Grails one to one relation

I tried to define one to one relation in 2 different way:
Grails 2.0.3

Case 1:

class Car {
    String model  
    Engine eng  
    static constraints = {
        eng unique: true
    }
}

class Engine {
    Double capacity
    static belongsTo = [car : Car]
}

Case 2:

class Car {
    String model
    static hasOne = [eng : Engine]
    static constraints = {
        eng unique: true
    }
}


class Engine {
    Double capacity
    static belongsTo = [car : Car]
}

looks similar, and both provide one to one bidirectional mapping. Unfortunately DB has different structure in both cases.

Case 1:

Case 2:

Why once Car and once Engine keeps link to second table.

Where is my problem? When I am looking at the code, from DDD perspective, both cases suggest that Car class is more important and Car aggregates Engine. Unfortunately when I look from DB side on case 2 I would rather said that it is opposite - Engine aggregate Car. Of course I can use first approach, but most of publication I saw about grails, present second way for defining relation. Maybe I misunderstood something and I use hasOne in wrong way?