class Group {
...
static mapping = { people column:'person_id' }
}
class Person {
...
static mapping = { groups column:'group_id' }
}

class Actor {

static belongsTo = Film
static hasMany = [films:Film]

String firstName
String lastName

static constraints = { firstName(maxSize:45) lastName(maxSize:45) }
}
class Film {

static hasMany = [actors:Actor]

String title
String description
Integer releaseYear
Integer rentalDuration
BigDecimal rentalRate
Integer length
BigDecimal replacementCost
String rating

static constraints = { title(maxSize:255) description(blank:true, nullable:true, maxSize:65535) releaseYear(nullable:true, range:1900..2100) rentalDuration(min:1, max:127) rentalRate(min:0.00, max:99.00, scale:2) length(nullable:true, min:1, max:32767) replacementCost(min:0.00, max:999.00, scale:2) rating(inList:["G", "PG", "PG-13", "R", "NC-17"]) }

}

When you have the domain classes as above Grails will automatically create the film_actor table containing:

• actors_id
• films_id

Now the critical part is that it appears that actors_id references the film table and films_id references the actor table... Should be the other way around I think.

class Actor {
...
static mapping = { films column:'actor_id' }
}

class Film {
...
static mapping = { actors column:'film_id' }
}