Filtering on a relation with multiple values doesn't work as documented (RESTful API Handler)

[zupamario]

Hello Zenstack Team,

I think I found an issue in the rest api handler.

According to the documentation, it should be possible to filter by multiple values (see example 3: https://zenstack.dev/docs/reference/server-adapters/api-handlers/rest#examples-4). That works for "normal" attributes but not for relations. When filtering on relations it only filter on the first value and ignores the rest of the list.

I was able to reproduce the issue from the example model below. I tested the following requests. Specifically request number 2 is problematic in my use-case.

1. Filter on an int field with multiple values: localhost:3000/api/post?filter[rating]=2,5
   WORKS

2. Filter on relation field that has an integer id: localhost:3000/api/post?filter[page]=1,2
   does NOT work correctly -> always filters only by the first value in the list

3. Filter on a string field with multiple values: localhost:3000/api/post?filter[title]=Join the Prisma Slack,Follow Prisma on Twitter
   WORKS

4. Filter on relation field that has a string id: localhost:3000/api/post?filter[author]=cm8n6dhkr0000gu5ea1v6kwv5,cm8n6dhkt0002gu5e7t0my17d
   also does NOT work correctly, but this returns an empty list, so maybe I'm doing the query wrong

datasource db {
    provider = 'sqlite'
    url = 'file:./dev.db'
}

generator client {
    provider = "prisma-client-js"
}

/**
 * User model
 */
model User {
    id       String @id @default(cuid())
    email    String @unique @email @length(6, 32)
    name     String
    password String @password @omit
    posts    Post[]

    // everybody can signup
    @@allow('create', true)

    // full access by self
    @@allow('all', auth() == this)
}

model Page {
    id       Int    @id @default(autoincrement())
    name   String
    post Post[]
}

/**
 * Post model
 */
model Post {
    id        String   @id @default(cuid())
    createdAt DateTime @default(now())
    updatedAt DateTime @updatedAt
    title     String   @length(1, 256)
    content   String
    published Boolean  @default(false)
    author    User     @relation(fields: [authorId], references: [id])
    authorId  String
    rating    Int      @default(5)
    page      Page     @relation(fields: [pageId], references: [id])
    pageId    Int

    // allow read for all signin users
    @@allow('read', auth() != null && published)

    // full access by author
    @@allow('all', author == auth())
}```

**Environment (please complete the following information):**

-   ZenStack version: 2.13.0
-   Prisma version: 5.18.0
-   Database type: Postgresql

[ymc9]

Fixed in 2.14.0

[zupamario]

This is great. Thanks for your awesome work!