Add Sluggable concern and unique validation to Budget, Group and Heading

* What: We need to generate slug on Budget, Group and Heading classes, validating its unique for its scope

* How: Adding a presence and unique validation using Budget always as scope.

app/models/budget.rb

@@ -1,13 +1,15 @@
 class Budget < ActiveRecord::Base
 
   include Measurable
+  include Sluggable
 
   PHASES = %w(accepting reviewing selecting valuating balloting reviewing_ballots finished).freeze
   CURRENCY_SYMBOLS = %w(€ $ £ ¥).freeze
 
-  validates :name, presence: true
+  validates :name, presence: true, uniqueness: true
   validates :phase, inclusion: { in: PHASES }
   validates :currency_symbol, presence: true
+  validates :slug, presence: true, format: /\A[a-z0-9\-_]+\z/
 
   has_many :investments, dependent: :destroy
   has_many :ballots, dependent: :destroy

app/models/budget/group.rb

@@ -1,10 +1,14 @@
 class Budget
   class Group < ActiveRecord::Base
+    include Sluggable
+
     belongs_to :budget
 
     has_many :headings, dependent: :destroy
 
     validates :budget_id, presence: true
-    validates :name, presence: true
+    validates :name, presence: true, uniqueness: { scope: :budget }
+    validates :slug, presence: true, format: /\A[a-z0-9\-_]+\z/
+
   end
 end

app/models/budget/heading.rb

@@ -1,12 +1,15 @@
 class Budget
   class Heading < ActiveRecord::Base
+    include Sluggable
+
     belongs_to :group
 
     has_many :investments
 
     validates :group_id, presence: true
-    validates :name, presence: true
+    validates :name, presence: true, uniqueness: { if: :name_exists_in_budget_headings }
     validates :price, presence: true
+    validates :slug, presence: true, format: /\A[a-z0-9\-_]+\z/
 
     delegate :budget, :budget_id, to: :group, allow_nil: true
 
@@ -16,5 +19,9 @@ class Budget
       "#{group.name}: #{name}"
     end
 
+    def name_exists_in_budget_headings
+      group.budget.headings.where(name: name).any?
+    end
+
   end
 end