An example of bolt file for the article: https://medium.com/evenbit/securing-your-firebase-app-part-2-33260a84dc78

rules.bolt

type Message {
  message: String,
  author: FirebaseId,

  create() { isSignedIn() }
  read() { isSignedIn() }
  update() { isCurrentUser(this.author) }
  delete() { isCurrentUser(this.author) }
}

isCurrentUser(uid) { isSignedIn() && auth.uid === uid }
isSignedIn() { auth !== null }

rules.json

{
  "rules": {
    "messages": {
      "$message_id": {
        ".validate": "newData.hasChildren(['message', 'author'])",
        "message": {
          ".validate": "newData.isString()"
        },
        "author": {
          ".validate": "newData.isString()"
        },
        "$other": {
          ".validate": "false"
        },
        ".write": "data.val() == null && auth != null || data.val() != null && newData.val() != null && (auth != null && auth.uid == newData.child('author').val()) || data.val() != null && newData.val() == null && (auth != null && auth.uid == newData.child('author').val())",
        ".read": "auth != null"
      }
    }
  }
}