Firestore create documents if they don't exist, skip if they do

Question

I want to create Firestore documents if they don't exist - if they do exist, skip them (don't update). Here's the flow

var arrayOfRandomIds = [array of 500 random numbers];
for (var id of arrayOfRandomIds)
{
 var ref = db.collection("tickets").doc(id);
 batch.set(ref, {name: "My name", location: "Somewhere"}, { merge: true });
}
batch.commit();

I just want to know, would this overwrite any existing documents if they exist? I don't want anything overwritten, just skipped.

Thanks.

Answer 1

I think you can use security rules to accomplish that. That way you won't be charged for an additional document read to see if it already exists.

service cloud.firestore {
  match /databases/{database}/documents {
    match /tickets/{id} {
      allow create;
    }
  }
} 

Answer 2

Firestore doesn't have a native "create but don't overwrite" operation. Here are the only available operations:

• update: only change the contents of an existing document
• set without merge: create or overwrite
• set with merge: create or update if exists

Instead of a batch, what you can do instead is perform a transaction that checks to see if the document exists, then creates it conditionally if it does not already exist. You will have to write that logic inside your transaction handler.