at 12:31, Frederick C. wrote:

@resources = []
array.each{|id| @resources << Resource.find(id)}

But, this requires n Find calls which isn’t acceptable.

Resource.find @resources

Sorry, only read the body of the message, not the subject!
I would sort it after, eg

order_hash = {}
array.each_with_index {|id, index| order_hash[id]=index
Resource.find(array).sort_by {|r| order_hash[r.id]}

Fred

4

Frederick C. wrote:

On 4 Dec 2008, at 12:31, Frederick C. wrote:

@resources = []
array.each{|id| @resources << Resource.find(id)}

But, this requires n Find calls which isn’t acceptable.

Resource.find @resources

Sorry, only read the body of the message, not the subject!
I would sort it after, eg

order_hash = {}
array.each_with_index {|id, index| order_hash[id]=index
Resource.find(array).sort_by {|r| order_hash[r.id]}

Fred

ah yes, of course. Nice touch with the hash (as opposed to sorting by
array.index) :slight_smile:

Thanks a lot, Fred.

5

On Dec 4, 2008, at 7:46 AM, Max W. wrote:

Sorry, only read the body of the message, not the subject!

Thanks a lot, Fred.

YMMV, but why bother with the hash? Unless you have a truly huge list
of id’s

@resources = Resource.find(array).sort_by {|r| array.index(r.id)}

You could even hide the implementation:

class Resource
def self.find_by_ordered_ids(*array)
options = {}
options = array.pop if array.last.is_a? Hash
# pass an Array or a list of id arguments
array = array.flatten if array.first.is_a? Array
find(array, options).sort_by {|r| array.index(r.id)}
end
end

(Note, untested, but that doesn’t mean that it won’t work :wink:

-Rob

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

6

If you use MySQL, the “field” function allows you
to retrieve records in a specific id order:

http://groups.google.com/group/rubyonrails-talk/browse_thread/thread/158dc2d879b2fb1/af78ce75ddfa1ed1

–
Rails Wheels - Find Plugins, List & Sell Plugins -
http://railswheels.com

7

Mark Reginald J. wrote:

If you use MySQL, the “field” function allows you
to retrieve records in a specific id order:

http://groups.google.com/group/rubyonrails-talk/browse_thread/thread/158dc2d879b2fb1/af78ce75ddfa1ed1

–
Rails Wheels - Find Plugins, List & Sell Plugins -
http://railswheels.com

Ah, now that is a great tip, i’d not seen this before! Thanks a lot.

The weird thing is, i’ve gone to my console to try it out, and rails is
now always giving me the resources back in the order of the id array
anyway, whether i use the field option or not. It’s the same with any
of my AR classes! I’m sure that this didn’t used to happen though…i
think i’m going crazy. Can anyone tell me if this is normal? If so
then i just imagined this whole problem…

AModel.find([3,1,2]).collect(&:id)
=> [3,1,2]

or do you get

=> [1,2,3]

?

8

Max W. wrote:

The weird thing is, i’ve gone to my console to try it out, and rails is
now always giving me the resources back in the order of the id array
anyway, whether i use the field option or not. It’s the same with any
of my AR classes! I’m sure that this didn’t used to happen though…i
think i’m going crazy. Can anyone tell me if this is normal?

You are not going crazy.

MySQL may, but is not required to, return the objects in the order of
the array. So, the behavior will change without warning.

You still need to sort after the fetch.

9

On 23 August 2010 03:18, Robert H. [email protected] wrote:

Max W. wrote:

i think i’m going crazy. Can anyone tell me if this is normal?

You are not going crazy.

I am! This is a reply to a two-year-old thread… I just wasted 10
minutes trying to find out why I hadn’t received the previous
messages. I thought my email spam filter was being paranoid :-/