ActiveRecord::Relation#pluck method

activerecord/CHANGELOG.md

@@ -1,5 +1,14 @@
 ## Rails 3.2.0 (unreleased) ##
 
+
+*   Implemented ActiveRecord::Relation#pluck method
+
+    Method returns Array of column value from table under ActiveRecord model
+        
+        Client.pluck(:id)
+
+    *Bogdan Gusiev*
+
 *   Automatic closure of connections in threads is deprecated.  For example
     the following code is deprecated:
 

activerecord/lib/active_record/associations/collection_proxy.rb

@@ -39,7 +39,7 @@ module ActiveRecord
       instance_methods.each { |m| undef_method m unless m.to_s =~ /^(?:nil\?|send|object_id|to_a)$|^__|^respond_to|proxy_/ }
 
       delegate :group, :order, :limit, :joins, :where, :preload, :eager_load, :includes, :from,
-               :lock, :readonly, :having, :to => :scoped
+               :lock, :readonly, :having, :pluck, :to => :scoped
 
       delegate :target, :load_target, :loaded?, :scoped,
                :to => :@association

activerecord/lib/active_record/base.rb

@@ -449,7 +449,7 @@ module ActiveRecord #:nodoc:
       delegate :select, :group, :order, :except, :reorder, :limit, :offset, :joins,
                :where, :preload, :eager_load, :includes, :from, :lock, :readonly,
                :having, :create_with, :uniq, :to => :scoped
-      delegate :count, :average, :minimum, :maximum, :sum, :calculate, :to => :scoped
+      delegate :count, :average, :minimum, :maximum, :sum, :calculate, :pluck, :to => :scoped
 
       def inherited(child_class) #:nodoc:
         # force attribute methods to be higher in inheritance hierarchy than other generated methods

activerecord/lib/active_record/relation/calculations.rb

@@ -166,6 +166,23 @@ 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. 
+    #
+    # Examples:
+    #
+    #   Person.pluck(:id) # SELECT people.id FROM people
+    #   Person.uniq.pluck(:role) # SELECT DISTINCT role FROM people
+    #   Person.where(:confirmed => true).limit(5).pluck(:id)
+    #
+    def pluck(column_name)
+      scope = self.select(column_name)
+      self.connection.select_values(scope.to_sql).map! do |value|
+        type_cast_using_column(value, column_for(column_name))
+      end
+    end
+
     private
 
     def perform_calculation(operation, column_name, options = {})

activerecord/test/cases/calculations_test.rb

@@ -1,5 +1,6 @@
 require "cases/helper"
 require 'models/company'
+require "models/contract"
 require 'models/topic'
 require 'models/edge'
 require 'models/club'
@@ -446,4 +447,28 @@ class CalculationsTest < ActiveRecord::TestCase
     distinct_authors_for_approved_count = Topic.group(:approved).count(:author_name, :distinct => true)[true]
     assert_equal distinct_authors_for_approved_count, 2
   end
+
+  def test_pluck
+    assert_equal [1,2,3,4], Topic.order(:id).pluck(:id)
+  end
+
+  def test_pluck_type_cast
+    topic = topics(:first)
+    relation = Topic.where(:id => topic.id)
+    assert_equal [ topic.approved ], relation.pluck(:approved)
+    assert_equal [ topic.last_read ], relation.pluck(:last_read)
+    assert_equal [ topic.written_on ], relation.pluck(:written_on)
+
+  end
+
+  def test_pluck_and_uniq
+    assert_equal [50, 53, 55, 60], Account.order(:credit_limit).uniq.pluck(:credit_limit)
+  end
+
+  def test_pluck_in_relation
+    company = Company.first
+    contract = company.contracts.create!
+    assert_equal [contract.id], company.contracts.pluck(:id)
+  end
+
 end

railties/guides/source/active_record_querying.textile

@@ -1146,6 +1146,15 @@ h3. +select_all+
 Client.connection.select_all("SELECT * FROM clients WHERE id = '1'")
 </ruby>
 
+h3. +pluck+
+
+<tt>pluck</tt> can be used to query single column from table under model. It accepts column name as argument and returns Array of values of the specified column with corresponding data type.
+
+<ruby>
+Client.where(:active => true).pluck(:id) # SELECT id FROM clients WHERE clients.active
+Client.uniq.pluck(:role) # SELECT DISTINCT role FROM clients
+</ruby>
+
 h3. Existence of Objects
 
 If you simply want to check for the existence of the object there's a method called +exists?+. This method will query the database using the same query as +find+, but instead of returning an object or collection of objects it will return either +true+ or +false+.