title: Jolt Transform Nested Grouping
body: <p>I have a JSON that has this flat structure:</p>
  
  <pre><code>[{
      "PK": "1111",
      "SOURCE_DB": "Oracle",
      "CONTACT_TYPE": "Phone",
      "CONTACT_SUBTYPE": "Work",
      "EMAIL": null
      "PHONE_COUNTRY_CODE": "44",
      "PHONE_NUMBER": "12345678",
      "PHONE_EXT": "907643",
      "STATUS": "Active"
      }, {
      "PK": "1111",
      "SOURCE_DB": "Oracle",
      "CONTACT_TYPE": "Phone",
      "CONTACT_SUBTYPE": "Home",
      "EMAIL": null
      "PHONE_COUNTRY_CODE": "353",
      "PHONE_NUMBER": "87654321",
      "PHONE_EXT": null,
      "STATUS": "Active"
      }, {
      "PK": "1111",
      "SOURCE_DB": "",
      "CONTACT_TYPE": "Email",
      "CONTACT_SUBTYPE": "Personal",
      "EMAIL": "me@mail.com"
      "PHONE_COUNTRY_CODE": null,
      "PHONE_NUMBER": null,
      "PHONE_EXT": null,
      "STATUS": "Active"
      },
      {
      "PK": "2222",
      "SOURCE_DB": "DB2",
      "CONTACT_TYPE": "Phone",
      "CONTACT_SUBTYPE": "Home",
      "EMAIL": null
      "PHONE_COUNTRY_CODE": "44",
      "PHONE_NUMBER": "98761234",
      "PHONE_EXT": null,
      "STATUS": "Inactive"
      }, {
      "PK": "2222",
      "SOURCE_DB": "DB2",
      "CONTACT_TYPE": "Email",
      "CONTACT_SUBTYPE": "Work",
      "EMAIL": "you@mail.co.uk"
      "PHONE_COUNTRY_CODE": null,
      "PHONE_NUMBER": null,
      "PHONE_EXT": null,
      "STATUS": "Active"
      }
  ]
  </code></pre>
  
  <p>Then, I want to group them, first by Key (PK), then within each entry, ContactMethods will be grouped together. This is the output:</p>
  
  <pre><code>{
      "Accounts": [{
              "Reference": {
                  "Key": "1111",
                  "System": "Oracle"
              },
              "ContactMethods": {
                  "Phone": [{
                          "Subtype": "Work",
                          "CountryCode": "44",
                          "Number": "12345678",
                          "Extension": "907643",
                          "Active": true
                      }, {
                          "Subtype": "Home",
                          "CountryCode": "353",
                          "Number": "87654321",
                          "Extension": null,
                          "Active": true
                      }
                  ],
                  "Email": [{
                          "Subtype": "Personal",
                          "EmailAddress": "my@mail.com",
                          "Active": true
                      }
                  ]
              }
          }, {
              "Reference": {
                  "Key": "2222",
                  "System": "DB2"
              },
              "ContactMethods": {
                  "Phone": [{
                          "Subtype": "Home",
                          "CountryCode": "44",
                          "Number": "98761234",
                          "Extension": null,
                          "Active": false
                      }
                  ],
                  "Email": [{
                          "Subtype": "Work",
                          "EmailAddress": "you@mail.co.uk",
                          "Active": true
                      }
                  ]
              }
          }
      ]
  }
  </code></pre>
  
  <p>I am able to group this by PK, but I am having difficulty on the second part, on how to do the grouping within the nested structure. Can you show a sample spec and put some explanation?</p>