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To view this page for the v1 version of Schemars, which is still under development, click here.

Deriving JsonSchema with Fields Using Custom Serialization

Serde allows you to change how a field is (de)serialized by setting a #[serde(with = "path")] attribute, where $path::serialize and $path::deserialize must be functions with the correct signature. Schemars supports the same attribute, but path must be a type implementing JsonSchema.

In order to derive JsonSchema on a type which includes a #[serde(with = "path")] attribute where path is not a type implementing JsonSchema, you'll need to override it with a suitable #[schemars(with = "Type")] or #[schemars(schema_with = "path")] attribute.

use schemars::{schema_for, JsonSchema, Schema, SchemaGenerator};
use serde::{Deserialize, Serialize};

// `int_as_string` and `bool_as_string` use the schema for `String`.
#[derive(Default, Deserialize, Serialize, JsonSchema)]
pub struct MyStruct {
    #[serde(default = "eight", with = "as_string")]
    #[schemars(with = "String")]
    pub int_as_string: i32,

    #[serde(default = "eight")]
    pub int_normal: i32,

    #[serde(default, with = "as_string")]
    #[schemars(schema_with = "make_custom_schema")]
    pub bool_as_string: bool,

    #[serde(default)]
    pub bool_normal: bool,
}

fn make_custom_schema(generator: &mut SchemaGenerator) -> Schema {
    let mut schema = String::json_schema(generator);
    schema
        .ensure_object()
        .insert("format".into(), "boolean".into());
    schema
}

fn eight() -> i32 {
    8
}

// This module serializes values as strings
mod as_string {
    use serde::{de::Error, Deserialize, Deserializer, Serializer};

    pub fn serialize<T, S>(value: &T, serializer: S) -> Result<S::Ok, S::Error>
    where
        T: std::fmt::Display,
        S: Serializer,
    {
        serializer.collect_str(value)
    }

    pub fn deserialize<'de, T, D>(deserializer: D) -> Result<T, D::Error>
    where
        T: std::str::FromStr,
        D: Deserializer<'de>,
    {
        let string = String::deserialize(deserializer)?;
        string
            .parse()
            .map_err(|_| D::Error::custom("Input was not valid"))
    }
}

fn main() {
    let schema = schema_for!(MyStruct);
    println!("{}", serde_json::to_string_pretty(&schema).unwrap());
}

Click to see the output JSON schema...

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "MyStruct",
  "type": "object",
  "properties": {
    "bool_as_string": {
      "type": "string",
      "format": "boolean",
      "default": "false"
    },
    "bool_normal": {
      "type": "boolean",
      "default": false
    },
    "int_as_string": {
      "type": "string",
      "default": "8"
    },
    "int_normal": {
      "type": "integer",
      "format": "int32",
      "default": 8
    }
  }
}

Note that the default values in the schema are serialized as strings where appropriate.