arel more nicely supports EXISTS queries

lib/arel/nodes/exists.rb

@@ -1,11 +1,7 @@
 module Arel
   module Nodes
-    class Exists
-      attr_reader :select_stmt
-
-      def initialize select_stmt
-        @select_stmt = select_stmt
-      end
+    class Exists < Arel::Nodes::Function
+      alias :select_stmt :expressions
     end
   end
 end

lib/arel/select_manager.rb

@@ -22,6 +22,12 @@ def skip amount
       self
     end
 
+    ###
+    # Produces an Arel::Nodes::Exists node
+    def exists
+      Arel::Nodes::Exists.new @ast
+    end
+
     def where_clauses
       #warn "where_clauses is deprecated" if $VERBOSE
       to_sql = Visitors::ToSql.new @engine

lib/arel/tree_manager.rb

@@ -4,7 +4,7 @@ class TreeManager
     include Arel::Relation
 
     attr_accessor :visitor
-    attr_reader :ast
+    attr_reader :ast, :engine
 
     def initialize engine
       @engine  = engine

lib/arel/visitors/to_sql.rb

@@ -62,7 +62,8 @@ def visit_Arel_Nodes_InsertStatement o
       end
 
       def visit_Arel_Nodes_Exists o
-        "EXISTS (#{visit o.select_stmt})"
+        "EXISTS (#{visit o.select_stmt})#{
+          o.alias ? " AS #{visit o.alias}" : ''}"
       end
 
       def visit_Arel_Nodes_Values o

test/test_select_manager.rb

@@ -56,9 +56,7 @@ def execute sql, name = nil, *args
           manager.project SqlLiteral.new '*'
           manager.from table
           manager.order :foo
-          manager.to_sql.must_be_like %{
-          SELECT * FROM "users" ORDER BY foo
-          }
+          manager.to_sql.must_be_like %{ SELECT * FROM "users" ORDER BY foo }
         end
       end
 
@@ -68,9 +66,7 @@ def execute sql, name = nil, *args
           manager = Arel::SelectManager.new Table.engine
           manager.from table
           manager.group :foo
-          manager.to_sql.must_be_like %{
-          SELECT FROM "users" GROUP BY foo
-          }
+          manager.to_sql.must_be_like %{ SELECT FROM "users" GROUP BY foo }
         end
       end
 
@@ -130,6 +126,26 @@ def execute sql, name = nil, *args
       end
     end
 
+    describe 'exists' do
+      it 'should create an exists clause' do
+        table = Table.new(:users)
+        manager = Arel::SelectManager.new Table.engine, table
+        manager.project SqlLiteral.new '*'
+        m2 = Arel::SelectManager.new(manager.engine)
+        m2.project manager.exists
+        m2.to_sql.must_be_like %{ SELECT EXISTS (#{manager.to_sql}) }
+      end
+
+      it 'can be aliased' do
+        table = Table.new(:users)
+        manager = Arel::SelectManager.new Table.engine, table
+        manager.project SqlLiteral.new '*'
+        m2 = Arel::SelectManager.new(manager.engine)
+        m2.project manager.exists.as('foo')
+        m2.to_sql.must_be_like %{ SELECT EXISTS (#{manager.to_sql}) AS foo }
+      end
+    end
+
     describe 'ast' do
       it 'should return the ast' do
         table   = Table.new :users