Arel doesn't has to know if a table or column exists.

lib/arel/algebra/relations/relation.rb

@@ -84,7 +84,8 @@ def on(*predicates)
 
     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 @@ def [](index)
       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)

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 @@ def initialize(name, options = {})
       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 @@ def as(table_alias)
       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
 

spec/arel/algebra/unit/relations/relation_spec.rb

@@ -16,10 +16,9 @@ module Arel
       end
 
       describe 'when given a', Symbol, String do
-        it "returns the attribute with the same name, if it exists" do
+        it "returns the attribute with the same name" do
           check @relation[:id].should == @attribute1
           check @relation['id'].should == @attribute1
-          @relation[:does_not_exist].should be_nil
         end
       end
     end

spec/arel/algebra/unit/relations/table_spec.rb

@@ -10,7 +10,6 @@ module Arel
       describe 'when given a', Symbol do
         it "manufactures an attribute if the symbol names an attribute within the relation" do
           check @relation[:id].should == Attribute.new(@relation, :id)
-          @relation[:does_not_exist].should be_nil
         end
       end
 

spec/arel/engines/sql/unit/primitives/attribute_spec.rb

@@ -9,7 +9,7 @@ module Arel
 
     describe '#column' do
       it "returns the corresponding column in the relation" do
-        @attribute.column.should == @relation.column_for(@attribute)
+         @attribute.column.should == @relation.column_for(@attribute)
       end
     end
 
@@ -31,6 +31,25 @@ module Arel
           end
         end
       end
+
+      describe 'for an inexistent attribute' do
+        it "manufactures sql" do
+          sql = @relation[:does_not_exist].to_sql
+
+          adapter_is :mysql do
+            sql.should be_like(%Q{`users`.`does_not_exist`})
+          end
+
+          adapter_is :oracle do
+            sql.should be_like(%Q{"USERS"."DOEST_NOT_EXIST"})
+          end
+
+          adapter_is_not :mysql, :oracle do
+            sql.should be_like(%Q{"users"."does_not_exist"})
+          end
+        end
+      end
+
     end
   end
 end