has_many .build does not set reverse reflection

class Category < ActiveRecord::Base

  has_many :things, :dependent => :destroy
end

class Thing < ActiveRecord::Base
  belongs_to :category
  validates_presence_of :category
end


>> c=Category.new
>> t=c.things.build
>> c.save
=> false
>> pp c.errors
#<ActiveRecord::Errors:0x7f24100e2528
 @base=#<Category id: nil, created_at: nil, updated_at: nil, name: nil>,
 @errors=
  {"things"=>
    [#<ActiveRecord::Error:0x7f24100cdbc8
      @attribute=:things,
      @base=#<Category id: nil, created_at: nil, updated_at: nil, name: nil>,
      @message=:invalid,
      @options={},
      @type=:invalid>]}>
=> nil

This seems very wrong to me. Things has a category, it was created through a category!

I certainly expect t.category_id to return nil. However, I would expect t.category to return the category. To work around the problem you have to insert a

>> t.category = c

before the save. Which is probably fine for most people. But in my code, that ends up being:

>> record.send("#{owner.class.reverse_reflection(association_name).name}=", owner)

So I went "UGH!" and opened this bug.