feat: Ability to use "where" clause inside `insert` mutation to link existing data

_Note from Hasura team: issue closed in favour of https://github.com/hasura/graphql-engine/issues/1573_

I think this is a feature request.

Given the following schema:

CREATE TABLE "Person" (
  "id" uuid PRIMARY KEY DEFAULT gen_random_uuid()
);

CREATE TABLE "EmailAddress" (
  "personId" uuid NOT NULL REFERENCES "Person" ("id") ON DELETE CASCADE,
  "emailAddress" citext PRIMARY KEY
);

CREATE TABLE "Agreement" (
  "id" uuid PRIMARY KEY DEFAULT gen_random_uuid(),
  "text" citext NOT NULL
);

CREATE TABLE "PersonAgreement" (
  "personId" uuid NOT NULL REFERENCES "Person" ("id") ON DELETE CASCADE,
  "agreementId" uuid NOT NULL REFERENCES "Agreement" ("id") ON DELETE CASCADE,
  "value" boolean NOT NULL
);

A person can be uniquely found using an email address. I’d like to insert a new PersonAgreement into the database given an agreementId, emailAddress, and PersonAgreement value.

Using sql, the email address could be used to get the related personId which could be combined with the agreementId and value to insert a new PersonAgreement.

At the moment, it does not appear to be possible to execute this query within a transaction using Hasura. The problem is that the only way to associate the Person with the new PersonAgreement is to insert a new Person along with the PersonAgreement. Except in this case I don’t wish to create a new Person, I wish to find an existing person or have the mutation fail.

Example of the query I’d like to perform:

mutation AddPersonAgreement(
  $emailAddress: String!,
  $agreementId: uuid!,
  $value: Boolean!
) {
  insert_PersonAgreement(
    objects: {
      value: $value,
      agreementId: $agreementId,
      person: {
        where: { # <-- new API here
          emailAddresses: { emailAddress: { _eq: $emailAddress } }
        }
      }
    }
  ) {
    affected_rows
  }
}

[rikinsk]

Related to #3366

This could be resolved if it were possible to add nested objects via the update mutation.

Something roughly along the lines of:

mutation insertUserEvent($email: String, $eventId: Int) {
  update_users (
    where: { email: {_eq: $email}}
    _add: {
      event: {
        eventId: $eventId
      }
    }
  ) {
    affected_rows
  }
}

This is a feature being worked on and should be released soon.

[jorroll]

Yes, this query might translate to:

INSERT INTO "PersonAgreement" ("value", "agreementId", "personId")
VALUES (
  $1, 
  $2, 
  SELECT "id" FROM "Person"
    JOIN "EmailAddress" ON ("EmailAddress"."personId" = "Person"."id")
    WHERE "EmailAddress"."emailAddress" = $3
);

I’m not sure if this SQL is correct, but hopefully the idea is clear.