querying pocketbase relational data

Say you have a Posts collection. Every Post is related to a User. How can we fetch relational fields when we get Posts?

await pb.collection("posts").getFullList({
  filter: `user_id="${userId}"`, // get posts by user_id
  expand: "user_id",
});

expand is the option that lets us add the fields related to a user (we assume that the relational column is titled user_id). The data is returned to us as such:

{
	title: "Some title",
	content: "Some content",
	expand: {
		// username, email, created, etc. ALL the fields on a user record
	}
}

But maybe we don’t want all the fields. We can limit them by querying only certain fields:

await pb.collection("posts").getFullList({
  filter: `user_id="${userId}"`,
  expand: "user_id",
  fields: "title,content,expand.user_id.username",
});