TPersonRelation = class(TCollectionItem)
private
  FName: string;
  FDescription: string;
published
  property Name: string read FName write FName;
  property Description: string read FDescription write FDescription;
end;

TPersonRelations = class(TCollection)
private
  function GetItem(Index: Integer): TPersonRelation;
  procedure SetItem(Index: Integer; const Value: TPersonRelation);
public
  constructor Create;
  property Items[Index: Integer]: TPersonRelation read GetItem write SetItem; default;
end;

TPerson = class(TPersistent)
private
  FAddress: TPersonAddress;
  FColleagues: TStringList;
  FBirthDate: string;
  FName: string;
  FEmail: string;
  FTelephone: string;
  FGender: string;
  FNationality: string;
  FJobTitle: string;
  FURL: string;
  FRelations: TPersonRelations;
public
  constructor Create;
  destructor Destroy; override;
published
  property Address: TPersonAddress read FAddress;
  property Colleagues: TStringList read FColleagues;
  property Email: string read FEmail write FEmail;
  property JobTitle: string read FJobTitle write FJobTitle;
  property Name: string read FName write FName;
  property BirthDate: string read FBirthDate write FBirthDate;
  property Gender: string read FGender write FGender;
  property Nationality: string read FNationality write FNationality;
  property Telephone: string read FTelephone write FTelephone;
  property URL: string read FURL write FURL;
  property Relations: TPersonRelations read FRelations;
end;

var
  p: TPerson;
  r: TPersonRelation;
begin
  p := TPerson.Create;
  try
    p.JSON := jsonSample;

    r := p.Relations.Add;
    r.Name := 'John Doe';
    r.Description := 'Brother';

    r := p.Relations.Add;
    r.Name := 'Mia Reyes';
    r.Description := 'Mother';

    p.Log;
  finally
    p.Free;
  end;
end;

{
  "$type": "TPerson",
  "Address": {
    "$type": "TPersonAddress",
    "AddressLocality": "Colorado Springs",
    "AddressRegion": "CO",
    "PostalCode": "80840",
    "StreetAddress": "100 Main Street"
  },
  "BirthDate": "1979-10-12",
  "Colleagues": [],
  "Email": "info@example.com",
  "Gender": "female",
  "JobTitle": "Research Assistant",
  "Name": "Jane Doe",
  "Nationality": "Albanian",
  "Relations": [
    {
      "$type": "TPersonRelation",
      "Description": "Brother",
      "Name": "John Doe"
    },
    {
      "$type": "TPersonRelation",
      "Description": "Mother",
      "Name": "Mia Reyes"
    }
  ],
  "Telephone": "(123) 456-6789",
  "URL": "http://www.example.com"
}

const
  jsonSample =
    '{' +
      '"address":{' +
        '"addressLocality":"Colorado Springs",' +
        '"addressRegion":"CO",' +
        '"postalCode":"80840",' +
        '"streetAddress":"100 Main Street"' +
      '},' +
      '"colleague":[' +
        '"http://www.example.com/JohnColleague.html",' +
        '"http://www.example.com/JameColleague.html"' +
      '],' +
      '"email":"info@example.com",' +
      '"jobTitle":"Research Assistant",' +
      '"name":"Jane Doe",' +
      '"birthDate":"1979-10-12",' +
      '"gender":"female",' +
      '"nationality":"Albanian",' +
      '"relations": ['+
        '{'+
          '"Description": "Brother",'+
          '"Name": "John Doe"'+
        '},'+
        '{'+
          '"Description": "Mother",'+
          '"Name": "Mia Reyes"'+
        '}'+
      '],'+
      '"telephone":"(123) 456-6789",' +
      '"url":"http://www.example.com"' +
    '}';

This is because the way the JSON is loaded. When using the class helpers, the "$type" property is ignored. Basically, it is adapted to make sure it can load any kind of JSON, whether it's JSON coming directly from a predefined object structure, or an unknown structure that needs to be mapped on the object, without knowing the class types of the JSON objects inside the JSON structure. To fix this, we need to implement an interface on our TPersonRelations collection class, named ITMSFNCBaseListIO.

ITMSFNCBaseListIO = interface
['{FAB1D21E-D798-4CE0-B17B-9D75E4456AB4}']
  function GetItemClass: TClass;
end;

TPersonRelations = class(TCollection, ITMSFNCBaseListIO)
private
  function GetItem(Index: Integer): TPersonRelation;
  procedure SetItem(Index: Integer; const Value: TPersonRelation);
  function GetItemClass: TClass;
  function QueryInterface(const IID: TGUID; out obj): HResult; stdcall;
  function _AddRef: Integer; stdcall;
  function _Release: Integer; stdcall;
public
  constructor Create;
  property Items[Index: Integer]: TPersonRelation read GetItem write SetItem; default;
  function Add: TPersonRelation;
end;

ITMSFNCBasePersistenceIO = interface
  ['{91DEAFC3-8932-45F4-A3ED-5AAA0C0E9250}']
  function CreateObject(const AClassName: string; const ABaseClass: TClass): TObject;
end;

TPerson = class(TInterfacedPersistent, ITMSFNCBasePersistenceIO)
...
protected
  function CreateObject(const AClassName: string; const ABaseClass: TClass): TObject;
public
...

and the implementation

function TPerson.CreateObject(const AClassName: string;
  const ABaseClass: TClass): TObject;
begin
  Result := nil;
  if AClassName = 'TPersonRelation' then
    Result := TPersonRelation.Create(Relations);
end;

To load the data, we can now use our class helper and make sure we register our TPersonRelation class.

var
  p: TPerson;
begin
  p := TPerson.Create;
  try
    p.JSON := jsonSample;
    p.Log;
  finally
    p.Free;
  end;
end;

RegisterClass(TPersonRelation);

var
  p: TPerson;
begin
  p := TPerson.Create;
  try
    TTMSFNCObjectPersistence.LoadObjectFromString(p, jsonSample);
    p.Log;
  finally
    p.Free;
  end;
end;