Relation table for implicit m:n-relation not recognized by introspection

[nikolasburk]

Bug description

Relation table for implicit m:n-relation not recognized by introspection.

How to reproduce

I have this Prisma schema:

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

datasource db {
  provider = "postgresql"
  url      = "postgresql://nikolasburk:nikolasburk@localhost:5432/newdb"
}

model Category {
  id    Int    @default(autoincrement()) @id
  posts Post[]
}
model Post {
  id         Int        @default(autoincrement()) @id
  categories Category[]
}

Now I run:

prisma2 migrate save --experimental
prisma2 migrate up --experimental

This created this README.md for the migration:

# Migration `20200319192142-init`

This migration has been generated by Nikolas Burk at 3/19/2020, 7:21:42 PM.
You can check out the [state of the schema](./schema.prisma) after the migration.

## Database Steps

```sql
CREATE TABLE "public"."Category" (
    "id" SERIAL,
    PRIMARY KEY ("id")
) 

CREATE TABLE "public"."Post" (
    "id" SERIAL,
    PRIMARY KEY ("id")
) 

CREATE TABLE "public"."_CategoryToPost" (
    "A" integer  NOT NULL ,
    "B" integer  NOT NULL 
) 

CREATE UNIQUE INDEX "_CategoryToPost_AB_unique" ON "public"."_CategoryToPost"("A","B")

CREATE  INDEX "_CategoryToPost_B_index" ON "public"."_CategoryToPost"("B")

ALTER TABLE "public"."_CategoryToPost" ADD FOREIGN KEY ("A")REFERENCES "public"."Category"("id") ON DELETE CASCADE  ON UPDATE CASCADE

ALTER TABLE "public"."_CategoryToPost" ADD FOREIGN KEY ("B")REFERENCES "public"."Post"("id") ON DELETE CASCADE  ON UPDATE CASCADE
```

## Changes

```diff
diff --git schema.prisma schema.prisma
migration ..20200319192142-init
--- datamodel.dml
+++ datamodel.dml
@@ -1,0 +1,18 @@
+generator client {
+  provider = "prisma-client-js"
+}
+
+datasource db {
+  provider = "postgresql"
+  url      = "postgresql://nikolasburk:nikolasburk@localhost:5432/newdb"
+}
+
+model Category {
+  id    Int    @default(autoincrement()) @id
+  posts Post[]
+}
+
+model Post {
+  id         Int        @default(autoincrement()) @id
+  categories Category[]
+}
```

The DB looks as follows:

CREATE TABLE "_CategoryToPost" (
    "A" integer NOT NULL REFERENCES "Category"(id) ON DELETE CASCADE ON UPDATE CASCADE,
    "B" integer NOT NULL REFERENCES "Post"(id) ON DELETE CASCADE ON UPDATE CASCADE
);
CREATE UNIQUE INDEX "_CategoryToPost_AB_unique" ON "_CategoryToPost"("A" int4_ops,"B" int4_ops);
CREATE INDEX "_CategoryToPost_B_index" ON "_CategoryToPost"("B" int4_ops);
CREATE TABLE "Category" (
    id integer DEFAULT nextval('"Category_id_seq"'::regclass) PRIMARY KEY
);
CREATE UNIQUE INDEX "Category_pkey" ON "Category"(id int4_ops);
CREATE TABLE "Post" (
    id integer DEFAULT nextval('"Post_id_seq"'::regclass) PRIMARY KEY
);
CREATE UNIQUE INDEX "Post_pkey" ON "Post"(id int4_ops);

Then I remove the models from the Prisma schema and run:

prisma2 introspect

The resulting models in the Prisma schema look as follows:

model Category {
  id             Int              @default(autoincrement()) @id
  CategoryToPost CategoryToPost[] @relation("CategoryTo_CategoryToPost")
}

model Post {
  id             Int              @default(autoincrement()) @id
  CategoryToPost CategoryToPost[] @relation("PostTo_CategoryToPost")
}

model CategoryToPost {
  A Category @relation("CategoryTo_CategoryToPost")
  B Post     @relation("PostTo_CategoryToPost")

  @@index([B], name: "_CategoryToPost_B_index")
  @@map("_CategoryToPost")
  @@unique([A, B], name: "_CategoryToPost_AB_unique")
}

Expected behavior

I would except to obtain the same models as the initial schema with the implicit m:n-relation:

model Category {
  id    Int    @default(autoincrement()) @id
  posts Post[]
}

model Post {
  id         Int        @default(autoincrement()) @id
  categories Category[]
}

Tested with alpha and preview024 with same results.