From dbd0140974ed768705f3680d5d6e47a56305b965 Mon Sep 17 00:00:00 2001
From: Aaron Patterson <aaron.patterson@gmail.com>
Date: Fri, 5 Nov 2010 16:33:26 -0700
Subject: arel more nicely supports EXISTS queries

---
 lib/arel/nodes/exists.rb    |  8 ++------
 lib/arel/select_manager.rb  |  6 ++++++
 lib/arel/tree_manager.rb    |  2 +-
 lib/arel/visitors/to_sql.rb |  3 ++-
 test/test_select_manager.rb | 28 ++++++++++++++++++++++------
 5 files changed, 33 insertions(+), 14 deletions(-)

diff --git a/lib/arel/nodes/exists.rb b/lib/arel/nodes/exists.rb
index 167a345006..18ba8403b4 100644
--- a/lib/arel/nodes/exists.rb
+++ b/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
diff --git a/lib/arel/select_manager.rb b/lib/arel/select_manager.rb
index f2520f38d7..f6738bf26f 100644
--- a/lib/arel/select_manager.rb
+++ b/lib/arel/select_manager.rb
@@ -22,6 +22,12 @@ module Arel
       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
diff --git a/lib/arel/tree_manager.rb b/lib/arel/tree_manager.rb
index ca87def8d6..6176f8a250 100644
--- a/lib/arel/tree_manager.rb
+++ b/lib/arel/tree_manager.rb
@@ -4,7 +4,7 @@ module Arel
     include Arel::Relation
 
     attr_accessor :visitor
-    attr_reader :ast
+    attr_reader :ast, :engine
 
     def initialize engine
       @engine  = engine
diff --git a/lib/arel/visitors/to_sql.rb b/lib/arel/visitors/to_sql.rb
index 3c96b3e259..b6af47cc57 100644
--- a/lib/arel/visitors/to_sql.rb
+++ b/lib/arel/visitors/to_sql.rb
@@ -62,7 +62,8 @@ module Arel
       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
diff --git a/test/test_select_manager.rb b/test/test_select_manager.rb
index ebec92b1da..d63bec0093 100644
--- a/test/test_select_manager.rb
+++ b/test/test_select_manager.rb
@@ -56,9 +56,7 @@ module Arel
           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 @@ module Arel
           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 @@ module Arel
       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
-- 
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