Serde: custom variant tagging based on nested field

help
1

Hi, I'm trying to deserialize a json like this:

        {
            "field1": {
                "type": "type1",
                "id":"field1-id"
            },
            "att1": true,
            "att2": "A"
        }

Another example:

        {
            "field1": {
                "type": "type2",
                "id":"field1-id"
            },
            "att3": "aaaa",
            "att4": 123
        }

The basic gist of this being: the main json object has always a "field1" object, and one of its properties defines the main object's "type", which in turn determines a number of extra properties.

I thought about implementing with a struct, an enum tagged on "type" with variants to be flattened into the main struct:

pub struct Field1 {
    #[serde(rename = "type")]
    pub field_type: String,
    pub id: String,
}

pub struct MyStruct {
    pub field1: Field1,
    #[serde(flatten)]
    pub variant: Type,
}

#[serde(tag = "type")]
pub enum Type {
    Unknown,
    Type1 {
        att1: bool,
        att2: String,
    },
    Type2 {
        att3: String,
        att4: i32,
    },
}

But of course this doesn't work, as "type" is not part of the enum to be tagged, so my question is, is it possible to provide a custom way of determining the correct variant, or perhaps this approach is completely wrong and there's another way?

Thank you for your help!

2

Based on your examples and definition of the problem, what you could do is to define the main object's type as an enum, and each variant as a separate struct:


pub enum MyMainModel {
    Type1(Type1Struct),
    Type2(Type2Struct),
}

pub struct Field1 {
    #[serde(rename = "type")]
    pub field_type: Type,
    pub id: String,
}

pub struct Type1Struct {
    pub field1: Field1,
    pub att1: Bool,
    pub att2: String,
}

pub struct Type2Struct {
    pub field1: Field1,
    pub att3: String,
    pub att4: usize,
}

pub enum Type {
  type1,
  type2
}

I guess that gives you an idea of what to do next.

1 Like
3

Thank you for your reply, your solution actually did work, however while reviewing my code and documentation I ended up using serde(untagged) .

I had tried untagged previously, but that didn't quite work, but then I realized that was a trivial mistake on my side. Untagged tries to deserialize each variant in order: having Unknown with no fields as a first variant means it will always succeed.

I know it's a bit silly on my side, that'll teach me to read the docs more carefully.

Thanks!

4

Yes, I intentionally left the untagged part out of my post because I was just skimming it out really quick. That's why I thought you would figure out the rest :smiley:.

5

This topic was automatically closed 90 days after the last reply. We invite you to open a new topic if you have further questions or comments.