added support for json_array_length and jsonb_array_length functions

[zaira-bibi]

Added support for json_array_length and jsonb_array_length functions under #4216.

Help needed:

1. I can't figure out the correct signature for both of the functions. I have implemented a JsonOrNullableJson trait but it does not tell that the functions are for json arrays only.
2. Under the current signature, I can't seem to test for nullability as json!(null) gives a scalar.

I'd really appreciate some guidance on this. TIA.

[zaira-bibi]

@weiznich I would really appreciate some help on the issues mentioned in the PR description. Thanks!

[weiznich]

Sorry for taking a few days to write this answer, this somehow got lost between all the other PR's 😞

This is already mostly the correct signature. We cannot restrict this function to only accept json arrays at compile time as that's a purely runtime concept, so it's fine to accept JsonOrNullableJson here. What needs to be done is to make the nullability of the return value dependend on the input nullability, which means if you put in a Nullable<Json> we want to return a Nullable<Integer> as the function returns null for null inputs.

You can do that by changing the function signature as follows:

Suggested change

	fn json_array_length<E: JsonOrNullableJson + SingleValue>(e: E) -> Integer;
	fn json_array_length<E: JsonOrNullableJson + MaybeNullableValue<Integer>>(e: E) -> E::Out;

As for the test: You should be able to test for null values like this:

let result = diesel::select(json_array_length::<Nullable<Json>, _>(None::<serde_json::Value>)).get_result::<Option<i32>(conn);

(The same applies to the jsonb variant of this function)

[zaira-bibi]

Thank you so much for the explanation! I have pushed in a new commit for the required changed. Please let me know if anything else needs to be changed/fixed. Thanks once again!

[weiznich]

Looks good now, thanks 🙏