Cannot update document by index in FaunaDB - faunadb

Question

I'm attempting to update a document using an index in my FaunaDB collection using FQL.
Update(
Match(
Index('users_by_id'),
'user-1'
),
{
data: {
name: 'John'
}
}
)
This query gives me the following error:
Error: [
{
"position": [
"update"
],
"code": "invalid argument",
"description": "Ref expected, Set provided."
}
]
How can I update the document using the index users_by_id?

Answer 1

Match returns a set reference, not a document reference, because there could be zero or more matching documents.
If you are certain that there is a single document that matches, you can use Get. When you call Get with a set reference (instead of a document reference), the first item of the set is retrieved. Since Update requires a document reference, you can then use Select to retrieve the fetched document's reference.
For example:
Update(
Select(
"ref",
Get(Match(Index('users_by_id'), 'user-1'))
),
{
data: {
name: 'John'
}
}
)
If you have more than one match, you should use Paginate to "realize" the set into an array of matching documents, and then Map over the array to perform a bulk update:
Map(
Paginate(
Match(Index('users_by_id'), 'user-1')
),
Lambda(
"ref",
Update(
Var("ref"),
{
data: {
name: "John",
}
}
)
)
)
Note: For this to work, your index has to have an empty values definition, or it must explicitly define the ref field as the one and only value. If your index returns multiple fields, the Lambda function has to be updated to accept the same number of parameters as are defined in your index's values definition.