henrym:~/testapp$ rails c
Loading development environment (Rails 3.0.4)
r:001 > (some_firm = Firm.new).save   # Create and save a new Firm
 => true 
r:002 > some_firm.clients         # No clients yet
 => [] 
r:003 > some_firm.clients.new     # Create a new client
 => #<Client id: nil, firm_id: 1, created_at: nil, updated_at: nil> 
r:004 > some_firm.clients         # Still no clients
 => [] 
r:005 > some_firm.clients.build   # Create a new client with build
 => #<Client id: nil, firm_id: 1, created_at: nil, updated_at: nil> 
r:006 > some_firm.clients         # New client is added to clients 
 => [#<Client id: nil, firm_id: 1, created_at: nil, updated_at: nil>] 
r:007 > some_firm.save
 => true 
r:008 > some_firm.clients         # Saving firm also saves the attached client
 => [#<Client id: 1, firm_id: 1, created_at: "2011-02-11 00:18:47",
updated_at: "2011-02-11 00:18:47">] 

The only difference between some_firm.clients.new and some_firm.clients.build seems to be that build also adds the newly-created client to the clients collection.

build 与 new 的最大区别：

新的对象会在 collection 中，是这个样子吗？

reference http://stackoverflow.com/questions/4954313/build-vs-new-in-rails-3