Refactoring to dry up account subclasses, as discussed in PR mbulat#41

  - Refactors ::balance and #balance methods defined in each of Account's subclasses
    to the Account model.
  - Retain original documentation
  - Adds documentation for the methods in their new locations (with a note on intended use)
  - Prevents calling ::balance or #balance on the Account class/instance directly
  - Adds specs for preventing calling ::balance and #balance on the Account class/instance directly

app/models/plutus/account.rb

@@ -30,6 +30,8 @@ module Plutus
   #
   # @author Michael Bulat
   class Account < ActiveRecord::Base
+    class_attribute :normal_credit_balance
+
     has_many :credit_amounts, :extend => AmountsExtension, :class_name => 'Plutus::CreditAmount'
     has_many :debit_amounts, :extend => AmountsExtension, :class_name => 'Plutus::DebitAmount'
     has_many :credit_entries, :through => :credit_amounts, :source => :entry, :class_name => 'Plutus::Entry'
@@ -43,6 +45,32 @@ class Account < ActiveRecord::Base
       include Plutus::NoTenancy
     end
 
+    # The balance of the account. This instance method is intended for use only
+    # on instances of account subclasses.
+    #
+    # If the account has a normal credit balance, the debits are subtracted from the credits
+    # unless this is a contra account, in which case credits are substracted from debits.
+    #
+    # For a normal debit balance, the credits are subtracted from the debits
+    # unless this is a contra account, in which case debits are subtracted from credits.
+    #
+    # @example
+    #   >> liability.balance
+    #   => #<BigDecimal:103259bb8,'0.2E4',4(12)>
+    #
+    # @return [BigDecimal] The decimal value balance
+    def balance
+      unless self.class == Plutus::Account
+        if self.normal_credit_balance ^ contra
+          credits_balance - debits_balance
+        else
+          debits_balance - credits_balance
+        end
+      else
+        raise(NoMethodError, "undefined method 'balance'")
+      end
+    end
+
     # The credit balance for the account.
     #
     # @example
@@ -65,6 +93,33 @@ def debits_balance
       debit_amounts.balance
     end
 
+    # This class method is used to return the balance of all accounts
+    # for a given class and is intended for use only on account subclasses.
+    #
+    # Contra accounts are automatically subtracted from the balance.
+    #
+    # @example
+    #   >> Plutus::Liability.balance
+    #   => #<BigDecimal:1030fcc98,'0.82875E5',8(20)>
+    #
+    # @return [BigDecimal] The decimal value balance
+    def self.balance
+      unless self.new.class == Plutus::Account
+        accounts_balance = BigDecimal.new('0')
+        accounts = self.all
+        accounts.each do |account|
+          unless account.contra
+            accounts_balance += account.balance
+          else
+            accounts_balance -= account.balance
+          end
+        end
+        accounts_balance
+      else
+        raise(NoMethodError, "undefined method 'balance'")
+      end
+    end
+
     # The trial balance of all accounts in the system. This should always equal zero,
     # otherwise there is an error in the system.
     #

app/models/plutus/asset.rb

@@ -9,6 +9,8 @@ module Plutus
   # @author Michael Bulat
   class Asset < Account
 
+    self.normal_credit_balance = false
+
     # The balance of the account.
     #
     # Assets have normal debit balances, so the credits are subtracted from the debits
@@ -20,11 +22,7 @@ class Asset < Account
     #
     # @return [BigDecimal] The decimal value balance
     def balance
-      unless contra
-        debits_balance - credits_balance
-      else
-        credits_balance - debits_balance
-      end
+      super
     end
 
     # This class method is used to return
@@ -38,16 +36,7 @@ def balance
     #
     # @return [BigDecimal] The decimal value balance
     def self.balance
-      accounts_balance = BigDecimal.new('0')
-      accounts = self.all
-      accounts.each do |asset|
-        unless asset.contra
-          accounts_balance += asset.balance
-        else
-          accounts_balance -= asset.balance
-        end
-      end
-      accounts_balance
+      super
     end
   end
 end

app/models/plutus/equity.rb

@@ -9,6 +9,8 @@ module Plutus
   # @author Michael Bulat
   class Equity < Account
 
