3 files changed, 70 insertions, 22 deletions

diff --git a/activerecord/lib/active_record/relation/calculations.rb b/activerecord/lib/active_record/relation/calculations.rb
index 3ce9995031..31d99f0192 100644
--- a/activerecord/lib/active_record/relation/calculations.rb
+++ b/activerecord/lib/active_record/relation/calculations.rb
@@ -108,34 +108,57 @@ module ActiveRecord
       0
     end
 
-    # This method is designed to perform select by a single column as direct SQL query
-    # Returns <tt>Array</tt> with values of the specified column name
-    # The values has same data type as column.
+    # Use <tt>pluck</tt> as a shortcut to select a single attribute without
+    # loading a bunch of records just to grab one attribute you want.
+    #
+    #   Person.pluck(:name)
+    #
+    # instead of
+    #
+    #   Person.all.map(&:name)
+    #
+    # Pluck returns an <tt>Array</tt> of attribute values type-casted to match
+    # the plucked column name, if it can be deduced. Plucking a SQL fragment
+    # returns String values by default.
     #
     # Examples:
     #
-    #   Person.pluck(:id) # SELECT people.id FROM people
-    #   Person.uniq.pluck(:role) # SELECT DISTINCT role FROM people
-    #   Person.where(:age => 21).limit(5).pluck(:id) # SELECT people.id FROM people WHERE people.age = 21 LIMIT 5
+    #   Person.pluck(:id)
+    #   # SELECT people.id FROM people
+    #   # => [1, 2, 3]
+    #
+    #   Person.uniq.pluck(:role)
+    #   # SELECT DISTINCT role FROM people
+    #   # => ['admin', 'member', 'guest']
+    #
+    #   Person.where(:age => 21).limit(5).pluck(:id)
+    #   # SELECT people.id FROM people WHERE people.age = 21 LIMIT 5
+    #   # => [2, 3]
+    #
+    #   Person.pluck('DATEDIFF(updated_at, created_at)')
+    #   # SELECT DATEDIFF(updated_at, created_at) FROM people
+    #   # => ['0', '27761', '173']
     #
     def pluck(column_name)
-      key = column_name.to_s.split('.', 2).last
-
       if column_name.is_a?(Symbol) && column_names.include?(column_name.to_s)
         column_name = "#{table_name}.#{column_name}"
       end
 
       result = klass.connection.select_all(select(column_name).arel, nil, bind_values)
-      types  = result.column_types.merge klass.column_types
-      column = types[key]
+
+      key    = result.columns.first
+      column = klass.column_types.fetch(key) {
+        result.column_types.fetch(key) {
+          Class.new { def type_cast(v); v; end }.new
+        }
+      }
 
       result.map do |attributes|
-        value = klass.initialize_attributes(attributes)[key]
-        if column
-          column.type_cast value
-        else
-          value
-        end
+        raise ArgumentError, "Pluck expects to select just one attribute: #{attributes.inspect}" unless attributes.one?
+
+        value = klass.initialize_attributes(attributes).values.first
+
+        column.type_cast(value)
       end
     end
 
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