A sample Firebase database rules file for the article: https://medium.com/evenbit/securing-your-firebase-app-part-2-33260a84dc78

api-key.database.rules.json

{
  "rules": {
    "queue": {
      "tasks": {
        "$task_id": {
          ".validate": "newData.hasChildren(['apiKey', 'someValue', 'anotherValue']) && root.child('apiKeys').child(apiKey).val() != null",
          "apiKey": {
            ".validate": "newData.isString()"
          },
          "someValue": {
            ".validate": "newData.isNumber()"
          },
          "anotherValue": {
            ".validate": "newData.hasChildren()"
          },
          "$other": {
            ".validate": "false"
          }
        }
      }
    },
    "apiKeys": {
      "$apiKey": {
        ".validate": "newData.isString()"
      },
      ".validate": "newData.hasChildren()"
    }
  }
}