[
   {
      "relationships" : {
         "checked_in_by" : [
            {
               "type" : "Person",
               "id" : "..."
            }
         ],
         "checked_in_at" : [
            {
               "type" : "Station",
               "id" : "..."
            }
         ],
         "options" : [],
         "event_times" : [
            {
               "type" : "EventTime",
               "id" : "..."
            }
         ],
         "person" : [
            {
               "type" : "Person",

let data = try Data(contentsOf: URL(fileURLWithPath: file))
guard let json = try JSONSerialization.jsonObject(with: data) as? [[String:[String:Any]]] else {
    throw ConversionError.jsonNotArrayOfDictionaries
}
//Get the relationships with more than one value.  These will require join tables.
let joinTables = json.lazy
    .flatMap { ($0["relationships"] as? [String:[[String:Any]]])?.filter { $0.value.count > 1} }
    .map { $0.keys }
    .joined()