has_many: collection.build doesn't allow passing a block to set attributes

Given:

class Foo < ActiveRecord::Base
  has_many :bars
end

class Bar < ActiveRecord::Base
end

It is possible to set attributes of a newly created Bar, using

@bar = Bar.new do |b|
  b.something = "whatever"
end
@bar = Bar.create do |b|
  b.something = "whatever"
end
@bar = Foo.bars.create do |b|
  b.something = "whatever"
end

However, this doesn't work:

@bar = Foo.bars.build do |b|
  b.something = "whatever"
end