Crate schemars

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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> },
}

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"
                        }
                    },
                    "additionalProperties": false
                },
                {
                    "type": "object",
                    "required": [
                        "StructVariant"
                    ],
                    "properties": {
                        "StructVariant": {
                            "type": "object",
                            "required": [
                                "floats"
                            ],
                            "properties": {
                                "floats": {
                                    "type": "array",
                                    "items": {
                                        "type": "number",
                                        "format": "float"
                                    }
                                }
                            }
                        }
                    },
                    "additionalProperties": false
                }
            ]
        }
    }
}

§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", deny_unknown_fields)]
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> },
}

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"
        }
    },
    "additionalProperties": false,
    "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.

§Schema from Example Value

If you want a schema for a type that can’t/doesn’t implement JsonSchema, but does implement serde::Serialize, then you can generate a JSON schema from a value of that type. However, this schema will generally be less precise than if the type implemented JsonSchema - particularly when it involves enums, since schemars will not make any assumptions about the structure of an enum based on a single variant.

use schemars::schema_for_value;
use serde::Serialize;

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

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

let schema = schema_for_value!(MyStruct {
    my_int: 123,
    my_bool: true,
    my_nullable_enum: Some(MyEnum::StringNewType("foo".to_string()))
});
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",
    "examples": [
        {
            "my_bool": true,
            "my_int": 123,
            "my_nullable_enum": {
                "StringNewType": "foo"
            }
        }
    ],
    "type": "object",
    "properties": {
        "my_bool": {
            "type": "boolean"
        },
        "my_int": {
            "type": "integer"
        },
        "my_nullable_enum": true
    }
}

§Feature Flags

  • derive (enabled by default) - provides #[derive(JsonSchema)] macro
  • impl_json_schema - implements JsonSchema for Schemars types themselves
  • preserve_order - keep the order of struct fields in Schema and SchemaObject

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

For example, to implement JsonSchema on types from chrono, enable it as a feature in the schemars dependency in your Cargo.toml like so:

[dependencies]
schemars = { version = "0.8", features = ["chrono"] }

Modules§

  • gen
    JSON Schema generator and settings.
  • schema
    JSON Schema types.
  • visit
    Contains the Visitor trait, used to recursively modify a constructed schema and its subschemas.

Macros§

Traits§

  • JsonSchema
    A type which can be described as a JSON Schema document.

Type Aliases§

  • Map
    The map type used by schemars types.
  • MapEntry
    A view into a single entry in a map, which may either be vacant or occupied. This is constructed from the entry method on BTreeMap or IndexMap, depending on whether the preserve_order feature flag is set.
  • Set
    The set type used by schemars types.

Derive Macros§