如果Firebase实时数据库中存在数据库节点，请使用云功能检查

Question

I have a onUpdate Cloud Function and I need to check if a database node exists.

For my the ideal way to do it would be:

exports.watchTeamMemberUpdates = functions.database
  .ref('/teams/{teamName}')
  .onUpdate((change) => {
     if (!change.before.exists()) {
       console.log('exists error');
       return null;
     }
});

But it requires a snapshot and i'm giving it a promise.

Is it possible to check if the file exists with the promise in any way?

Also is there a better way to make snapshots other than .once.then?

Answer 1

I make the assumption that by writing "I need to check if a file exists" you mean checking if a database node exists (ie the team member saved under team name). I've taken the liberty to modify your question title and content accordingly.

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Note that if you want to detect if the team member didn't exist with an onUpdate() trigger, you will not succeed , because this Cloud Function will only be triggered if the node existed before , since we watch for updates (not for creation).

So you should probably use an onWrite() trigger. Below is the code for the two types of trigger. I would suggest you try several writes and updates in your database (after having deployed theses two functions, of course) and look what is printed to the Cloud Function log.

exports.watchTeamMemberUpdates = functions.database
  .ref('/teams/{teamName}')
  .onUpdate((change, context) => {
    if (!change.before.exists()) {
      console.log('watchTeamMemberUpdates: exists error');
      return null;
    } else {
      console.log('watchTeamMemberUpdates: Team member exists');
      return null;
    }
  });

exports.watchTeamMemberWrites = functions.database
  .ref('/teams/{teamName}')
  .onWrite((change, context) => {
    if (!change.before.exists()) {
      console.log('watchTeamMemberWrites: exists error');
      return null;
    } else {
      console.log('watchTeamMemberWrites: Team member exists');
      return null;
    }
  });

PS: Note that the handler to be passed to the onUpdate(handler) method is a function with two arguments, see https://firebase.google.com/docs/reference/functions/functions.database.RefBuilder#onUpdate . However the second is optional. This is the same with the onWrite(handler) method.