Question

I have two tables: Region and District. So region_id is the foreign key into the table District (a region has one or many districts). So, when I select a region on my list I only want to display the districts associated with that particular region.
My correct code displays all the districts independently of region:

def list = {
        params.max = Math.min(params.max? params.int('max') : 20, 100)
        [districtInstanceList : District.list(params),
        districtInstanceTotal: District.count()]
    } 

Does someone know how to only display based on the foreign key constraint? I know I could write a SQL query in my list closure, but I suppose grails probably has a way to do it.
My database is MySQL, and the grails version is 2.0.1.
My District domain is:

class District {

def scaffold = true

String name
String description
String logo
String homepage
// defines the 1:n constrain with the Region table
static belongsTo = [region : Region]
// defines the 1: constraint with the Stream table
static hasMany = [streams : Stream]

static constraints ={
    name(blank:false, minSize:6, maxSize:30)
    description(blank: false, maxSize:100)
}

public String toString(){
    name
}
 }

Answer 1

You can use GORM:

def list = {
    params.max = Math.min(params.max? params.int('max') : 20, 100)
    Region region = Region.get(params.id) // or what parameter you're using
    List districts = District.findAllByRegion(region)
    [districtInstanceList : districts,
    districtInstanceTotal: District.count()]
} 

You can read about Grails GORM here: http://grails.org/doc/latest/guide/GORM.html