implemented support for math operations in numeric attributes

README.markdown

@@ -74,6 +74,23 @@ The `AND` operator behaves similarly.
 
 The examples above are fairly simple and other libraries match or come close to matching the expressiveness of Arel (e.g., `Sequel` in Ruby).
 
+#### Inline math operations
+
+Suppose we have a table `products` with prices in different currencies. And we have a table currency_rates, of constantly changing currency rates. In Arel:
+
+    products = Arel::Table.new(:products)
+    products.columns # => [products[:id], products[:name], products[:price], products[:currency_id]]
+
+    currency_rates = Arel::Table.new(:currency_rates)
+    currency_rates.columns # => [currency_rates[:from_id], currency_rates[:to_id], currency_rates[:date], currency_rates[:rate]]
+
+Now, to order products by price in user preferred currency simply call:
+
+    products.
+      join(:currency_rates).on(products[:currency_id].eq(currency_rates[:from_id])).
+      where(currency_rates[:to_id].eq(user_preferred_currency), currency_rates[:date].eq(Date.today)).
+      order(products[:price] * currency_rates[:rate])
+
 #### Complex Joins
 
 Where Arel really shines in its ability to handle complex joins and aggregations. As a first example, let's consider an "adjacency list", a tree represented in a table. Suppose we have a table `comments`, representing a threaded discussion:

lib/arel.rb

@@ -3,6 +3,7 @@ require 'arel/factory_methods'
 
 require 'arel/expressions'
 require 'arel/predications'
+require 'arel/math'
 require 'arel/table'
 require 'arel/attributes'
 require 'arel/compatibility/wheres'

lib/arel/attributes/attribute.rb

@@ -5,12 +5,16 @@ module Arel
       include Arel::Predications
     end
 
+    class NumericAttribute < Attribute
+      include Arel::Math
+    end
+
     class String    < Attribute; end
     class Time      < Attribute; end
     class Boolean   < Attribute; end
-    class Decimal   < Attribute; end
-    class Float     < Attribute; end
-    class Integer   < Attribute; end
+    class Decimal   < NumericAttribute; end
+    class Float     < NumericAttribute; end
+    class Integer   < NumericAttribute; end
     class Undefined < Attribute; end
   end
 

lib/arel/math.rb

@@ -0,0 +1,21 @@
+module Arel
+  module Math
+
+    def *(other)
+      Arel::Nodes::Multiplication.new(self, other)
+    end
+
+    def +(other)
+      Arel::Nodes::Addition.new(self, other)
+    end
+
+    def -(other)
+      Arel::Nodes::Subtraction.new(self, other)
+    end
+
+    def /(other)
+      Arel::Nodes::Division.new(self, other)
+    end
+  
+  end
+end

lib/arel/nodes.rb

@@ -19,6 +19,7 @@ require 'arel/nodes/join_source'
 require 'arel/nodes/ordering'
 require 'arel/nodes/delete_statement'
 require 'arel/nodes/table_alias'
+require 'arel/nodes/math_operation'
 
 # nary
 require 'arel/nodes/and'

lib/arel/nodes/math_operation.rb

@@ -0,0 +1,15 @@
+module Arel
+  module Nodes
+    class MathOperation < Binary
+      include Arel::Expressions
+      include Arel::Predications
+      include Arel::Math
+    end
+
+    class Multiplication < MathOperation; end
+    class Division < MathOperation; end
+    class Addition < MathOperation; end
+    class Subtraction < MathOperation; end
+
+  end
+end

lib/arel/visitors/to_sql.rb

@@ -395,6 +395,22 @@ key on UpdateManager using UpdateManager#key=
       alias :visit_Time                          :quoted
       alias :visit_TrueClass                     :quoted
 
+      def visit_Arel_Nodes_Multiplication o
+        "#{visit o.left} * #{visit o.right}"
+      end
+
+      def visit_Arel_Nodes_Division o
+        "#{visit o.left} / #{visit o.right}"
+      end
+
+      def visit_Arel_Nodes_Addition o
+        "(#{visit o.left} + #{visit o.right})"
+      end
+
+      def visit_Arel_Nodes_Subtraction o
+        "(#{visit o.left} - #{visit o.right})"
+      end
+
       def visit_Array o
         o.empty? ? 'NULL' : o.map { |x| visit x }.join(', ')
       end

test/support/fake_record.rb

@@ -6,13 +6,17 @@ module FakeRecord
     attr_reader :tables
 
     def initialize
-      @tables = %w{ users photos developers }
+      @tables = %w{ users photos developers products}
       @columns = {
         'users' => [
           Column.new('id', :integer),
           Column.new('name', :string),
           Column.new('bool', :boolean),
-          Column.new('created_at', :date),
+          Column.new('created_at', :date)
+        ],
+        'products' => [
+          Column.new('id', :integer),
+          Column.new('price', :decimal)
         ]
       }
       @primary_keys = {

test/visitors/test_to_sql.rb

@@ -188,6 +188,28 @@ module Arel
         end
       end
 
+      describe "Nodes::MathOperation" do
+        it "should handle Multiplication" do
+          node = Arel::Attributes::Decimal.new(Table.new(:products), :price) * Arel::Attributes::Decimal.new(Table.new(:currency_rates), :rate)
+          @visitor.accept(node).must_equal %("products"."price" * "currency_rates"."rate")
+        end
+
+        it "should handle Division" do
+          node = Arel::Attributes::Decimal.new(Table.new(:products), :price) / 5
+          @visitor.accept(node).must_equal %("products"."price" / 5)
+        end
+
+        it "should handle Addition" do
+          node = Arel::Attributes::Decimal.new(Table.new(:products), :price) + 6
+          @visitor.accept(node).must_equal %(("products"."price" + 6))
+        end
+
+        it "should handle Subtraction" do
+          node = Arel::Attributes::Decimal.new(Table.new(:products), :price) - 7
+          @visitor.accept(node).must_equal %(("products"."price" - 7))
+        end
+      end
+
       describe "Nodes::NotIn" do
         it "should know how to visit" do
           node = @attr.not_in [1, 2, 3]