DeepJson

class Parent {
        String name
        Integer age
        Date dob

        Child child
    }

    class Child {
        String name
        Integer age
        Date dob
        static hasMany = [qualities: Quality]
        GrandChild grandChild
    }

    class GrandChild {
        String name
        Integer noOfTeeth
        Date dob
        Address address
    }

    class Quality {
        Integer key
        String value
    }

    class Address {
        String street
        String apt
        String city
        Integer zip
        String country
    }

    //Parent Controller
    import grails.converters.JSON
    import grails.converters.XML
    def index() {
            JSON.use('deep')
            render Parent.all as JSON

            //XML.use('deep')
            //render Parent.all as XML
        }

    //Bootstrap
        def fastQuality = new Quality(key: 1, value: "Fast").save()
        def furiousQuality = new Quality(key: 2, value: "Furious").save()
        def programmerQuality = new Quality(key: 3, value: "Programmer").save()

        def address = new Address(street: 'Main St', apt: '123', zip: 43230, city: 'Maple Shade', country: 'USA').save()

        def gc = new GrandChild(name: "GrandChild", noOfTeeth: 10, dob: new Date(), address: address).save()

        def child =  new Child(name: "Child", age: 30, dob: new Date(), grandChild: gc).save()
        [fastQuality, furiousQuality, programmerQuality].each{child.addToQualities(it)}

        def parent = new Parent(name: "Parent", age: 70, dob: new Date(), child: child).save(flush: true, failOnError: true)


//JSON output
[
    {
        "class": "com.example.Parent",
        "id": 1,
        "age": 70,
        "child": {
            "class": "com.example.Child",
            "id": 1,
            "age": 30,
            "dob": "2013-06-28T05:53:58Z",
            "grandChild": {
                "class": "com.example.GrandChild",
                "id": 1,
                "address": {
                    "class": "com.example.Address",
                    "id": 1,
                    "apt": "123",
                    "city": "Maple Shade",
                    "country": "USA",
                    "street": "Main St",
                    "zip": 43230
                },
                "dob": "2013-06-28T05:53:58Z",
                "name": "GrandChild",
                "noOfTeeth": 10
            },
            "name": "Child",
            "qualities": [
                {
                    "class": "com.example.Quality",
                    "id": 1,
                    "key": 1,
                    "value": "Fast"
                },
                {
                    "class": "com.example.Quality",
                    "id": 2,
                    "key": 2,
                    "value": "Furious"
                },
                {
                    "class": "com.example.Quality",
                    "id": 3,
                    "key": 3,
                    "value": "Programmer"
                }
            ]
        },
        "dob": "2013-06-28T05:53:58Z",
        "name": "Parent"
    }
]

//XML Output
<?xml version="1.0" encoding="UTF-8"?>
<list>
  <parent id="1">
    <age>70</age>
    <child id="1">
      <age>30</age>
      <dob>2013-06-28 02:09:06.793 EDT</dob>
      <grandChild id="1">
        <address id="1">
          <apt>123</apt>
          <city>Maple Shade</city>
          <country>USA</country>
          <street>Main St</street>
          <zip>43230</zip>
        </address>
        <dob>2013-06-28 02:09:06.782 EDT</dob>
        <name>GrandChild</name>
        <noOfTeeth>10</noOfTeeth>
      </grandChild>
      <name>Child</name>
      <qualities>
        <quality id="1">
          <key>1</key>
          <value>Fast</value>
        </quality>
        <quality id="3">
          <key>3</key>
          <value>Programmer</value>
        </quality>
        <quality id="2">
          <key>2</key>
          <value>Furious</value>
        </quality>
      </qualities>
    </child>
    <dob>2013-06-28 02:09:06.807 EDT</dob>
    <name>Parent</name>
  </parent>
</list>