Remove @unique from id relationship does not remove the unique index.

Bug description

I have a relationship between to Models. They and linked through a id. Initial I had set up that ID to be unique but the logic changed so I removed it. However, it does not remove the unique index from the MySQL table and still fails on that constraint. I had to manually remove it.

How to reproduce

1. Create a model like the below.

model Envelope {
    ClaimedCoupon    ClaimedCoupon?
}

model ClaimedCoupon {
    Envelope      Envelope  @relation(fields: [envelopeId], references: [id])
    envelopeId   @unique
}

1. Then change the model to this.

model Envelope {
    ClaimedCoupon    ClaimedCoupon[]
}

model ClaimedCoupon {
    Envelope      Envelope  @relation(fields: [envelopeId], references: [id])
    envelopeId
}

Also tried:

model Envelope {
    ClaimedCoupon    ClaimedCoupon[]
}

model ClaimedCoupon {
    Envelope      Envelope  @relation(fields: [envelopeId], references: [id])
    envelopeId
    
    @@index([envelopeId], name: "envelopeId")
}

1. Run create migration.
2. No change happens.
3. The unique index remains.

Expected behavior

The unique index should get deleted.

Environment & setup

• OS: MacOS and Ubuntu 18
• Database: MySQL
• Node.js version: 15.3.0
• Prisma version: ^2.14.0

[albertoperdomo]

OK, I re-read and did some testing again @ssmith-14Four

This is what I think is happening:

• In the first schema you share you have a 1:1 relation: there is a relation scalar (envelopeId) and on the *opposite side there is an optional back-relation ClaimedCoupon? See docs

• In this case, Prisma will create a unique index, even if none is defined in the schema. But it happens that you also have @unique in there. If you omit it, the resulting DB schema is the same.

• Then you change it to a 1:n relation, by removing the @unique attribute from the foreign key envelopeId and making the back-relation ClaimedCoupon[] an array. At this point you would expect for the index to be dropped, but it seems you are hitting what looks like a bug that is unrelated to your previous relation syntax.

I just created another issue for that here. I will add that for the 1:1 relation, prisma already adds a uniqueness constraint anyway so it’s not necessary. You can omit the @unique in your first schema and the result there is the same.

What happens is when you transition to the second schema you want to remove your explicit @unique and this is not happening.

Does this make sense to you?

[tomhoule]

Ah, this index is there because you have a 1:1 relation, ti’s automatically inserted.