Svarog is a CLI that helps you protect your Firestore schema from unwanted mutations. It generates a set of of helper functions based on JSON Schema files that you can use in your Security Rules to validate user input.

Svarog was designed to be compatible with JSON Schema 7 - the latest draft of the JSON Schema standard. To get started, create a folder in your project directory, place your schema in a *.json file and give it an $id:

{
  "$schema": "http://json-schema.org/draft-07/schema",
  "$id": "Apple",
  "type": "object",
  "properties": {
    "color": {
      "type": "string",
      "enum": ["green", "red"]
    }
  },
  "required": ["color"]
}

You can use any built-in type to describe your database schema. However, you should also keep in mind that not all of the JSON Schema features are supported at the moment.

Svarog includes basic support for Timestamp, Bytes, LatLng and Path Firestore field types. To enable type checking on such fields, register the appropriate schemas in definitions section and then reference them in your main schema with $ref like this:

{
  "$schema": "http://json-schema.org/draft-07/schema",
  "$id": "FirestoreExample",
  "type": "object",
  "definitions": {
    "Timestamp": {},
  },
  "properties": {
    "date": { "$ref": "#/definitions/Timestamp" },
  }
}

Once you have your schema ready, install and run Svarog:

$ npm i -g svarog
$ svarog "schema/*.json" "firestore.rules" --verbose

The last command will pull every schema in schema folder, run the compiler and append a minified code snippet to the firestore.rules file. You can run this command every time you update your schema, and it will replace the generated snippet for you automatically if both old and new helpers were created with the compatible versions of CLI.

The code we generated in the previous step exposes isValid($id: string): boolean function that you can use in your Security Rules together with other assertions:

match /apples/{appleId} {
  allow write: if isValid("Apple");
}

Svarog will apply a strict schema check when a document is created (assuring that all required properties are present and nothing out of the schema is added), and a loose one on each update (when some properties defined in schema may be missing from the patch object).

USAGE
  $ svarog INPUT [OUTPUT]

ARGUMENTS
  INPUT   input file containing JSON Schema or a glob pattern
  OUTPUT  target file where Svarog will output security rule helpers

OPTIONS
  -f, --force    overwrites existing Svarog code unconditionally
  -h, --help     displays this message
  -v, --verbose  enables progress logs during compilation