Relation#from to accept other Relation objects

Record.from("(#{sub_query.to_sql})")    -> Record.from(sub_query)
Record.from("(#{sub_query.to_sql}) a")  -> Record.from(sub_query, :a)

activerecord/CHANGELOG.md

@@ -1,5 +1,12 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Added ability to ActiveRecord::Relation#from to accept other ActiveRecord::Relation objects
+
+      Record.from(subquery)
+      Record.from(subquery, :a)
+
+    *Radoslav Stankov*
+
 *   Added custom coders support for ActiveRecord::Store. Now you can set
     your custom coder like this:
 

activerecord/lib/active_record/relation/merger.rb

@@ -43,7 +43,7 @@ module ActiveRecord
       def normal_values
         Relation::SINGLE_VALUE_METHODS +
           Relation::MULTI_VALUE_METHODS -
-          [:where, :order, :bind, :reverse_order, :lock, :create_with, :reordering]
+          [:where, :order, :bind, :reverse_order, :lock, :create_with, :reordering, :from]
       end
 
       def merge
@@ -76,6 +76,7 @@ module ActiveRecord
       end
 
       def merge_single_values
+        relation.from_value          = values[:from] unless relation.from_value
         relation.lock_value          = values[:lock] unless relation.lock_value
         relation.reverse_order_value = values[:reverse_order]
 

activerecord/lib/active_record/relation/query_methods.rb

@@ -300,12 +300,25 @@ module ActiveRecord
       self
     end
 
-    def from(value)
-      spawn.from!(value)
+    # Specifies table from which the records will be fetched. For example:
+    #
+    #   Topic.select('title').from('posts')
+    #   #=> SELECT title FROM posts
+    #
+    # Can accept other relation objects. For example:
+    #
+    #   Topic.select('title').from(Topics.approved)
+    #   # => SELECT title FROM (SELECT * FROM topics WHERE approved = 't') subquery
+    #
+    #   Topics.select('a.title').from(Topics.approved, :a)
+    #   # => SELECT a.title FROM (SELECT * FROM topics WHERE approved = 't') a
+    #
+    def from(value, subquery_name = nil)
+      spawn.from!(value, subquery_name)
     end
 
-    def from!(value)
-      self.from_value = value
+    def from!(value, subquery_name = nil)
+      self.from_value = [value, subquery_name]
       self
     end
 
@@ -415,7 +428,7 @@ module ActiveRecord
       build_select(arel, select_values.uniq)
 
       arel.distinct(uniq_value)
-      arel.from(from_value) if from_value
+      arel.from(build_from) if from_value
       arel.lock(lock_value) if lock_value
 
       arel
@@ -464,6 +477,17 @@ module ActiveRecord
       end
     end
 
+    def build_from
+      opts, name = from_value
+      case opts
+      when Relation
+        name ||= 'subquery'
+        opts.arel.as(name.to_s)
+      else
+        opts
+      end
+    end
+
     def build_joins(manager, joins)
       buckets = joins.group_by do |join|
         case join

activerecord/test/cases/relation_test.rb

@@ -203,13 +203,18 @@ module ActiveRecord
       assert_equal [], relation.extending_values
     end
 
-    (Relation::SINGLE_VALUE_METHODS - [:lock, :reordering, :reverse_order, :create_with]).each do |method|
+    (Relation::SINGLE_VALUE_METHODS - [:from, :lock, :reordering, :reverse_order, :create_with]).each do |method|
       test "##{method}!" do
         assert relation.public_send("#{method}!", :foo).equal?(relation)
         assert_equal :foo, relation.public_send("#{method}_value")
       end
     end
 
+    test '#from!' do
+      assert relation.from!('foo').equal?(relation)
+      assert_equal ['foo', nil], relation.from_value
+    end
+
     test '#lock!' do
       assert relation.lock!('foo').equal?(relation)
       assert_equal 'foo', relation.lock_value

activerecord/test/cases/relations_test.rb

@@ -133,6 +133,13 @@ class RelationTest < ActiveRecord::TestCase
     assert topics.loaded?
   end
 
+  def test_finiding_with_subquery
+    relation = Topic.where(:approved => true)
+    assert_equal relation.to_a, Topic.select('*').from(relation).to_a
+    assert_equal relation.to_a, Topic.select('subquery.*').from(relation).to_a
+    assert_equal relation.to_a, Topic.select('a.*').from(relation, :a).to_a
+  end
+
   def test_finding_with_conditions
     assert_equal ["David"], Author.where(:name => 'David').map(&:name)
     assert_equal ['Mary'],  Author.where(["name = ?", 'Mary']).map(&:name)