Ordering a rails model with string outline notation - ruby-on-rails

Question

I have a rails model that the primary field that the user wants to sort on is a Line Item that is stored in dot-notation format as a string (i.e.: 2.1.4, 2.1.4.1, 2.1.4.5, etc). Ordering alphabetically works great, except that 2.1.4.10 comes before 2.1.4.2 alphabetically. What I want to call 'dot-based numeric order' would put 2.1.4.10 after 2.1.4.9, and 2.4.1.10.1 would precede 2.4.1.11
The question is this: What is The Rails Way™ to set the default order on the model so that the Line Items appear in the correct order, according to 'dot-based numeric order'.
Presume a simple case:
class LineItem < ActiveRecord::Base
validates :line_item, :presence => true, :uniqueness => true
end
and that :line_item is a string.

Answer 1

I assume you are using PostgreSQL and if you really want to set default order for your model, add this default_scope to your LineItem model:
default_scope -> { order("STRING_TO_ARRAY(line_item, '.')::int[] ASC") }
Otherwise I suggest you to use named scope, it can be override and chained:
scope :version_order, -> { order("STRING_TO_ARRAY(line_item, '.')::int[] ASC") }

Answer 2

To do this yourself:
lines = ['3.3.3.3', '3.54.3.3', '3.3.3.20']
sorted = lines.sort do |a, b|
a.split('.').zip(b.split('.')).inject(0) do |res, val|
(res == 0)? val[0].to_i <=> val[1].to_i : res
end
end #=> ["3.3.3.3", "3.3.3.20", "3.54.3.3"]
How it works:
To sort, we pass an array and a block, that blocks gives us 2 arguments that are next to each other in the list, and we can return 0, -1, or 1, which tells Ruby which directions to swap the numbers.
[4,3,-1,2].sort do |x, y|
if x > y
1
elsif x < y
-1
else
0
end
end #=> [-1, 2, 3, 4]
Instead of doing that long logic, Ruby provides a nice operator for us: <=>. Zero means no change, -1 means it's in ascending order, and 1 means the two numbers are in descending order. Ruby repeats that a bunch, and sorts the list.
4 <=> 4 #=> 0
3 <=> 5 #=> -1
5 <=> 3 #=> 1
7 <=> -1 #-> 1
So, we should give higher items (in terms of dots) priority:
#Pseudo Code:
33.44 > 22.55 #=> true
33.44 < 44.33
The easiest way to integrate through all the numbers is an #inject, which gives you a value, and the item you are on. You can do things like this:
[4,4,4].inject(0) {|sum, i| sum + i} #=> 12
[4,4,4].inject(0) {|sum, i| sum - i} #=> -12
['Hello', "I'm penne12"] {|new_word, word| new_word + "-" + word} #=> "Hello-I'm penne12"
So, we'll use an inline if:
(true)? "it's true" : "true is now false. Yay!" #=> "it's true"
(4 > 5)? "logic is weird" : "4 > 5" #=> "4 > 5"
Like this:
.inject(0) do |res, val|
(res == 0)? val[0].to_i <=> val[1].to_i : res
end
We'll split both strings by the ., to get a list:
"Hello. This. Is. A. Test.".split('.') #=> ["Hello", " This", " Is", " A", "Test"]
"4.4.4.4" #=> [4,4,4,4]
And join the two lists together by element using ruby's #Zip (it's really weird.)
[4,4,4,4].zip([5,5,5,5]) #=> [[4,5], [4,5], [4,5], [4,5]]
You can change what item a and b are, if you want to sort by a different property. Ruby doesn't care what you do to either variable, it only cares about the return value.
a, b = a.line_item, b.line_item
On a model:
class LineItem < ActiveRecord::Base
validates :line_item, :presence => true, :uniqueness => true
def self.sort_by_dbno
self.all.sort do |a, b|
a, b = a.line_item, b.line_item
a.split('.').zip(b.split('.')).inject(0) do |res, val|
(res == 0)? val[0].to_i <=> val[1].to_i : res
end
end
end
end

Answer 3

I overrode the <=> operator with #Penne12's code:
def <=>(y)
self.line_item.split('.').zip(y.line_item.split('.')).inject(0) do |res, val|
(res == 0)? val[0].to_i <=> val[1].to_i : res
end
end
Sorting works on any enumerable collection, with no sort block:
bobs_items = LineItem.where(:owner => bob, :complete => false)
"Bob's workload: #{bobs_items.sort.map { |li| li.line_item }.join(', ')}"