Importing JSON into TFDMemTable

Question

I'm attempting to import JSON from a TMemo to a TFDMemTable. Upon execution I get a "Exception EAccessViolation..." error. The line of code that appears to be causing the problem is FDMemTable1.FieldByName('userId').AsString := oProd.GetValue('PutRequest.Item.userId').Value; which is where I'm appending my first entry to the FDMemTable. I checked the JSON and it appears to be formatted correctly.

Here's my JSON array:

{"Jobs":[{"PutRequest":{"Item":{"userId":{"S":"1"},"WorkOrder":{"S":"29236"},"ServiceDate":{"S":"4/12/2019"}}}},{"PutRequest":{"Item":{"userId":{"S":"1"},"WorkOrder":{"S":"29237"},"ServiceDate":{"S":"4/12/2019"}}}}]}

Here's a snippet from my code:

uses
  System.JSON;

procedure TForm1.FormCreate(Sender: TObject);
var
  oJson: TJSONObject;
  oArr: TJsonArray;
  oPair: TJSONPair;
  i: Integer;
  oProd: TJSONObject;
begin
  FDMemTable1.FieldDefs.Add('userId', ftString, 5);
  FDMemTable1.FieldDefs.Add('WorkOrder', ftString, 5);
  FDMemTable1.FieldDefs.Add('ServiceDate', ftString, 10);
  FDMemTable1.Active := True;

  oJson := TJSONObject.ParseJSONValue(TEncoding.ASCII.GetBytes(Memo1.Lines.Text), 0) as TJSONObject;
  try
    oArr := oJson.Get('Jobs').JsonValue as TJSONArray;
    for i := 0 to oArr.Count - 1 do begin
      oProd := oArr.Items[i] as TJSONObject;
      FDMemTable1.Append;
      FDMemTable1.FieldByName('userId').AsString := oProd.GetValue('PutRequest.Item.userId').Value;
      FDMemTable1.FieldByName('WorkOrder').AsString := oProd.GetValue('PutRequest.Item.WorkOrder').Value;
      FDMemTable1.FieldByName('ServiceDate').AsString := oProd.GetValue('PutRequest.Item.ServiceDate').Value;
      FDMemTable1.Post;
    end;
  finally
    oJson.Free;
  end;
end;

I have checked the assignments and values of "oArr" and "oProd" and they appear correct. I'm guessing that I'm not addressing the JSON properly when I'm trying to assign the value to the FDMemTable. How do I resolve this?

Answer 1

You forgot to add

`FDMemTable1.CreateDataSet;`

under

FDMemTable1.FieldDefs.Add('ServiceDate', ftString, 10);

which means your dataSet Fields are not created yet and thus the call to

FDMemTable1.FieldByName('userId').AsString

will return nil and throw an EV when assigning to it

add that line and you are good to go

see this guide by Jim McKeeth (the code under the video) for proper way to use TMemTable and what it can do.