ConversionError when mediumint column is null

Bug description

Prisma throws a ConversionError when attempting to retrieve rows from a MySQL database if one of the columns is of type mediumint and a row contains a NULL value in that column.

How to reproduce

Create a table that has a column with type mediumint. Insert at least one row with NULL in the mediumint column. Then, use the .findMany() function to attempt to retrieve data:

CREATE TABLE `items` (
  `item_id` int(10) UNSIGNED NOT NULL,
  `problematic_column` mediumint(8) UNSIGNED DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

ALTER TABLE `items`
  ADD PRIMARY KEY (`item_id`);

INSERT INTO `items` SET `item_id` = 123, `problematic_column` = NULL;

model Item {
  @@map("items")

  id                Int  @id @map("item_id")
  problematicColumn Int? @map("problematic_column")
}

await prisma.item.findMany();

Expected behavior

No error; NULL values in the mediumint column should be retrieved like NULL values in other column types.

Prisma information

Client Version 2.5.1 Engine Version c88925ce44a9b89b4351aec85ba6a28979d2658e

Environment & setup

• OS: Debian
• Database: MySQL 5.7
• Node.js version: Node.js v12
• Prisma version: see above

Other notes

See https://github.com/prisma/prisma/discussions/3537 for a full discussion of the issue.

[janpio]

I could reproduce the issue in this PR to our integration tests: https://github.com/prisma/prisma/pull/3583 👍

[WesCossick]

Okay, so it looks like the problem isn’t if the mediumint column has an integer, it’s if the column is set to NULL. I’ve updated the “how to reproduce” section above accordingly.