Added "from" method, allowing to specify custom from clauses.

lib/arel/algebra/relations.rb

@@ -5,6 +5,7 @@ require 'arel/algebra/relations/utilities/externalization'
 require 'arel/algebra/relations/row'
 require 'arel/algebra/relations/writes'
 require 'arel/algebra/relations/operations/alias'
+require 'arel/algebra/relations/operations/from'
 require 'arel/algebra/relations/operations/group'
 require 'arel/algebra/relations/operations/join'
 require 'arel/algebra/relations/operations/order'

lib/arel/algebra/relations/operations/from.rb

@@ -0,0 +1,6 @@
+module Arel
+  class From < Compound
+    attributes :relation, :sources
+    deriving :initialize, :==
+  end
+end

lib/arel/algebra/relations/relation.rb

@@ -43,7 +43,7 @@ module Arel
         join(other_relation, OuterJoin)
       end
 
-      [:where, :project, :order, :take, :skip, :group].each do |operation_name|
+      [:where, :project, :order, :take, :skip, :group, :from].each do |operation_name|
         class_eval <<-OPERATION, __FILE__, __LINE__
           def #{operation_name}(*arguments, &block)
             arguments.all?(&:blank?) && !block_given?? self : #{operation_name.to_s.classify}.new(self, *arguments, &block)
@@ -130,6 +130,7 @@ module Arel
       def joins(formatter = nil); nil end # FIXME
       def taken;                  nil end
       def skipped;                nil end
+      def sources;                []  end
     end
     include DefaultOperations
   end

lib/arel/algebra/relations/utilities/compound.rb

@@ -2,7 +2,7 @@ module Arel
   class Compound < Relation
     attr_reader :relation
     delegate :joins, :join?, :inserts, :taken, :skipped, :name, :externalizable?,
-             :column_for, :engine,
+             :column_for, :engine, :sources,
              :to => :relation
 
     [:attributes, :wheres, :groupings, :orders].each do |operation_name|

lib/arel/engines/memory/relations/operations.rb

@@ -32,6 +32,12 @@ module Arel
     end
   end
 
+  class From < Compound
+    def eval
+      unoperated_rows[sources..-1]
+    end
+  end
+
   class Group < Compound
     def eval
       raise NotImplementedError

lib/arel/engines/sql/formatters.rb

@@ -98,6 +98,10 @@ module Arel
             (table.name != name_for(table) ? " AS " + quote_table_name(name_for(table)) : '')
         end
       end
+
+      def attribute(attribute)
+        attribute
+      end
     end
 
     class Attribute < WhereCondition

lib/arel/engines/sql/relations/relation.rb

@@ -13,7 +13,7 @@ module Arel
 
         query = build_query \
           "SELECT     #{select_clauses.kind_of?(::Array) ? select_clauses.join("") : select_clauses.to_s}",
-          "FROM       #{table_sql(Sql::TableReference.new(self))}",
+          "FROM       #{from_clauses}",
           (joins(self)                                   unless joins(self).blank? ),
           ("WHERE     #{where_clauses.join("\n\tAND ")}" unless wheres.blank?      ),
           ("GROUP BY  #{group_clauses.join(', ')}"       unless groupings.blank?   )
@@ -27,7 +27,7 @@ module Arel
       else
         build_query \
           "SELECT     #{select_clauses.join(', ')}",
-          "FROM       #{table_sql(Sql::TableReference.new(self))}",
+          "FROM       #{from_clauses}",
           (joins(self)                                   unless joins(self).blank? ),
           ("WHERE     #{where_clauses.join("\n\tAND ")}" unless wheres.blank?      ),
           ("GROUP BY  #{group_clauses.join(', ')}"       unless groupings.blank?   ),
@@ -51,6 +51,10 @@ module Arel
       parts.compact.join(" ")
     end
 
+    def from_clauses
+      sources.blank? ? table_sql(Sql::TableReference.new(self)) : sources
+    end
+
     def select_clauses
       attributes.collect { |a| a.to_sql(Sql::SelectClause.new(self)) }
     end

spec/arel/engines/sql/unit/relations/from_spec.rb

@@ -0,0 +1,50 @@
+require 'spec_helper'
+
+module Arel
+  describe Table do
+    before do
+      @relation = Table.new(:users)
+    end
+
+    describe '#to_sql' do
+      it "manufactures a simple select query" do
+        sql = @relation.from("workers").to_sql
+
+        adapter_is :mysql do
+          sql.should be_like(%Q{
+            SELECT `users`.`id`, `users`.`name`
+            FROM workers
+          })
+        end
+
+        adapter_is_not :mysql do
+          sql.should be_like(%Q{
+            SELECT "users"."id", "users"."name"
+            FROM workers
+          })
+        end
+      end
+    end
+
+    describe '#to_sql' do
+      it "overrides and use last from clause given " do
+        sql = @relation.from("workers").from("users").to_sql
+
+        adapter_is :mysql do
+          sql.should be_like(%Q{
+            SELECT `users`.`id`, `users`.`name`
+            FROM users
+          })
+        end
+
+        adapter_is_not :mysql do
+          sql.should be_like(%Q{
+            SELECT "users"."id", "users"."name"
+            FROM users
+          })
+        end
+      end
+    end
+
+  end
+end