Add ballot_lines_count counter_cache to ballot -> ballot line relationship

app/models/budget/ballot/line.rb

@@ -1,7 +1,7 @@
 class Budget
   class Ballot
     class Line < ApplicationRecord
-      belongs_to :ballot
+      belongs_to :ballot, counter_cache: :ballot_lines_count
       belongs_to :investment, counter_cache: :ballot_lines_count
       belongs_to :heading
       belongs_to :group

db/migrate/20170628211547_add_ballot_line_counter_cached_to_budget_ballots.rb

@@ -0,0 +1,5 @@
+class AddBallotLineCounterCachedToBudgetBallots < ActiveRecord::Migration
+  def change
+    add_column :budget_ballots, :ballot_lines_count, :integer, default: 0
+  end
+end

db/schema.rb

@@ -133,6 +133,7 @@ ActiveRecord::Schema.define(version: 20190411090023) do
     t.integer  "budget_id"
     t.datetime "created_at",                         null: false
     t.datetime "updated_at",                         null: false
+    t.integer  "ballot_lines_count", default: 0
     t.boolean  "physical",           default: false
     t.integer  "poll_ballot_id"
   end