I am struggling to find a solution to prevent clients from just creating random fields with values in a document where they have write access to in Firestore. Since you cannot restrict access to single fields in Firestore like you could with the realtime database, this seems hard to achieve.
A solution would maybe be to not allow creation of fields and just letting clients update fields, but this would mean you would have to precreate the fields for documents which is not really a good solution in my opinion, especially if you have documents per user, which are dynamically created and having to use cloud functions to precreate fields in a document just seems unjustified.
Does anyone have a better solution?
As said in the Firebase Firestore documentation, you actually can prevent or allow writes or reads in certain fields. This can be achieved by adding a rule similar to this:
match /collection/{doc} {
allow update: if request.resource.data.field == resource.data.field;
}
Which would basically check if that specific field will have the exact same value after the update. You can also add rules to check if the requested value is between a range or equals to (your predefined value).
allow update: if request.resource.data.field > 0 && request.resource.data.field > 100;
You can inspect the keys of the request.resource and only have it pass if it doesn't contain a field that you want to keep read-only (meaning that the request isn't trying to update that field). For example:
allow update: if !request.resource.data.keys().hasAny(['my_field'])
(Thanks to James Qualls for the inspiration!)
