validate related records in the same validation context as parent.

E.G.:

```ruby
class Parent < ActiveRecord::Base
  has_one :child
  validates_presence_of :name, :on => "custom_context"
  validates_associated :child
end

class Child < ActiveRecord::Base
  belongs_to :parent
  validates_presence_of :name, :on => "custom_context"
end

p = Parent.new(:name => "Montoto", :child => Child.new)
p.valid?(:custom_context) # => Returns true, even though the child is not valid under the same context.
```

activerecord/lib/active_record/autosave_association.rb

@@ -295,7 +295,7 @@ def validate_collection_association(reflection)
     def association_valid?(reflection, record)
       return true if record.destroyed? || record.marked_for_destruction?
 
-      unless valid = record.valid?
+      unless valid = record.valid?(validation_context)
         if reflection.options[:autosave]
           record.errors.each do |attribute, message|
             attribute = "#{reflection.name}.#{attribute}"

activerecord/lib/active_record/validations/associated.rb

@@ -2,7 +2,7 @@ module ActiveRecord
   module Validations
     class AssociatedValidator < ActiveModel::EachValidator
       def validate_each(record, attribute, value)
-        if Array.wrap(value).reject {|r| r.marked_for_destruction? || r.valid?}.any?
+        if Array.wrap(value).reject {|r| r.marked_for_destruction? || r.valid?(record.validation_context) }.any?
           record.errors.add(attribute, :invalid, options.merge(:value => value))
         end
       end

activerecord/test/cases/validations/association_validation_test.rb

@@ -118,4 +118,21 @@ def test_validates_presence_of_belongs_to_association__existing_parent
     end
   end
 
+  def test_validates_associated_models_in_the_same_context
+    Topic.validates_presence_of :title, :on => :custom_context
+    Topic.validates_associated :replies
+    Reply.validates_presence_of :title, :on => :custom_context
+
+    t = Topic.new('title' => '')
+    r = t.replies.new('title' => '')
+
+    assert t.valid?
+    assert !t.valid?(:custom_context)
+
+    t.title = "Longer"
+    assert !t.valid?(:custom_context), "Should NOT be valid if the associated object is not valid in the same context."
+
+    r.title = "Longer"
+    assert t.valid?(:custom_context), "Should be valid if the associated object is not valid in the same context."
+  end
 end