feat: use enum values as keys for map

[MicaiahReid]

This PR fixes GREsau/okapi#128.

When generating a schema for a Map with a key that is an enum, the generated schema only allows keys based off of the enum options. For example,

use schemars::{schema_for, JsonSchema};
use serde::Serialize;
use std::collections::BTreeMap;

#[derive(serde::Serialize, JsonSchema, Debug)]
struct EnumKeyStruct {
    key: BTreeMap<Thing, SomeOtherStruct>,
}

#[derive(serde::Serialize, JsonSchema, Debug)]
enum Thing {
    Option1,
    Option2,
}

#[derive(serde::Serialize, JsonSchema, Debug)]
struct SomeOtherStruct {
    key1: String,
    key2: u64,
    key3: bool,
}

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

yields:

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "EnumKeyStruct",
  "type": "object",
  "required": [
    "key"
  ],
  "properties": {
    "key": {
      "oneOf": [
        {
          "type": "object",
          "required": [
            "\"Option1\""
          ],
          "properties": {
            "\"Option1\"": {
              "$ref": "#/definitions/SomeOtherStruct"
            }
          }
        },
        {
          "type": "object",
          "required": [
            "\"Option2\""
          ],
          "properties": {
            "\"Option2\"": {
              "$ref": "#/definitions/SomeOtherStruct"
            }
          }
        }
      ]
    }
  },
  "definitions": {
    "SomeOtherStruct": {
      "type": "object",
      "required": [
        "key1",
        "key2",
        "key3"
      ],
      "properties": {
        "key1": {
          "type": "string"
        },
        "key2": {
          "type": "integer",
          "format": "uint64",
          "minimum": 0.0
        },
        "key3": {
          "type": "boolean"
        }
      }
    },
    "Thing": {
      "type": "string",
      "enum": [
        "Option1",
        "Option2"
      ]
    }
  }
}

This also maintains the original behavior for maps with non-enum keys:

BTreeMap<String, String>

Code

use schemars::{schema_for, JsonSchema};
use serde::Serialize;
use std::collections::BTreeMap;

#[derive(Serialize, JsonSchema, Debug)]
struct PlainMap {
    key: BTreeMap<String, String>,
}

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

Output

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "PlainMap",
  "type": "object",
  "required": [
    "key"
  ],
  "properties": {
    "key": {
      "type": "object",
      "additionalProperties": {
        "type": "string"
      }
    }
  }
}

BTreeMap<SomeStruct, String>

Code

use schemars::{schema_for, JsonSchema};
use serde::Serialize;
use std::collections::BTreeMap;

#[derive(serde::Serialize, JsonSchema, Debug)]
struct SomeStruct {
    key: BTreeMap<SomeOtherStruct, String>,
}

#[derive(serde::Serialize, JsonSchema, Debug)]
struct SomeOtherStruct {
    key1: String,
    key2: u64,
    key3: bool,
}
fn main() {
    let schema = schema_for!(SomeOtherStruct);
    println!("{}", serde_json::to_string_pretty(&schema).unwrap());
}

Output

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "SomeOtherStruct",
  "type": "object",
  "required": [
    "key1",
    "key2",
    "key3"
  ],
  "properties": {
    "key1": {
      "type": "string"
    },
    "key2": {
      "type": "integer",
      "format": "uint64",
      "minimum": 0.0
    },
    "key3": {
      "type": "boolean"
    }
  }
}

[MicaiahReid]

@GREsau I think I've got this working, I'd appreciate a review!

[MicaiahReid]

This definitely feels a bit hacky, I'm open to suggestions.

[GREsau]

Thanks for the PR, but unfortunately I don't think the schemas created by this change are correct.

In your example:

• according to the schema, key must be either an object with an Option1 property, or an object with an Option2 property (but not both). This means the schema would reject empty maps, or a map with both Option1 and Option2.
• it probably shouldn't add Thing to definitions since it's not actually referenced anywhere in the schema.
• according to the schema, the property names include double quotes (e.g. "Option1" instead of Option1), which is not correct

I think the correct schema would be something like:

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "EnumKeyStruct",
  "type": "object",
  "required": [
    "key"
  ],
  "properties": {
    "key": {
      "type": "object",
      "properties": {
        "Option1": {
          "$ref": "#/definitions/SomeOtherStruct"
        },
        "Option2": {
          "$ref": "#/definitions/SomeOtherStruct"
        }
      }
    }
  },
  "definitions": {
    "SomeOtherStruct": {      
      /* ... */
    }
  }
}