From b706f690b611d647aae15a85caa19d167814ae3c Mon Sep 17 00:00:00 2001
From: Emilio Tagua <miloops@gmail.com>
Date: Wed, 10 Mar 2010 18:16:07 -0300
Subject: Arel doesn't has to know if a table or column exists.

---
 lib/arel/algebra/relations/relation.rb  |  5 +++--
 lib/arel/engines/sql/relations/table.rb | 18 +++++++++++++++---
 2 files changed, 18 insertions(+), 5 deletions(-)

(limited to 'lib')

diff --git a/lib/arel/algebra/relations/relation.rb b/lib/arel/algebra/relations/relation.rb
index e848c8aa1c..4cd3f8e109 100644
--- a/lib/arel/algebra/relations/relation.rb
+++ b/lib/arel/algebra/relations/relation.rb
@@ -84,7 +84,8 @@ module Arel
 
     module AttributeAccessable
       def [](index)
-        case index
+        @cached_attributes ||= {}
+        @cached_attributes[index] ||= case index
         when Symbol, String
           find_attribute_matching_name(index)
         when Attribute, Expression
@@ -96,7 +97,7 @@ module Arel
       end
 
       def find_attribute_matching_name(name)
-        attributes.detect { |a| a.named?(name) }
+        attributes.detect { |a| a.named?(name) } || Attribute.new(self, name)
       end
 
       def find_attribute_matching_attribute(attribute)
diff --git a/lib/arel/engines/sql/relations/table.rb b/lib/arel/engines/sql/relations/table.rb
index e4b75d6c3d..d10b761ea3 100644
--- a/lib/arel/engines/sql/relations/table.rb
+++ b/lib/arel/engines/sql/relations/table.rb
@@ -2,7 +2,7 @@ module Arel
   class Table < Relation
     include Recursion::BaseCase
 
-    cattr_accessor :engine
+    cattr_accessor :engine, :tables
     attr_reader :name, :engine, :table_alias, :options
 
     def initialize(name, options = {})
@@ -19,6 +19,7 @@ module Arel
       if @engine.connection
         begin
           require "arel/engines/sql/compilers/#{@engine.adapter_name.downcase}_compiler"
+          @@tables ||= engine.tables
         rescue LoadError
           raise "#{@engine.adapter_name} is not supported by Arel."
         end
@@ -29,9 +30,20 @@ module Arel
       Table.new(name, options.merge(:as => table_alias))
     end
 
+    def table_exists?
+      if @table_exists
+        true
+      else
+        @table_exists = @@tables.include?(name) || engine.table_exists?(name)
+      end
+    end
+
     def attributes
-      @attributes ||= columns.collect do |column|
-        Attribute.new(self, column.name.to_sym)
+      return @attributes if defined?(@attributes)
+      if table_exists?
+        @attributes = columns.collect { |column| Attribute.new(self, column.name.to_sym) }
+      else
+        []
       end
     end
 
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