You should cut out your steps in a transaction to delete the tags not in the query before inserting the new ones. Then you can create and update your data and return your post

With interactiveTransactions

In your schema.prisma you should enable interactiveTransactions, for this:

generator client {
  provider        = "prisma-client-js"
  previewFeatures = ["interactiveTransactions"]
}

Then cut out your steps in a transaction

// Remove <Post> if you aren't in typescript
await prisma.$transaction<Post>(async (trx) => {
  const existingTags = tags.filter(({ id }) => id);
  const newTags = tags.filter(({ id }) => !id);

  await trx.tag.deleteMany({
    where: {
      id: {
        notIn: existingTags.map(({ id }) => id),
      },
    },
  });
  // Update the existing tags
  await Promise.all(
    tags.map((tag) =>
      trx.tag.update({
        where: {
          id: tag.id,
        },
        data: {
          color: tag.color,
          name: tag.name,
        },
      })
    )
  );
  await trx.tag.createMany({
    data: newTags.map((tag) => ({
      ...tag,
      creator: {
        connect: {
          id: session.user.id,
        },
      },
    })),
  });
  return trx.post.update({
    where: {
      id: postId,
    },
    data: {
      title,
      content,
      updatedAt: new Date(),
    },
    include: {
      tags: true,
    },
  });
});

Sorry for this, I have changed and found another way to do this. If anyone was curious on how i changed it (for future use if anyone gets stuck and gets the same results like this)

    const prisma = new PrismaClient()
    const { username, email, hashPassword } = await request.json();
    let verified = crypto.randomBytes(32).toString('hex');

    const userExist = await prisma.users.findFirst({ where: { OR: [{ username }, { email }] } })
    if (userExist === null) {
        await prisma.users.create({ data: { username, email, hashPassword, verified } })
        return new NextResponse('Success!', { status: 201 })
    } else {
        return new NextResponse("Sorry, username and or email already exists!", { status: 500 })
    }

If I understand your question correctly, the request body may optionally contain an id or be undefined. If the id is present in the request, you want to update the corresponding record. If it isn't, you want to create a new record.

upsert functionality requires a unique identifier that can be used by the database to check whether there is a matching record. The database will then decide what to do. However with an undefined value, prisma cannot work.

You could use a key that will not exist in the database in case the value of id is undefined to make upsert insert the record in this case.

await prisma.profile.upsert({
  where: { id: id || '' },
  // ...
});

Or you could differentiate the cases and treat them appropriately:

if (id) {
  await prisma.profile.update({
    where: { id },
    data: {
      username,
      about,
      // ...
    },
  });
} else {
  await prisma.profile.create({
    data: {
      username,
      about,
      // ...
    },
  });
}

You might want to think about the case that there is an id given in the body, but it does exist in the database. Do you want the update to fail and reply with an error? Or do you want the database to silently insert? Then use upsert instead of update above.

I'm providing my solution based on the clarifications you provided in the comments. First I would make the following changes to your Schema.

Changing the schema

model A_User {
  id        Int          @id
  username  String
  age       Int
  bio       String       @db.VarChar(1000)
  createdOn DateTime     @default(now())
  features  A_Features[]
}

model A_Features {
  id          Int      @id @default(autoincrement())
  description String   @unique
  users       A_User[]
}

Notably, the relationship between A_User and A_Features is now many-to-many. So a single A_Features record can be connected to many A_User records (as well as the opposite).

Additionally, A_Features.description is now unique, so it's possible to uniquely search for a certain feature using just it's description.

You can read the Prisma Guide on Relations to learn more about many-to-many relations.

Writing the update query

Again, based on the clarification you provided in the comments, the update operation will do the following:

• Overwrite existing features in a A_User record. So any previous features will be disconnected and replaced with the newly provided ones. Note that the previous features will not be deleted from A_Features table, but they will simply be disconnected from the A_User.features relation.

• Create the newly provided features that do not yet exist in the A_Features table, and Connect the provided features that already exist in the A_Features table.

You can perform this operation using two separate update queries. The first update will Disconnect all previously connected features for the provided A_User. The second query will Connect or Create the newly provided features in the A_Features table. Finally, you can use the transactions API to ensure that both operations happen in order and together. The transactions API will ensure that if there is an error in any one of the two updates, then both will fail and be rolled back by the database.

//inside async function
const disconnectPreviouslyConnectedFeatures =  prisma.a_User.update({
    where: {id: 1},
    data: {
        features: {
            set: []  // disconnecting all previous features
        }
    }
})

const connectOrCreateNewFeatures =  prisma.a_User.update({
    where: {id: 1},
    data: {
        features: {
            // connect or create the new features
            connectOrCreate: [
                {
                    where: {
                        description: "'first feature'"
                    }, create: {
                        description: "'first feature'"
                    }
                },
                {
                    where: {
                        description: "second feature"
                    }, create: {
                        description: "second feature"
                    }
                }
            ]
        }
    }
})

// transaction to ensure either BOTH operations happen or NONE of them happen.
await prisma.$transaction([disconnectPreviouslyConnectedFeatures, connectOrCreateNewFeatures ])

If you want a better idea of how connect, disconnect and connectOrCreate works, read the Nested Writes section of the Prisma Relation queries article in the docs.

Prisma doesn't natively support upsertMany.

There is a Feature Request to provide the upsertMany method.

As of now the best approach would be to loop over the data and invoke upsert in the loop along with using $transaction.

Example:

const collection = await prisma.$transaction(
    userData.map(cur =>
      prisma.cur.upsert({
        where: { id: cur.id },
        update: {},
        create: { id: cur.id },
      })
    )
  )

Here's a reference to $transaction API which should be helpful.