Firestore Field Default Timestamp

I created a model for the chat message like this.

class Message extends Identifiable {
  ...
  DateTime createdAt;

  Message(
      {...,
      DateTime? createdAt})
      : createdAt = createdAt ?? DateTime.now();

I assigned default timestamp now to createdAt.
But I’m curious it is possible to set it on server side.

Actually, I am using a function to generate the map to pass in set and update of Firestore.

Map<String, dynamic> toRow() {
    return {
      MessageFieldNames.authorId: authorId,
      MessageFieldNames.authorName: authorName,
      MessageFieldNames.content: content
    };
  }

To order the server set default value to the field, I specified serverTimestamp.

Therefore I checked if createdAt is null and gave server timestamp.

Map<String, dynamic> toRow() {
    return {
      ...
      MessageFieldNames.createdAt: createdAt ?? FieldValue.serverTimestamp(),
    };
  }

Of course I had to make createdAt optional

class Message extends Identifiable {
  ...
  DateTime? createdAt;

  Message(
      {...,
      this.createdAt});

Now I can see creadtedAt field set by server current timestamp.

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References

FieldValue | Firebase