Add method to easily access a goal by code

Similar to what we do with settings, only for settings we return the
value of the setting (which is what we're going to need most of the
time), and here we return the object.

app/models/sdg/goal.rb

@@ -8,4 +8,8 @@ class SDG::Goal < ApplicationRecord
   def description
     I18n.t("sdg.goals.goal_#{code}.description")
   end
+
+  def self.[](code)
+    find_by!(code: code)
+  end
 end

spec/models/sdg/goal_spec.rb

@@ -3,7 +3,7 @@ require "rails_helper"
 describe SDG::Goal do
   describe "validations" do
     it "is valid with an existent code" do
-      goal = SDG::Goal.where(code: "1").first_or_initialize
+      goal = SDG::Goal[1]
 
       expect(goal).to be_valid
     end
@@ -21,8 +21,18 @@ describe SDG::Goal do
     end
   end
 
+  describe ".[]" do
+    it "finds existing goals by code" do
+      expect(SDG::Goal[1].code).to be 1
+    end
+
+    it "raises an exception for non-existing codes" do
+      expect { SDG::Goal[100] }.to raise_exception ActiveRecord::RecordNotFound
+    end
+  end
+
   it "translates title" do
-    goal = SDG::Goal.where(code: "1").first_or_create!
+    goal = SDG::Goal[1]
 
     expect(goal.title).to eq "No Poverty"
 
@@ -32,7 +42,7 @@ describe SDG::Goal do
   end
 
   it "translates description" do
-    goal = SDG::Goal.where(code: "1").first_or_create!
+    goal = SDG::Goal[1]
 
     expect(goal.description).to eq "End poverty in all its forms, everywhere."