Calculate burden by member

app/models/expense.rb

@@ -10,4 +10,8 @@ class Expense < ApplicationRecord
   def monthly
     payment * PERIOD_OCCURENCES[period.to_sym] / 12
   end
+
+  def self.total
+    Expense.sum(&:payment)
+  end
 end

app/models/income.rb

@@ -1,3 +1,7 @@
 class Income < ApplicationRecord
   belongs_to :member
+
+  def self.total
+    Income.where(included: true).sum(&:amount)
+  end
 end

app/models/member.rb

@@ -1,2 +1,23 @@
 class Member < ApplicationRecord
+  has_many :incomes
+
+  def income
+    incomes.sum(&:amount)
+  end
+
+  def included_income
+    incomes.where(included: true).sum(&:amount)
+  end
+
+  def others_included_income
+    Income.where(included: true) - included_income
+  end
+
+  def burden_percent
+    (Income.total - others_included_income) / Income.total
+  end
+
+  def burden_amount
+    burden_percent * Expense.total
+  end
 end