Insert permissions and actions

If an action is used for data validation, it is hard to re-use constraints specified through insert permissions.

Motivation

Let's consider a table with the following schema:

create table message (
  id serial primary key,
  content text not null,
  channel_id integer not null references channel(id)
)
create table channel (..)
create table channel_members (..)

Let's say you want to allow a user role to insert into message table if the following check constraint holds:

{"channel": {"members": {"user_id": "x-hasura-user-id"}}}

i.e, a user can only post a message to a channel if they are a member of that particular channel. Now let's say you want to do some sort of validation on the content of the message (maybe through some external service), this is a problem that would broadly fall under the 'data validation' category (data cannot be validated with graphql-engine's permissions or through Postgres's check constraints).

We recommend actions in such cases, we would suggest the developer to remove the permission for message table on user and then do the entire validation in the webhook for the action. However the webhook now has to do more than validation of the message's content - it also has to check this constraint {"channel": {"members": {"user_id": "x-hasura-user-id"}}} which was part of insert permissions earlier. This is inconvenient and the reusability of hasura's permission system is lost.

Why should the insert permission be removed? Because if the insert permission is defined, the insert_message mutation is generated which can be directly used by anyone with the user role and thus bypassing the validation of the message content.

This problem is originally reported on discord.

Proposed solution

Introduce a new field in insert permission called admin_only, this will remove the insert mutation field for the table from mutation_root for that role but the mutation can still be accessed when admin-secret is specified. The action's handler can now specify the admin-secret, set x-hasura-role as the user's role and execute the request so that the constraints defined on the role in insert permissions are enforced.

Currently admin-secret + x-hasura-role is used in GraphiQL to mimic a request as a particular role. So admin_only insert mutations will be allowed when a request is made through GraphiQL with admin-secret + x-hasura-role. We'll need to make this clear in our documentation.