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, 2 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/associations/collection_association.rb b/activerecord/lib/active_record/associations/collection_association.rb
index fe3e5b00f7..96270ec0e9 100644
--- a/activerecord/lib/active_record/associations/collection_association.rb
+++ b/activerecord/lib/active_record/associations/collection_association.rb
@@ -174,6 +174,8 @@ module ActiveRecord
       # 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]