Firestore Rules Example in Google Console

firestore.rules

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /shoppinglist/{lists} {
      function isSignedIn() {
        return request.auth != null;
      }
      function isEmailVerified() {
        return request.auth.token.email_verified != false;
      }
      function isKnownUser() {
        return isSignedIn() && request.auth.uid != "" && isEmailVerified();
      }
      function UserId() {
        return request.auth.uid;
      }
      function getOwner(rsc) {
        // Read Owner in the resource (rsc).
        return rsc.data.owner;
      }
      function getShares(rsc) {
        // Read Shares in the resource (rsc).
        return rsc.data.shares;
      }
      function isOwner(rsc) {
          // Determine if the user is the owner
        return isKnownUser() && (getOwner(rsc) == UserId());
      }
      function isShared(rsc) {
          // Determine if the user was shared the list
        return isKnownUser() && (UserId() in getShares(rsc));
      }
      function getParentDoc(){
          // Get the parent document
        return get(/databases/$(database)/documents/shoppinglist/$(lists));
      }
    	allow read:  if isOwner(resource) || isShared(resource);
      allow write: if isOwner(resource);
      
        match /items/{items} {
          allow read:  if isOwner(getParentDoc()) || isShared(getParentDoc());
          allow write: if isOwner(getParentDoc());
        }
    }
  }
}