Move logic to reject slugs that are just numbers into the slug module

app/models/category.rb

@@ -85,9 +85,6 @@ class Category < ActiveRecord::Base
     if name.present?
       self.slug = Slug.for(name)
 
-      # Reject slugs that only contain numbers, because that's indistinguishable from an id.
-      self.slug = '' unless self.slug =~ /[^\d]/
-
       return if self.slug.blank?
 
       # If a category with that slug already exists, set the slug to nil so the category can be found

lib/slug.rb

@@ -6,7 +6,8 @@
 module Slug
 
   def self.for(string)
-    string.parameterize.gsub("_", "-")
+    slug = string.parameterize.gsub("_", "-")
+    slug =~ /[^\d]/ ? slug : '' # Reject slugs that only contain numbers, because they would be indistinguishable from id's.
   end
 
 end

spec/components/slug_spec.rb

@@ -43,5 +43,13 @@ describe Slug do
     Slug.for("o_o_o").should == "o-o-o"
   end
 
+  it "doesn't generate slugs that are just numbers" do
+    Slug.for('123').should be_blank
+  end
+
+  it "doesn't generate slugs that are just numbers" do
+    Slug.for('電車男 2').should be_blank
+  end
+
 end