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.

3 files changed, 18 insertions, 0 deletions

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 f3520d43e0..42f5b69d4e 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
@@ -799,6 +799,12 @@ 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
diff --git a/activerecord/test/cases/associations/has_many_associations_test.rb b/activerecord/test/cases/associations/has_many_associations_test.rb
index 4b56037a08..50c23c863f 100644
--- a/activerecord/test/cases/associations/has_many_associations_test.rb
+++ b/activerecord/test/cases/associations/has_many_associations_test.rb
@@ -262,6 +262,12 @@ 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
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 d4ceae6f80..f0582a3090 100644
--- a/activerecord/test/cases/associations/has_many_through_associations_test.rb
+++ b/activerecord/test/cases/associations/has_many_through_associations_test.rb
@@ -767,6 +767,12 @@ 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)