Don't short-circuit reject_if proc

When updating an associated record via nested attribute hashes the
reject_if proc could be bypassed if the _destroy flag was set in the
attribute hash and allow_destroy was set to false.

The fix is to only short-circuit if the _destroy flag is set and the
option allow_destroy is set to true. It also fixes an issue where
a new record wasn't created if _destroy was set and the option
allow_destroy was set to false.

CVE-2015-7577

activerecord/lib/active_record/nested_attributes.rb

@@ -523,7 +523,7 @@ def has_destroy_flag?(hash)
     # has_destroy_flag? or if a <tt>:reject_if</tt> proc exists for this
     # association and evaluates to +true+.
     def reject_new_record?(association_name, attributes)
-      has_destroy_flag?(attributes) || call_reject_if(association_name, attributes)
+      will_be_destroyed?(association_name, attributes) || call_reject_if(association_name, attributes)
     end
 
     # Determines if a record with the particular +attributes+ should be
@@ -532,7 +532,8 @@ def reject_new_record?(association_name, attributes)
     #
     # Returns false if there is a +destroy_flag+ on the attributes.
     def call_reject_if(association_name, attributes)
-      return false if has_destroy_flag?(attributes)
+      return false if will_be_destroyed?(association_name, attributes)
+
       case callback = self.nested_attributes_options[association_name][:reject_if]
       when Symbol
         method(callback).arity == 0 ? send(callback) : send(callback, attributes)
@@ -541,6 +542,15 @@ def call_reject_if(association_name, attributes)
       end
     end
 
+    # Only take into account the destroy flag if <tt>:allow_destroy</tt> is true
+    def will_be_destroyed?(association_name, attributes)
+      allow_destroy?(association_name) && has_destroy_flag?(attributes)
+    end
+
+    def allow_destroy?(association_name)
+      self.nested_attributes_options[association_name][:allow_destroy]
+    end
+
     def raise_nested_attributes_record_not_found!(association_name, record_id)
       raise RecordNotFound, "Couldn't find #{self.class._reflect_on_association(association_name).klass.name} with ID=#{record_id} for #{self.class.name} with ID=#{id}"
     end

activerecord/test/cases/nested_attributes_test.rb

@@ -146,6 +146,19 @@ def test_destroy_works_independent_of_reject_if
     assert man.reload.interests.empty?
   end
 
+  def test_reject_if_is_not_short_circuited_if_allow_destroy_is_false
+    Pirate.accepts_nested_attributes_for :ship, reject_if: ->(a) { a[:name] == "The Golden Hind" }, allow_destroy: false
+
+    pirate = Pirate.create!(catchphrase: "Stop wastin' me time", ship_attributes: { name: "White Pearl", _destroy: "1" })
+    assert_equal "White Pearl", pirate.reload.ship.name
+
+    pirate.update!(ship_attributes: { id: pirate.ship.id, name: "The Golden Hind", _destroy: "1" })
+    assert_equal "White Pearl", pirate.reload.ship.name
+
+    pirate.update!(ship_attributes: { id: pirate.ship.id, name: "Black Pearl", _destroy: "1" })
+    assert_equal "Black Pearl", pirate.reload.ship.name
+  end
+
   def test_has_many_association_updating_a_single_record
     Man.accepts_nested_attributes_for(:interests)
     man = Man.create(name: 'John')