average node is added

lib/arel/attributes/attribute.rb

@@ -20,6 +20,10 @@ module Arel
       def maximum
         Nodes::Max.new [self], Nodes::SqlLiteral.new('max_id')
       end
+
+      def average
+        Nodes::Avg.new [self], Nodes::SqlLiteral.new('avg_id')
+      end
     end
 
     class String  < Attribute; end

lib/arel/nodes.rb

@@ -4,9 +4,11 @@ require 'arel/nodes/or'
 require 'arel/nodes/and'
 
 require 'arel/nodes/in'
+require 'arel/nodes/function'
 require 'arel/nodes/count'
 require 'arel/nodes/sum'
 require 'arel/nodes/max'
+require 'arel/nodes/avg'
 require 'arel/nodes/having'
 require 'arel/nodes/sql_literal'
 require 'arel/nodes/select_core'

lib/arel/nodes/avg.rb

@@ -0,0 +1,6 @@
+module Arel
+  module Nodes
+    class Avg < Arel::Nodes::Function
+    end
+  end
+end

lib/arel/nodes/count.rb

@@ -1,22 +1,11 @@
 module Arel
   module Nodes
-    class Count
-      attr_accessor :expressions, :distinct, :alias
+    class Count < Arel::Nodes::Function
+      attr_accessor :distinct
 
-      def initialize expr, distinct = false
-        @expressions = expr
-        @distinct    = distinct
-        @alias       = nil
-      end
-
-      def as aliaz
-        self.alias = SqlLiteral.new(aliaz)
-        self
-      end
-
-      def to_sql
-        viz = Visitors::ToSql.new Table.engine
-        viz.accept self
+      def initialize expr, distinct = false, aliaz = nil
+        super(expr, aliaz)
+        @distinct = distinct
       end
     end
   end

lib/arel/nodes/function.rb

@@ -0,0 +1,22 @@
+module Arel
+  module Nodes
+    class Function
+      attr_accessor :expressions, :alias
+
+      def initialize expr, aliaz = nil
+        @expressions = expr
+        @alias       = aliaz
+      end
+
+      def as aliaz
+        self.alias = SqlLiteral.new(aliaz)
+        self
+      end
+
+      def to_sql
+        viz = Visitors::ToSql.new Table.engine
+        viz.accept self
+      end
+    end
+  end
+end

lib/arel/nodes/max.rb

@@ -1,22 +1,6 @@
 module Arel
   module Nodes
-    class Max
-      attr_accessor :expressions, :alias
-
-      def initialize expr, aliaz = nil
-        @expressions = expr
-        @alias       = aliaz
-      end
-
-      def as aliaz
-        self.alias = SqlLiteral.new(aliaz)
-        self
-      end
-
-      def to_sql
-        viz = Visitors::ToSql.new Table.engine
-        viz.accept self
-      end
+    class Max < Arel::Nodes::Function
     end
   end
 end

lib/arel/nodes/sum.rb

@@ -1,22 +1,6 @@
 module Arel
   module Nodes
-    class Sum
-      attr_accessor :expressions, :alias
-
-      def initialize expr, aliaz = nil
-        @expressions = expr
-        @alias       = aliaz
-      end
-
-      def as aliaz
-        self.alias = SqlLiteral.new(aliaz)
-        self
-      end
-
-      def to_sql
-        viz = Visitors::ToSql.new Table.engine
-        viz.accept self
-      end
+    class Sum < Arel::Nodes::Function
     end
   end
 end

lib/arel/visitors/to_sql.rb

@@ -86,6 +86,11 @@ module Arel
           visit x }.join(', ')})#{o.alias ? " AS #{visit o.alias}" : ''}"
       end
 
+      def visit_Arel_Nodes_Avg o
+        "AVG(#{o.expressions.map { |x|
+          visit x }.join(', ')})#{o.alias ? " AS #{visit o.alias}" : ''}"
+      end
+
       def visit_Arel_Nodes_TableAlias o
         "#{visit o.relation} #{quote_table_name o.name}"
       end

spec/arel/attributes/attribute_spec.rb

@@ -3,6 +3,23 @@ require 'spec_helper'
 module Arel
   module Attributes
     describe 'attribute' do
+      describe '#average' do
+        it 'should create a AVG node' do
+          relation = Table.new(:users)
+          relation[:id].average.should be_kind_of Nodes::Avg
+        end
+
+        # FIXME: backwards compat. Is this really necessary?
+        it 'should set the alias to "avg_id"' do
+          relation = Table.new(:users)
+          mgr = relation.project relation[:id].average
+          mgr.to_sql.should be_like %{
+            SELECT AVG("users"."id") AS avg_id
+            FROM "users"
+          }
+        end
+      end
+
       describe '#maximum' do
         it 'should create a MAX node' do
           relation = Table.new(:users)