passing an ActiveRecord object to exists? is deprecated.

Pass the id of the object to the method by calling `.id` on the AR
object.

activerecord/CHANGELOG.md

@@ -1,3 +1,6 @@
+* Passing an Active Record object to `exists?` is now deprecated.  Call `.id`
+  on the object first.
+
 *   Only use BINARY for mysql case sensitive uniqueness check when column has a case insensitive collation.
 
     *Ryuta Kamizono*

activerecord/lib/active_record/associations/collection_association.rb

@@ -370,7 +370,7 @@ def include?(record)
           if record.new_record?
             include_in_memory?(record)
           else
-            loaded? ? target.include?(record) : scope.exists?(record)
+            loaded? ? target.include?(record) : scope.exists?(record.id)
           end
         else
           false

activerecord/lib/active_record/relation/finder_methods.rb

@@ -1,3 +1,5 @@
+require 'active_support/deprecation'
+
 module ActiveRecord
   module FinderMethods
     ONE_AS_ONE = '1 AS one'
@@ -280,7 +282,12 @@ def forty_two!
     #   Person.exists?(false)
     #   Person.exists?
     def exists?(conditions = :none)
-      conditions = conditions.id if Base === conditions
+      if Base === conditions
+        conditions = conditions.id
+        ActiveSupport::Deprecation.warn "You are passing an instance of ActiveRecord::Base to `exists?`." \
+          "Please pass the id of the object by calling `.id`"
+      end
+
       return false if !conditions
 
       relation = apply_join_dependency(self, construct_join_dependency)

activerecord/test/cases/finder_test.rb

@@ -56,7 +56,7 @@ def test_exists
     assert_equal true, Topic.exists?(id: [1, 9999])
 
     assert_equal false, Topic.exists?(45)
-    assert_equal false, Topic.exists?(Topic.new)
+    assert_equal false, Topic.exists?(Topic.new.id)
 
     assert_raise(NoMethodError) { Topic.exists?([1,2]) }
   end

activerecord/test/cases/relations_test.rb

@@ -769,7 +769,7 @@ def test_exists
     assert ! davids.exists?(authors(:mary).id)
     assert ! davids.exists?("42")
     assert ! davids.exists?(42)
-    assert ! davids.exists?(davids.new)
+    assert ! davids.exists?(davids.new.id)
 
     fake  = Author.where(:name => 'fake author')
     assert ! fake.exists?