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)

2 files changed, 31 insertions, 6 deletions

diff --git a/activerecord/lib/active_record/relation/merger.rb b/activerecord/lib/active_record/relation/merger.rb
index 3f880ce5e9..36f98c6480 100644
--- a/activerecord/lib/active_record/relation/merger.rb
+++ b/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]
 
diff --git a/activerecord/lib/active_record/relation/query_methods.rb b/activerecord/lib/active_record/relation/query_methods.rb
index 29536b16c4..19fe8155d9 100644
--- a/activerecord/lib/active_record/relation/query_methods.rb
+++ b/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