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.

1 files changed, 17 insertions, 0 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 8ff4c4706c..2a0f1bf92a 100644
--- a/activerecord/CHANGELOG.md
+++ b/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.