Rails/ActiveRecord: Return order with ActiveRecord#find(Array)

activerecordruby-on-rails

I'm using Ruby on Rails/ActiveRecord and am having trouble with an ActiveRecord#find call. I'm storing in the database a serialized array of recently viewed documents IDs. The IDs are stored in descending order of when they were last viewed, so the most recently viewed document ID is first in the array. The array includes 10 IDs maximum.

So far, so good. The problem is that ActiveRecord#find(Array) seems to ignore the order in which the ideas appear in the array. So if I type Document.find([1, 2, 3]), I get the same result as if I do Document.find([3, 2, 1]).

My question, then, is this: how can I get an ActiveRecord result array that is in the same order as the IDs I passed to #find? Or, if ActiveRecord doesn't make this possible directly, how can I sort the resulting Array after the fact?

Thanks so much for any answers people can contribute!

Best Answer

ActiveRecord is an interface to your database and returns the records to you in the same order that the database returns them. If you don't supply an 'order' parameter then the returned order is (effectively) random.

If your order is by id ascending or descending:

results = SomeModelName.find([1,2,3], :order => "id") # ascending order
results = SomeModelName.find([1,2,3], :order => "id desc") # descending order

If id order is not ascending or descending:

ids = [1, 3, 2]
r = SomeModelName.find(ids)
results = ids.map{|id| r.detect{|each| each.id == id}}

Related Solutions

Ruby-on-rails – Rails: How to set default values in ActiveRecord

There are several issues with each of the available methods, but I believe that defining an after_initialize callback is the way to go for the following reasons:

default_scope will initialize values for new models, but then that will become the scope on which you find the model. If you just want to initialize some numbers to 0 then this is not what you want.
Defining defaults in your migration also works part of the time... As has already been mentioned this will not work when you just call Model.new.
Overriding initialize can work, but don't forget to call super!
Using a plugin like phusion's is getting a bit ridiculous. This is ruby, do we really need a plugin just to initialize some default values?
Overriding after_initialize is deprecated as of Rails 3. When I override after_initialize in rails 3.0.3 I get the following warning in the console:

DEPRECATION WARNING: Base#after_initialize has been deprecated, please use Base.after_initialize :method instead. (called from /Users/me/myapp/app/models/my_model:15)

Therefore I'd say write an after_initialize callback, which lets you default attributes in addition to letting you set defaults on associations like so:

  class Person < ActiveRecord::Base
    has_one :address
    after_initialize :init

    def init
      self.number  ||= 0.0           #will set the default value only if it's nil
      self.address ||= build_address #let's you set a default association
    end
  end

Now you have just one place to look for initialization of your models. I'm using this method until someone comes up with a better one.

Caveats:

For boolean fields do:

self.bool_field = true if self.bool_field.nil?

See Paul Russell's comment on this answer for more details
If you're only selecting a subset of columns for a model (ie; using select in a query like Person.select(:firstname, :lastname).all) you will get a MissingAttributeError if your init method accesses a column that hasn't been included in the select clause. You can guard against this case like so:

self.number ||= 0.0 if self.has_attribute? :number

and for a boolean column...

self.bool_field = true if (self.has_attribute? :bool_value) && self.bool_field.nil?

Also note that the syntax is different prior to Rails 3.2 (see Cliff Darling's comment below)

Ruby-on-rails – Clean way to find ActiveRecord objects by id in the order specified

It's not that MySQL and other DBs sort things on their own, it's that they don't sort them. When you call Model.find([5, 2, 3]), the SQL generated is something like:

SELECT * FROM models WHERE models.id IN (5, 2, 3)

This doesn't specify an order, just the set of records you want returned. It turns out that generally MySQL will return the database rows in 'id' order, but there's no guarantee of this.

The only way to get the database to return records in a guaranteed order is to add an order clause. If your records will always be returned in a particular order, then you can add a sort column to the db and do Model.find([5, 2, 3], :order => 'sort_column'). If this isn't the case, you'll have to do the sorting in code:

ids = [5, 2, 3]
records = Model.find(ids)
sorted_records = ids.collect {|id| records.detect {|x| x.id == id}}

Best Answer

Related Solutions

Ruby-on-rails – Rails: How to set default values in ActiveRecord

Ruby-on-rails – Clean way to find ActiveRecord objects by id in the order specified

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