This feature is required because has_one associations meets often in Rails app. If we want to have possibility to seamlesly port applications to Grails, this is musthave feature

Fixed. Not this fix applies only to bidirectional one-to-ones. A unidirectional one-to-one is still mapped as a many-to-one by default. You can make it unique if you don't like this, which results in a true one-to-one:

class Face {
    Nose nose

    static mapping = {
        nose unique:true
    }
}

I downloaded 1.1 RC1 and groovy 1.6 and create two classes:

class Face {
String eyes;
String mouth;
Nose nose

static mapping = { nose unique:true }
}

class Nose { Face face String nostril1 String nostril2 }

When i bring up the app, i still see two foreign key columns:
alter table face add constraint FK2FD65D7F0070FA foreign key (nose_id) references nose
alter table nose add constraint FK33AFD3BF056CBA foreign key (face_id) references face

In order to get the single column mapping you need to define an owner:

class Face {
String eyes;
String mouth;
Nose nose

static mapping = { nose unique:true }
}

class Nose { 
   static belongsTo [face:Face] // THIS IS THE CHANGE
   String nostril1 
   String nostril2 
}

Sorry should have been clear on that in the issue

Thanks, for the clarification. If i understand what you are saying (and i tested it and it works!), then this means that we in order to have the one FK, we are forced to have the cascade in this direction and not the reverse. In other words, if the FK were in Nose instead, then Face would need to belongTo Nose and cascading would be reversed. Or is there some way around this?