Add #extract, which produces ANSI SQL function EXTRACT(<field> from <expr>).

lib/arel/expressions.rb

@@ -19,5 +19,9 @@ module Arel
     def average
       Nodes::Avg.new [self], Nodes::SqlLiteral.new('avg_id')
     end
+
+    def extract field
+      Nodes::Extract.new [self], field
+    end
   end
 end

lib/arel/nodes.rb

@@ -36,6 +36,7 @@ require 'arel/nodes/and'
 # We should make Function a Unary node and deprecate the use of "aliaz"
 require 'arel/nodes/function'
 require 'arel/nodes/count'
+require 'arel/nodes/extract'
 require 'arel/nodes/values'
 require 'arel/nodes/named_function'
 

lib/arel/nodes/extract.rb

@@ -0,0 +1,23 @@
+module Arel
+  module Nodes
+
+    class Extract < Arel::Nodes::Unary
+      include Arel::Expression
+      include Arel::Predications
+
+      attr_accessor :field
+      attr_accessor :alias
+
+      def initialize expr, field, aliaz = nil
+        super(expr)
+        @field = field
+        @alias = aliaz && SqlLiteral.new(aliaz)
+      end
+
+      def as aliaz
+        self.alias = SqlLiteral.new(aliaz)
+        self
+      end
+    end
+  end
+end

lib/arel/visitors/dot.rb

@@ -103,6 +103,12 @@ module Arel
       alias :visit_Arel_Nodes_Avg    :function
       alias :visit_Arel_Nodes_Sum    :function
 
+      def extract o
+        visit_edge o, "expressions"
+        visit_edge o, "alias"
+      end
+      alias :visit_Arel_Nodes_Extract :extract
+
       def visit_Arel_Nodes_NamedFunction o
         visit_edge o, "name"
         visit_edge o, "expressions"

lib/arel/visitors/to_sql.rb

@@ -272,6 +272,10 @@ key on UpdateManager using UpdateManager#key=
         }.join(', ')})#{o.alias ? " AS #{visit o.alias}" : ''}"
       end
 
+      def visit_Arel_Nodes_Extract o
+        "EXTRACT(#{o.field.to_s.upcase} FROM #{visit o.expr})#{o.alias ? " AS #{visit o.alias}" : ''}"
+      end
+
       def visit_Arel_Nodes_Count o
         "COUNT(#{o.distinct ? 'DISTINCT ' : ''}#{o.expressions.map { |x|
           visit x

test/nodes/test_extract.rb

@@ -0,0 +1,19 @@
+require 'helper'
+
+describe Arel::Nodes::Extract do
+  it "should extract field" do
+    table = Arel::Table.new :users
+    table[:timestamp].extract('date').to_sql.must_be_like %{
+      EXTRACT(DATE FROM "users"."timestamp")
+    }
+  end
+
+  describe "as" do
+    it 'should alias the extract' do
+      table = Arel::Table.new :users
+      table[:timestamp].extract('date').as('foo').to_sql.must_be_like %{
+        EXTRACT(DATE FROM "users"."timestamp") AS foo
+      }
+    end
+  end
+end