12 files changed, 87 insertions, 26 deletions
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index dee398177c..580a580ba5 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,7 +1,16 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*  `#pluck` can be used on a relation with `select` clause
+   Fix #7551
+
+   Example:
+
+        Topic.select([:approved, :id]).order(:id).pluck(:id)
+
+   *Yves Senn*
+
 *   Do not create useless database transaction when building `has_one` association.
-    
+
     Example:
 
         User.has_one :profile
diff --git a/activerecord/lib/active_record/base.rb b/activerecord/lib/active_record/base.rb
index a694a292fe..83047c1845 100644
--- a/activerecord/lib/active_record/base.rb
+++ b/activerecord/lib/active_record/base.rb
@@ -332,7 +332,6 @@ module ActiveRecord #:nodoc:
     extend Translation
     extend DynamicMatchers
     extend Explain
-    extend ConnectionHandling
 
     include Persistence
     include ReadonlyAttributes
diff --git a/activerecord/lib/active_record/railtie.rb b/activerecord/lib/active_record/railtie.rb
index 4ba35fe513..5464ca6066 100644
--- a/activerecord/lib/active_record/railtie.rb
+++ b/activerecord/lib/active_record/railtie.rb
@@ -149,8 +149,10 @@ module ActiveRecord
 
       ActiveSupport.on_load(:active_record) do
         ActionDispatch::Reloader.send(hook) do
-          ActiveRecord::Base.clear_reloadable_connections!
-          ActiveRecord::Base.clear_cache!
+          if ActiveRecord::Base.connected?
+            ActiveRecord::Base.clear_reloadable_connections!
+            ActiveRecord::Base.clear_cache!
+          end
         end
       end
     end
diff --git a/activerecord/lib/active_record/relation/calculations.rb b/activerecord/lib/active_record/relation/calculations.rb
index a7d2f4bd24..df27318678 100644
--- a/activerecord/lib/active_record/relation/calculations.rb
+++ b/activerecord/lib/active_record/relation/calculations.rb
@@ -165,7 +165,9 @@ module ActiveRecord
       if has_include?(column_names.first)
         construct_relation_for_association_calculations.pluck(*column_names)
       else
-        result  = klass.connection.select_all(select(column_names).arel, nil, bind_values)
+        relation = spawn
+        relation.select_values = column_names
+        result = klass.connection.select_all(relation.arel, nil, bind_values)
         columns = result.columns.map do |key|
           klass.column_types.fetch(key) {
             result.column_types.fetch(key) {
diff --git a/activerecord/lib/active_record/relation/delegation.rb b/activerecord/lib/active_record/relation/delegation.rb
index ab8b36c8ab..dbfa92bbbd 100644
--- a/activerecord/lib/active_record/relation/delegation.rb
+++ b/activerecord/lib/active_record/relation/delegation.rb
@@ -1,3 +1,4 @@
+require 'thread'
 
 module ActiveRecord
   module Delegation # :nodoc:
@@ -6,6 +7,8 @@ module ActiveRecord
     delegate :table_name, :quoted_table_name, :primary_key, :quoted_primary_key,
              :connection, :columns_hash, :auto_explain_threshold_in_seconds, :to => :klass
 
+    @@delegation_mutex = Mutex.new
+
     def self.delegate_to_scoped_klass(method)
       if method.to_s =~ /\A[a-zA-Z_]\w*[!?]?\z/
         module_eval <<-RUBY, __FILE__, __LINE__ + 1
@@ -32,13 +35,28 @@ module ActiveRecord
 
