Schemars

Generate JSON Schema documents from Rust code

Basic Usage

If you don't really care about the specifics, the easiest way to generate a JSON schema for your types is to #[derive(JsonSchema)] and use the schema_for! macro. All fields of the type must also implement JsonSchema - Schemars implements this for many standard library types.

use schemars::{schema_for, JsonSchema};

#[derive(JsonSchema)]
pub struct MyStruct {
    pub my_int: i32,
    pub my_bool: bool,
    pub my_nullable_enum: Option<MyEnum>,
}

#[derive(JsonSchema)]
pub enum MyEnum {
    StringNewType(String),
    StructVariant { floats: Vec<f32> },
}

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

Click to see the output JSON schema...

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "MyStruct",
  "type": "object",
  "required": [
    "my_bool",
    "my_int"
  ],
  "properties": {
    "my_bool": {
      "type": "boolean"
    },
    "my_int": {
      "type": "integer",
      "format": "int32"
    },
    "my_nullable_enum": {
      "anyOf": [
        {
          "$ref": "#/definitions/MyEnum"
        },
        {
          "type": "null"
        }
      ]
    }
  },
  "definitions": {
    "MyEnum": {
      "anyOf": [
        {
          "type": "object",
          "required": [
            "StringNewType"
          ],
          "properties": {
            "StringNewType": {
              "type": "string"
            }
          }
        },
        {
          "type": "object",
          "required": [
            "StructVariant"
          ],
          "properties": {
            "StructVariant": {
              "type": "object",
              "required": [
                "floats"
              ],
              "properties": {
                "floats": {
                  "type": "array",
                  "items": {
                    "type": "number",
                    "format": "float"
                  }
                }
              }
            }
          }
        }
      ]
    }
  }
}

Serde Compatibility

One of the main aims of this library is compatibility with Serde. Any generated schema should match how serde_json would serialize/deserialize to/from JSON. To support this, Schemars will check for any #[serde(...)] attributes on types that derive JsonSchema, and adjust the generated schema accordingly.

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

#[derive(Deserialize, Serialize, JsonSchema)]
#[serde(rename_all = "camelCase")]
pub struct MyStruct {
    #[serde(rename = "myNumber")]
    pub my_int: i32,
    pub my_bool: bool,
    #[serde(default)]
    pub my_nullable_enum: Option<MyEnum>,
}

#[derive(Deserialize, Serialize, JsonSchema)]
#[serde(untagged)]
pub enum MyEnum {
    StringNewType(String),
    StructVariant { floats: Vec<f32> },
}

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

Click to see the output JSON schema...

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "MyStruct",
  "type": "object",
  "required": [
    "myBool",
    "myNumber"
  ],
  "properties": {
    "myBool": {
      "type": "boolean"
    },
    "myNullableEnum": {
      "default": null,
      "anyOf": [
        {
          "$ref": "#/definitions/MyEnum"
        },
        {
          "type": "null"
        }
      ]
    },
    "myNumber": {
      "type": "integer",
      "format": "int32"
    }
  },
  "definitions": {
    "MyEnum": {
      "anyOf": [
        {
          "type": "string"
        },
        {
          "type": "object",
          "required": [
            "floats"
          ],
          "properties": {
            "floats": {
              "type": "array",
              "items": {
                "type": "number",
                "format": "float"
              }
            }
          }
        }
      ]
    }
  }
}

#[serde(...)] attributes can be overriden using #[schemars(...)] attributes, which behave identically (e.g. #[schemars(rename_all = "camelCase")]). You may find this useful if you want to change the generated schema without affecting Serde's behaviour, or if you're just not using Serde.

Feature Flags

• impl_json_schema - implements JsonSchema for Schemars types themselves

Optional Dependencies

Schemars can implement JsonSchema on types from several popular crates, enabled via optional dependencies (dependency versions are shown in brackets):

• chrono (^0.4)
• indexmap (^1.2)
• either (^1.3)
• uuid (^0.8)
• smallvec (^1.0)
• arrayvec (^0.5)