Can I update a FaunaDB document without knowing its ID? - faunadb

Question

FaunaDB's documentation covers how to update a document, but their example assumes that I'll have the id to pass into Ref:
Ref(schema_ref, id)
client.query(
q.Update(
q.Ref(q.Collection('posts'), '192903209792046592'),
{ data: { text: "Example" },
)
)
However, I'm wondering if it's possible to update a document without knowing its id. For instance, if I have a collection of users, can I find a user by their email, and then update their record? I've tried this, but Fauna returns a 400 (Database Ref expected, String provided):
client
.query(
q.Update(
q.Match(
q.Index("users_by_email", "me#example.com")
),
{ name: "Em" }
)
)

Answer 1

Although Bens comments are correct, (that's the way you do it), I wanted to note that the error you are receiving is because you are missing a bracket here: "users_by_email"), "me#example.com"
The error is logical if you know that Index takes an optional database reference as second argument.
To clarify what Ben said:
If you do this you'll get another error:
Update(
Match(
Index("accounts_by_email"), "test#test.com"
),
{ data: { email: "test2#test.com"} }
)
Since Match could potentially return more then one element. It returns a set of references called a SetRef. Think of setrefs as lists that are not materialized yet. If you are certain there is only one match for that e-mail (e.g. if you set a uniqueness constraint) you can materialize it using Paginate or Get:
Get:
Update(
Select(['ref'], Get(Match(
Index("accounts_by_email"), "test#test.com"
))),
{ data: { email: 'test2#test.com'} }
)
The Get returns the complete document, we need to specify that we require the ref with Select(['ref']..
Paginate:
Update(
Select(['data', 0],
Paginate(Match(
Index("accounts_by_email"), "test#test.com"
))
),
{ data: { email: "testchanged#test.com"} }
)

Answer 2

You are very close! Update does require a ref. You can get one via your index though. Assuming your index has a default values setting (i.e. paging a match returns a page of refs) and you are confident that the there is a single match or the first match is the one you want then you can do Select(["ref"], Get(Match(Index("users_by_email"), "me#example.com"))) to transform your set ref to a document ref. This can then be passed into update (or to any other function that wants a document ref, like Delete).