Sqlite json/jsonb patch

[JakobKlocker]

added json_patch & jsonb_patch with the test cases provided by sqlite.
I'm not completely certain if the jsonb test cases are correct, since jsonb_patch returns a jsonb, yet they pass if I compare it to a json. This was done in the existing jsonb function as well, so I guess it should be correct.

#4366 (comment)

[weiznich]

Thanks for working on this and sorry for taking that long to review.

The implementation itself already looks on the right way, but we want to adjust the signatures slightly to better handle Null values as explained in the review comments.

[weiznich]

The return type is not correct. This function (and also jsonb_patch) returns Null if either the first or second argument is nullable.

We might want something like

Suggested change

	fn json_patch<J: JsonOrNullableJson + SingleValue>(j: J, patch: J) -> Json;
	fn json_patch<J1: JsonOrNullableJson + SingleValue + CombinedNullableValue<J2, Json>, J2: JsonOrNullableJson + SingleValue>(j: J1, patch: J1) -> J1::Out;

with the CombinedNullableValue trait defined here:

diesel/diesel/src/pg/expression/expression_methods.rs

Lines 3936 to 3949 in c5cdaa4

	pub trait CombinedNullableValue<O, Out>: SingleValue {
	type Out: SingleValue;
	}
	impl<T, O, Out> CombinedNullableValue<O, Out> for T
	where
	T: SingleValue,
	O: SingleValue,
	T::IsNull: OneIsNullable<O::IsNull>,
	<T::IsNull as OneIsNullable<O::IsNull>>::Out: MaybeNullableType<Out>,
	<<T::IsNull as OneIsNullable<O::IsNull>>::Out as MaybeNullableType<Out>>::Out: SingleValue,
	{
	type Out = <<T::IsNull as OneIsNullable<O::IsNull>>::Out as MaybeNullableType<Out>>::Out;
	}

(That needs to be moved to diesel/src/expressions/nullable.rs into a pub(crate) private module.

[weiznich]

It would be great to test for patch_json::<Nullable<Json>, _>(None, json!({something})) and patch_json(json!{something}, None) here. (Also for patch_jsonb)