     def method_missing(method, *args, &block)
       if @klass.respond_to?(method)
-        ::ActiveRecord::Delegation.delegate_to_scoped_klass(method)
+        @@delegation_mutex.synchronize do
+          unless ::ActiveRecord::Delegation.method_defined?(method)
+            ::ActiveRecord::Delegation.delegate_to_scoped_klass(method)
+          end
+        end
+
         scoping { @klass.send(method, *args, &block) }
       elsif Array.method_defined?(method)
-        ::ActiveRecord::Delegation.delegate method, :to => :to_a
+        @@delegation_mutex.synchronize do
+          unless ::ActiveRecord::Delegation.method_defined?(method)
+            ::ActiveRecord::Delegation.delegate method, :to => :to_a
+          end
+        end
+
         to_a.send(method, *args, &block)
       elsif arel.respond_to?(method)
-        ::ActiveRecord::Delegation.delegate method, :to => :arel
+        @@delegation_mutex.synchronize do
+          unless ::ActiveRecord::Delegation.method_defined?(method)
+            ::ActiveRecord::Delegation.delegate method, :to => :arel
+          end
+        end
+
         arel.send(method, *args, &block)
       else
         super
diff --git a/activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb b/activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb
index 42f5b69d4e..1b1b479f1a 100644
--- a/activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb
+++ b/activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb
@@ -799,12 +799,6 @@ class HasAndBelongsToManyAssociationsTest < ActiveRecord::TestCase
     assert_equal 1, developer.projects.count
   end
 
-  def test_counting_should_not_fire_sql_if_parent_is_unsaved
-    assert_no_queries do
-      assert_equal 0, Developer.new.projects.count
-    end
-  end
-
   unless current_adapter?(:PostgreSQLAdapter)
     def test_count_with_finder_sql
       assert_equal 3, projects(:active_record).developers_with_finder_sql.count
@@ -862,4 +856,15 @@ class HasAndBelongsToManyAssociationsTest < ActiveRecord::TestCase
     def klass.name; 'Foo'; end
     assert_deprecated { klass.has_and_belongs_to_many :posts, :delete_sql => 'lol' }
   end
+
+  test "has and belongs to many associations on new records use null relations" do
+    projects = Developer.new.projects
+    assert_no_queries do
+      assert_equal [], projects
+      assert_equal [], projects.where(title: 'omg')
+      assert_equal [], projects.pluck(:title)
+      assert_equal 0, projects.count
+    end
+  end
+
 end
diff --git a/activerecord/test/cases/associations/has_many_associations_test.rb b/activerecord/test/cases/associations/has_many_associations_test.rb
index 45b0a76f5a..b1066fb24c 100644
--- a/activerecord/test/cases/associations/has_many_associations_test.rb
+++ b/activerecord/test/cases/associations/has_many_associations_test.rb
@@ -262,12 +262,6 @@ class HasManyAssociationsTest < ActiveRecord::TestCase
     assert_equal firm.limited_clients.length, firm.limited_clients.count
   end
 
-  def test_counting_should_not_fire_sql_if_parent_is_unsaved
-    assert_no_queries do
-      assert_equal 0, Person.new.readers.count
-    end
-  end
-
   def test_finding
     assert_equal 2, Firm.all.merge!(:order => "id").first.clients.length
   end
@@ -1665,6 +1659,7 @@ class HasManyAssociationsTest < ActiveRecord::TestCase
       assert_equal [], post.comments.where(body: 'omg')
       assert_equal [], post.comments.pluck(:body)
       assert_equal 0,  post.comments.sum(:id)
+      assert_equal 0,  post.comments.count
     end
   end
 end
diff --git a/activerecord/test/cases/associations/has_many_through_associations_test.rb b/activerecord/test/cases/associations/has_many_through_associations_test.rb
index b2a5d9d6f7..8e52ce1d91 100644
--- a/activerecord/test/cases/associations/has_many_through_associations_test.rb
+++ b/activerecord/test/cases/associations/has_many_through_associations_test.rb
@@ -766,12 +766,6 @@ class HasManyThroughAssociationsTest < ActiveRecord::TestCase
     assert_equal 1, authors(:mary).categories.general.count
   end
 
