Toql structs usually have a lot of Option types to make fields selectable with a query. Let's look how to attribute them with serde for smooth interaction.

It's nice to omit unselected fields. This can easily achieved with #[serde(skip_serializing_if = "Option::is_none")]

# #![allow(unused_variables)]
#fn main() {
    #[serde(skip_serializing_if = "Option::is_none")]
    age: Option<u8>

    #[serde(skip_serializing_if = "Option::is_none")]
    address: Option<Option<Join<Address>>> // Selectable left join
#}

Your server needs deserializing either

• when creating a new item
• or when updating an existing item

# #![allow(unused_variables)]
#fn main() {
    #[derive(Toql)]
    #[toql(auto_key = true)]
    struct User {
    
        // 'default' allows missing field 'id' in Json
        //  Needed typically for insert
        #[serde(default)] 
        #[toql(key)]
        id: u64
        
        // No Serde attribute:
        // Field must always be present in Json, but may be null -> None
        name: Option<String>

        // Never deserialize expressions
        #[serde(skip_deserializing)]  
        #[toql(sql = "(SELECT COUNT(*) From Book b WHERE b.author_id = ..id)")]
        pub number_of_books: Option<u64>,
    
        // See comment below
        #[serde(default, deserialize_with="des_double_option")]
        address: Option<Option<Join<Address>>> 
    }
#}

Notice the double Option on the selectable left join address. When deserializing from JSON the following mapping works:

To make this happen you need a custom deserialization function:

# #![allow(unused_variables)]
#fn main() {
use serde::{Deserializer, Deserialze};

pub fn des_double_option<'de, T, D>(de: D) -> Result<Option<Option<T>>, D::Error>
where
    T: Deserialize<'de>,
    D: Deserializer<'de>,
{
    Deserialize::deserialize(de).map(Some)
}
#}

Now you get the following:

• If you omit address in your JSON #[serde(default)] kicks in and you get None.
• If you send "addess": null, you get Some(None).
• If you send "address: {"id": 5}", you get Some(Some(Join::Key(AddressKey{id:5}))).
• If you send "address: {"id": 5, ...}", you get Some(Some(Join::Entity(Address{id:5, ...}))).

Toql update will now work as expected.