Support for null values

[yonojoy]

We have a server that delivers the following JSON (for TClient):

{
	"id": 6718,
	"first_name": "Hans",
	"last_name": "Bauer",
	"telephone": null,
	"email": null,
	"gender": null
}

If I read this into a TClient variable and then send this to the server again, this is translated to

{
	"id": 6718,
	"first_name": "Hans",
	"last_name": "Bauer",
	"telephone": "",
	"email": "",
	"gender": 0
}

which the server does not like: "0 is not a valid value for gender".

There are two solutions to this problem:

1. Implement handling for null values
2. Provide an option to treat null as not ...HasValue

While I think 1 is the correct solution, 2 can be implemented more easily by modifying OpenApiJson.pas, for example for USEDBX:

function TJsonWrapper.ObjContains(JObj: TJSONValue; const Name: string; out Value: TJSONValue): Boolean;
var
  Pair: TJSONPair;
begin
  Pair := TJSONObject(JObj).Get(Name);
  if Assigned(Pair) then
  begin
	Value := Pair.JsonValue;
	if Value.Null and FOptionTreatNullAsNotHasValue then
	  Value := nil;
  end
  else
	Value := nil;
end;

Solution 2 would result in the following JSON, which the server happily processes:

{
	"id": 6718,
	"first_name": "Hans",
	"last_name": "Bauer"
}

@wlandgraf What do you think?

[wlandgraf]

@yonojoy I think the correct way is 1, to handle null values.
HasValue properties have a different meaning, and the meaning is exactly "is it present in JSON" (when reading) or "was it set from code" (when writing, which ends up in the property not being included in JSON).

So null should have a different treatment. I believe all nullable properties (and this should be rightfully inferred from the JSON schema) should have an additional XxxIsNull helper which indicates if the value is null or not.

And the property setter should set FIsNull to False the same way it sets FHasValue to True. The JSON serialization/deserialization will also get more complex.

[yonojoy]

@wlandgraf Yes, I completely agree with your analysis. But it's a big amount of work. I won't have the resources to implement this myself.

But for me and maybe for others, solution 2 solves the immediate problems. Therefore I would suggest to add a define TREAT_NULL_VALUES_AS_NOT_HAS_VALUE, that implements 2 until solution 1 will be available.