{
  "id": "d7913a57-f458-4b81-8d6e-10f81e2b1dc2",
  "firstName": "John",
  "lastName": "Doe",
  "email": "johndoe@gmail.com",
  "companies" : [
      {
        "companyId": "6059be51-1e66-4f3a-9c8f-ced3bc89d562",
        "createdAt": 321321,
        "histories": [
          {
           "companyHistoryId": "dc9d1a57-fb82-4c25-a6cf-d27dfe114f8f",
           "content": "Company A was created.",
           "createdAt": 321321,
           "createdBy": "d7913a57-f458-4b81-8d6e-10f81e2b1dc2"
          }
        ],
     },
  ]
}

{
"version" : "2017-02-28",
"operation" : "UpdateItem",
"key" : {
    "id": { "S": "${context.arguments.id}" },
},
"update" : {
    "expression" : "SET companies = list_append(if_not_exists(companies, :emptyList), :newCompany)",
    "expressionValues" : {
        ":emptyList": { "L" : [] },
        ":newCompany" : { "L" : [
            { "M": {
                "name": { "S" : "${context.arguments.name}" },
                "createdAt": { "N" : "${context.arguments.createdAt}" },
                "companyId": { "S": "$util.autoId()" },
                "isActive": { "BOOL" : true },
                "histories": { "L": [
                    { "M": {
                        "companyHistoryId": { "S": "$util.autoId()" },
                        "content" : { "S" : "${context.arguments.name} was created." },
                        "createdAt": { "N" : "${context.arguments.createdAt}" },
                        "createdBy": { "S": "${context.arguments.id}" }
                    }}
                ] }
            }}
        ] },
    }
}