Support table relationships in service generators

[cannikin]

When generating an SDL, if any tables reference each other via prisma's @relation syntax, we will create the equivalent expected resolver syntax in the generated service.

Example

The following GraphQL query is not currently supported out of the box with Redwood's implementation of SDL + services. For the given schema.prisma:

model User {
  id    Int     @id @default(autoincrement())
  email String  @unique
  name  String?
  posts Post[]
}

model Post {
  id     Int     @id @default(autoincrement())
  title  String  @unique
  body   String
  user   User    @relation(fields: [userId], references: [id])
  userId Int
}

You should be able to write the following GraphQL query:

posts {
  id
  title
  body
  user {
    name
    email
  }
}

We will create the following additional functions in the posts.js service:

export const Post = {
  user: (_obj, { root }) => db.post.findOne({ where: { id: root.id } }).user(),
}

And the following functions in the users.js service:

export const User = {
  posts: (_obj, { root }) => db.user.findOne({ where: { id: root.id } }).posts(),
}

Apollo will automatically use these relationships to make the GraphQL query above work.

Closes #316
Closes #327

[cannikin]

Hmm, I have a dilemma. While this does add relationships to the services, the scaffolds created when there's a relationship are busted. Two reasons:

1. The foreign key field like userId is a string, when the GraphQL expects an Int (related to Cannot use scaffolded form to insert Float datatype into database #392)
2. Even if the ID is cast properly, Prisma blows up when trying to assign userId a value. It expects you to connect to an actual User record (?????)

So we could release this along with a note that scaffolds won't work with it yet. Instead of relationships being 100% broken, they're now only 50% broken. :) Getting data OUT of a service works great, it's inserting that's killing us.

Or don't do anything until both problems above are fixed. That probably means more complaints about relationships not being supported in the meantime. :(

[gfpacheco]

I was working on this but I got stuck because prisma's getDMMF doesn't give us the fields argument.

They just told me that it now does, but it's still in the alpha branch, and I thought Id$ wasn't a reliable way to do it, but I guess that's good enough, I'm glad someone with more time was able to finish it.

[weaversam8]

Great work getting this out the door @cannikin! 🎉

Can't wait to try it out!

[weaversam8]

@cannikin - This closed #327, but I think this actually should have closed #324. #327 was the issue for the generated UI stuff not working, while #324 was the issue for services not properly resolving relation fields.

If that's the case, #324 should definitely be closed by this issue, but #327 should probably be reopened, unless this PR is bigger than I've noticed so far.

[cannikin]

#327 was was actually fixed a couple PRs ago. The generated query on the display pages excludes other objects now and only pulls scalar fields. So if you had a Post with a userId you would get that in your query, no user. You can add that yourself, of course, but the scaffolded pages will only include scalars for editing. https://github.com/redwoodjs/redwood/blob/master/packages/cli/src/commands/generate/scaffold/scaffold.js#L149