Count returns 0 without querying if parent is not saved

Patches `CollectionAssociation#count` to return 0 without querying
if the parent record is new. Consider the following code:

    class Account
      has_many :dossiers
    end

    class Dossier
      belongs_to :account
    end

    a = Account.new
    a.dossiers.build

    # before patch
    a.dossiers.count
    # SELECT COUNT(*) FROM "dossiers" WHERE "dossiers"."account_id" IS NULL
    # => 0

    # after
    a.dosiers.count # fires without sql query
    # => 0

Fixes #1856.

activerecord/CHANGELOG.md

@@ -1,5 +1,22 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   `CollectionAssociation#count` returns 0 without querying if the
+    parent record is new.
+
+    Before:
+
+        person.pets
+        # SELECT COUNT(*) FROM "pets" WHERE "pets"."person_id" IS NULL
+        # => 0
+
+    After:
+
+        person.pets
+        # fires without sql query
+        # => 0
+
+    *Francesco Rodriguez*
+
 *   Fix `reset_counters` crashing on `has_many :through` associations.
     Fix #7822.
 

activerecord/lib/active_record/associations/collection_association.rb

@@ -174,6 +174,8 @@ def sum(*args)
       # association, it will be used for the query. Otherwise, construct options and pass them with
       # scope to the target class's +count+.
       def count(column_name = nil, count_options = {})
+        return 0 if owner.new_record?
+
         column_name, count_options = nil, column_name if column_name.is_a?(Hash)
 
         if options[:counter_sql] || options[:finder_sql]

activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb

@@ -799,6 +799,12 @@ def test_count_with_counter_sql
     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

activerecord/test/cases/associations/has_many_associations_test.rb

@@ -262,6 +262,12 @@ def test_counting_with_association_limit
     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

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -767,6 +767,12 @@ def test_count_has_many_through_with_named_scope
     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)