Note that this is assuming that you've used the "generate-all" command to create the default scaffolding. The screen listing the Posts also shows the User:
However, as we can see from the above screenshot the values in the drop down and the User field don't seem to make much sense. The reason for the values are that by default the scaffolding generator calls the "toString" method on the object. Which is set to return the class name of the object, plus it's unique id. While this makes sense to have as the default, we need to change it in order to make it easier for people to read.
Fortunately, changing it is quite straightforward, with us only needing to define (override) the toString method. Simply add the following to the User domain class:
After putting this into the User controller, restart the grails application and you should see the usernames of the users in the drop down when creating a new Post:
This works for any class that you can think of. Simply override the toString method to make it more presentable to the people using the site. I do this almost automatically for all of my domain classes.