`Option<T>` defaults to `None` when missing fields

[emirhantasdeviren]

Even if #[serde(default)] is not specified on a field with type Option<T> it still falls to None rather returning an error if the field is missing. If this is intentional why does that attribute exist?

Here is the playground link on version 1.0.202 https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=7ecdefd0f0b00f70d969571b36a5f50e

[valaphee]

Option<T> is one of the basic buildings block for allowing optional values, when a value is not present its Option<T>::None, therefore doesn't require Default.

#[serde(default)] just maps None to T's Default, which would also be None in the case of Option<T>, and therefore redundant.

And it's easier to write Option<T> instead of:

#[serde(skip_serializing_if = "Option::is_none")]
#[serde(default)]
_: Option<T>

[emirhantasdeviren]

Then, I have a question. In the case of a JSON payload how will I know if given field is absent or set as null.

For example in a PATCH endpoint of a resource I need to know if given field is set to null I will update the entity in my database if the field is absent I won't do any change in the database.

In a PUT endpoint I specifically need the field present in the payload even it is Option<T>, it must be set to null or some valid value.

[valaphee]

Ah, there is already an issue for this use case https://github.com/serde-rs/serde/issues/1042

atm you would have to create your own type https://stackoverflow.com/questions/44331037/how-can-i-distinguish-between-a-deserialized-field-that-is-missing-and-one-that

[valaphee]

Here is a more convenient method https://docs.rs/serde_with/latest/serde_with/rust/double_option/