Inappropiate migration generated for 'default: null'

[thefat32]

Issue type:

[ ] question
[X] bug report
[ ] feature request
[ ] documentation issue

Database system/driver:

[ ] cordova
[ ] mongodb
[ ] mssql
[X] mysql / mariadb
[ ] oracle
[ ] postgres
[ ] cockroachdb
[ ] sqlite
[ ] sqljs
[ ] react-native
[ ] expo

TypeORM version:

[X] latest
[ ] @next
[ ] 0.x.x (or put your version here)

Steps to reproduce or a small repository showing the problem:

When using the option default: null for an entity's column, and generating a migration using typeorm migration:generate, the generated migrations get into a loop. The generated migration is always the same, because 'null' default value is being normalized as null instead of NULL. When typeorm checks default value for the column it finds 'NULL' !== 'null'.

This bug is related to #3345

In order to reproduce with a simple User entity:

import {
  Entity,
  PrimaryGeneratedColumn,
  Column,
  Unique,
  CreateDateColumn,
  UpdateDateColumn
} from 'typeorm';
import { Length } from 'class-validator';

@Entity()
@Unique(['username'])
export default class User {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  @Length(4, 20)
  username: string;

  @Column()
  @Length(4, 100)
  password: string;

  @Column({ nullable: true, default: null })
  country: string;

  @Column('simple-array')
  @IsNotEmpty()
  roles: string[];

  @Column()
  @CreateDateColumn()
  createdAt!: Date;

  @Column()
  @UpdateDateColumn()
  updatedAt: Date;
}

• Sync Schema

ts-node ./node_modules/typeorm/cli.js schema:sync

• Generate Migration

ts-node ./node_modules/typeorm/cli.js migration:generate -n DefaultNullTest

Obtained:

• Up

ALTER TABLE `user` CHANGE `country` `country` varchar(255) NULL DEFAULT null

• Down

ALTER TABLE `user` CHANGE `country` `country` varchar(255) NULL DEFAULT NULL

I think modifying MysqlDriver.ts line 593 to:

return `NULL`;

should fix the problem

[thefat32]

Some change araised this bug again! Code line

    /**
     * Normalizes "default" value of the column.
     */
    normalizeDefault(columnMetadata: ColumnMetadata): string | undefined {
        const defaultValue = columnMetadata.default;

        if (defaultValue === null) {
            return undefined