Return false for exists? with new records - fixes #6199.

activerecord/lib/active_record/relation/finder_methods.rb

@@ -170,9 +170,8 @@ module ActiveRecord
     #   Person.exists?(['name LIKE ?', "%#{query}%"])
     #   Person.exists?
     def exists?(id = false)
-      return false if id.nil?
-
       id = id.id if ActiveRecord::Model === id
+      return false if id.nil?
 
       join_dependency = construct_join_dependency_for_association_find
       relation = construct_relation_for_association_find(join_dependency)

activerecord/test/cases/finder_test.rb

@@ -32,6 +32,7 @@ class FinderTest < ActiveRecord::TestCase
     assert Topic.exists?(:author_name => "Mary", :approved => true)
     assert Topic.exists?(["parent_id = ?", 1])
     assert !Topic.exists?(45)
+    assert !Topic.exists?(Topic.new)
 
     begin
       assert !Topic.exists?("foo")

activerecord/test/cases/relations_test.rb

@@ -644,6 +644,7 @@ class RelationTest < ActiveRecord::TestCase
     assert ! davids.exists?(authors(:mary).id)
     assert ! davids.exists?("42")
     assert ! davids.exists?(42)
+    assert ! davids.exists?(davids.new)
 
     fake  = Author.where(:name => 'fake author')
     assert ! fake.exists?