+    self.normal_credit_balance = true
+
     # The balance of the account.
     #
     # Equity accounts have normal credit balances, so the debits are subtracted from the credits
@@ -20,11 +22,7 @@ class Equity < Account
     #
     # @return [BigDecimal] The decimal value balance
     def balance
-      unless contra
-        credits_balance - debits_balance
-      else
-        debits_balance - credits_balance
-      end
+      super
     end
 
     # This class method is used to return
@@ -38,16 +36,7 @@ def balance
     #
     # @return [BigDecimal] The decimal value balance
     def self.balance
-      accounts_balance = BigDecimal.new('0')
-      accounts = self.all
-      accounts.each do |equity|
-        unless equity.contra
-          accounts_balance += equity.balance
-        else
-          accounts_balance -= equity.balance
-        end
-      end
-      accounts_balance
+      super
     end
   end
 end

app/models/plutus/expense.rb

@@ -9,6 +9,8 @@ module Plutus
   # @author Michael Bulat
   class Expense < Account
 
+    self.normal_credit_balance = false
+
     # The balance of the account.
     #
     # Expenses have normal debit balances, so the credits are subtracted from the debits
@@ -20,11 +22,7 @@ class Expense < Account
     #
     # @return [BigDecimal] The decimal value balance
     def balance
-      unless contra
-        debits_balance - credits_balance
-      else
-        credits_balance - debits_balance
-      end
+      super
     end
 
     # This class method is used to return
@@ -38,16 +36,7 @@ def balance
     #
     # @return [BigDecimal] The decimal value balance
     def self.balance
-      accounts_balance = BigDecimal.new('0')
-      accounts = self.all
-      accounts.each do |expense|
-        unless expense.contra
-          accounts_balance += expense.balance
-        else
-          accounts_balance -= expense.balance
-        end
-      end
-      accounts_balance
+      super
     end
   end
 end

app/models/plutus/liability.rb

@@ -9,6 +9,8 @@ module Plutus
   # @author Michael Bulat
   class Liability < Account
 
+    self.normal_credit_balance = true
+
     # The balance of the account.
     #
     # Liability accounts have normal credit balances, so the debits are subtracted from the credits
@@ -20,11 +22,7 @@ class Liability < Account
     #
     # @return [BigDecimal] The decimal value balance
     def balance
-      unless contra
-        credits_balance - debits_balance
-      else
-        debits_balance - credits_balance
-      end
+      super
     end
 
     # This class method is used to return
@@ -38,16 +36,7 @@ def balance
     #
     # @return [BigDecimal] The decimal value balance
     def self.balance
-      accounts_balance = BigDecimal.new('0')
-      accounts = self.all
-      accounts.each do |liability|
-        unless liability.contra
-          accounts_balance += liability.balance
-        else
-          accounts_balance -= liability.balance
-        end
-      end
-      accounts_balance
+      super
     end
   end
 end

app/models/plutus/revenue.rb

@@ -9,6 +9,8 @@ module Plutus
   # @author Michael Bulat
   class Revenue < Account
 
+    self.normal_credit_balance = true
+
     # The balance of the account.
     #
     # Revenue accounts have normal credit balances, so the debits are subtracted from the credits
@@ -20,11 +22,7 @@ class Revenue < Account
     #
     # @return [BigDecimal] The decimal value balance
     def balance
-      unless contra
-        credits_balance - debits_balance
-      else
-        debits_balance - credits_balance
-      end
+      super
     end
 
     # This class method is used to return
@@ -38,16 +36,7 @@ def balance
     #
     # @return [BigDecimal] The decimal value balance
     def self.balance
-      accounts_balance = BigDecimal.new('0')
-      accounts = self.all
-      accounts.each do |revenue|
-        unless revenue.contra
-          accounts_balance += revenue.balance
-        else
-          accounts_balance -= revenue.balance
-        end
-      end
-      accounts_balance
+      super
     end
   end
 end

spec/models/account_spec.rb

@@ -18,7 +18,13 @@ module Plutus
       end
     end
 
-    it { should_not respond_to(:balance) }
+    it "calling the instance method #balance should raise a NoMethodError" do
+      expect { subject.balance }.to raise_error NoMethodError, "undefined method 'balance'"
+    end
+
+    it "calling the class method ::balance should raise a NoMethodError" do
+      expect { subject.class.balance }.to raise_error NoMethodError, "undefined method 'balance'"
+    end
 
     describe ".trial_balance" do
       subject { Account.trial_balance }