How would I implement RESTful PUT URLs if the primary key is unknown before the resource is created?

I can see how this would work:

/user/456

with GET, POST, and DELETE but not with PUT unless the caller somehow knows the next primary key or they provide it themselves... how is this done?

I am going by what I read here: PUT vs POST in REST

The PUT me开发者_如何学JAVAthod requests that the enclosed entity be stored under the supplied Request-URI.

1. The client should send a POST request to /user to create the resource.
2. The server should then return a 201 CREATED response, with the URI of the resource in the Location header.
3. The client can then GET/PUT/DELETE from the URI it's been given to read/update/delete the resource.