GraphQL Query with multiple strings via 1 parameter

[Stefan_van_Roosmalen]

Hi there!

I am tryting to create a GraphQL query which is handling 1 parameter containing one or more string value(s).
In the schema it looks like this:

type Query {
  products(pcodes: [String!]!): [Product!]!
}

In order to handle this, I have written a function like this:

function TQuery.Products(pcodes: array of String): TArray<TProduct>;
begin
  [...]
end;

I have also tried

function TQuery.Products(pcodes: TArray<String>): TArray<TProduct>;

but whatever I do, I get an error message like Project Project1.exe raised exception class EInvalidCast with message 'Invalid class typecast'.

Any suggestions?

Thanks for your reply.

Stefan

[wlandgraf (Wagner Landgraf)]

Can you please provide the full code of the project? How are you registering such method in the GraphQL schema?

[Stefan_van_Roosmalen]

As a starting point, I have used the Bookshelf demo. From there I have altered my own code...

This is the model I am using:

type Product {
  id: Int!
  code: String!
  price: Float!
}

type ProductsOutput {
  products: [Product]
}

type Query {
  product(code: String!): Product
  products(codes: [String!]!): [Product!]!
}

And this is the unit in which the implementation of the functions is done:

unit GraphQL.StefansTest;

interface

uses
  Generics.Collections, GraphQL.Main, GraphQL.Schema, FireDAC.Comp.Client,
  FireDAC.Stan.Error, FireDAC.Stan.Param, System.SysUtils, System.StrUtils;

type
  TProduct = class
  private
    FId: Integer;
    FCode: String;
    FPrice: Double;
  public
    constructor Create(AId: Integer; ACode: String; APrice: Double);
    property Id: Integer read FId;
    property Code: String read FCode;
    property Price: Double read FPrice;
  end;

  TQuery = class
  public
    function Product(Code: String): TProduct;
    function Products(const Codes: TArray<string>): TArray<TProduct>;
  end;

implementation

uses
  DataConnModule;

var
  ProductList: TObjectList<TProduct>;

{ TProduct }

constructor TProduct.Create(AId: Integer; ACode: String; APrice: Double);
begin
  inherited Create;

  FId := AId;
  FCode := ACode;
  FPrice := APrice;
end;

{ TQuery }

function TQuery.Product(Code: String): TProduct;
var
  qWork: TFDQuery;
begin
  qWork := TFDQuery.Create(nil);
  qWork.Connection := dmDataConn.DBConn;

  with qWork do
  begin
    Close;
    SQL.Clear;
    SQL.Add('select top 1 ' +
            '       ID, ' +
            '       InternalNumber, ' +
            '       SellingPrice ' +
            'from Products ' +
            'where ID = ' + Code + ' ' +
            'order by ID desc ');
    Open;
    FetchAll;
  end;

  if qWork.RecordCount = 0 then
    Result := nil
  else
    Result := TProduct.Create(qWork.FieldByName('ID').AsInteger,
                              qWork.FieldByName('ID').AsString,
                              qWork.FieldByName('SellingPrice').AsFloat);

  qWork.Close;
  qWork.Free;
end;

function TQuery.Products(const Codes: TArray<string>): TArray<TProduct>;
var
  qWork: TFDQuery;
  tmpLine: String;
  i: Integer;
begin
  {
     to be completed
  }
  Result := ProductList.ToArray;
end;

initialization
  ProductList := TObjectList<TProduct>.Create(True);

finalization
  ProductList.Free;

end.

When I run a Product query (including parameter with 1 value only) it is working fine. However, when I run a Products query (including parameter with multiple values) the following error raises:
Project Project1.exe raised exception class EInvalidCast with message 'Invalid class typecast'.

Hopefully this is something that can be solved easily?
Thanks for helping.

Regards,
Stefan

[wlandgraf (Wagner Landgraf)]

To receive array values, item type should be TValue. So you should use it like this:

function TQuery.Products(pcodes: TArray<TValue>): TArray<TProduct>;

[Stefan_van_Roosmalen]

Thanks!
It's working perfect now.