database.rules.json

{
  "rules": {

    "invites": {
    	".indexOn": ["email"],
      ".read": "auth !== null",
      ".write": "auth !== null"
    },

    "users": {
      ".read": "auth !== null",
      "$uid": {
      	".write": "$uid === auth.uid",
        "boards": {
          ".validate": "auth.email_verified === true"
        }
      }
    },

    "boards": {
      ".read": "auth != null && auth.email_verified === true",
    	"$board": {
       	".write": "auth != null && auth.email_verified === true && (!data.exists() || !data.child('members').exists() || (data.child('members').val().length === 1 && data.child('members').hasChild(auth.uid)))",
       	"posts": {
          "$post": {
          	".write": "!data.exists() || data.child('authorUID').val() === auth.uid",

            "val": {
              ".write": "data.parent().parent().parent().child('members').hasChild(auth.uid)"
            },

            "col": {
              ".write": "data.parent().parent().parent().child('members').hasChild(auth.uid)"
            }
          }
        },
        "members": {
          ".write": "root.child('users').child(auth.uid).child('boards').child($board).exists() || data.parent().child('invites').hasChild(root.child('users').child(auth.uid).child('email').val().replace('.', '%2E'))"
        },
        "invites": {
          ".write": "data.parent().child('members').hasChild(auth.uid)"
        }
      }
    }

  }
}