-  def test_counting_should_not_fire_sql_if_parent_is_unsaved
-    assert_no_queries do
-      assert_equal 0, Person.new.posts.count
-    end
-  end
-
   def test_has_many_through_belongs_to_should_update_when_the_through_foreign_key_changes
     post = posts(:eager_other)
 
@@ -876,4 +870,17 @@ class HasManyThroughAssociationsTest < ActiveRecord::TestCase
     post = tags(:general).tagged_posts.create! :title => "foo", :body => "bar"
     assert_equal [tags(:general)], post.reload.tags
   end
+
+  test "has many through associations on new records use null relations" do
+    person = Person.new
+
+    assert_no_queries do
+      assert_equal [], person.posts
+      assert_equal [], person.posts.where(body: 'omg')
+      assert_equal [], person.posts.pluck(:body)
+      assert_equal 0,  person.posts.sum(:tags_count)
+      assert_equal 0,  person.posts.count
+    end
+  end
+
 end
diff --git a/activerecord/test/cases/base_test.rb b/activerecord/test/cases/base_test.rb
index 203e44857a..c02d8f7760 100644
--- a/activerecord/test/cases/base_test.rb
+++ b/activerecord/test/cases/base_test.rb
@@ -1442,6 +1442,13 @@ class BasicsTest < ActiveRecord::TestCase
     assert_match(/\/#{dev.id}$/, dev.cache_key)
   end
 
+  def test_cache_key_format_is_precise_enough
+    dev = Developer.first
+    key = dev.cache_key
+    dev.touch
+    assert_not_equal key, dev.cache_key
+  end
+
   def test_uniq_delegates_to_scoped
     scope = stub
     Bird.stubs(:all).returns(mock(:uniq => scope))
diff --git a/activerecord/test/cases/calculations_test.rb b/activerecord/test/cases/calculations_test.rb
index abbf2a765e..65d28ea028 100644
--- a/activerecord/test/cases/calculations_test.rb
+++ b/activerecord/test/cases/calculations_test.rb
@@ -580,4 +580,10 @@ class CalculationsTest < ActiveRecord::TestCase
 
     assert_equal ["Over There"], Possession.pluck(:where)
   end
+
+  def test_pluck_replaces_select_clause
+    taks_relation = Topic.select(:approved, :id).order(:id)
+    assert_equal [1,2,3,4], taks_relation.pluck(:id)
+    assert_equal [false, true, true, true], taks_relation.pluck(:approved)
+  end
 end
diff --git a/activerecord/test/cases/finder_test.rb b/activerecord/test/cases/finder_test.rb
index d44ac21b05..7db7953313 100644
--- a/activerecord/test/cases/finder_test.rb
+++ b/activerecord/test/cases/finder_test.rb
@@ -610,6 +610,11 @@ class FinderTest < ActiveRecord::TestCase
     assert_nil Topic.find_by_heading("The First Topic!")
   end
 
+  def test_find_by_one_attribute_bang_with_blank_defined
+    blank_topic = BlankTopic.create(title: "The Blank One")
+    assert_equal blank_topic, BlankTopic.find_by_title!("The Blank One")
+  end
+
   def test_find_by_one_attribute_with_conditions
     assert_equal accounts(:rails_core_account), Account.where('firm_id = ?', 6).find_by_credit_limit(50)
   end
diff --git a/activerecord/test/models/topic.rb b/activerecord/test/models/topic.rb
index ab31d85e46..f7f4cebc5a 100644
--- a/activerecord/test/models/topic.rb
+++ b/activerecord/test/models/topic.rb
@@ -107,6 +107,12 @@ class ImportantTopic < Topic
   serialize :important, Hash
 end
 
+class BlankTopic < Topic
+  def blank?
+    true
+  end
+end
+
 module Web
   class Topic < ActiveRecord::Base
     has_many :replies, :dependent => :destroy, :foreign_key => "parent_id", :class_name => 'Web::Reply'