title: TypeORM - table inheritance using enum discriminator
body: <p>Trying to use an enum to discriminate the  child entities:</p>
  <pre class="lang-js prettyprint-override"><code>export enum ActionCategory {
    USER = 'user',
    POSITION = 'position',
    NOTE = 'note',
    EMAIL = 'email',
  }
  
  @Index('action_pkey', ['id'], { unique: true })
  @Entity()
  @TableInheritance({
    column: { type: 'enum', enum: ActionCategory, name: 'category' },
  })
  export class Action {
    @PrimaryGeneratedColumn({
      name: 'action_id',
      type: 'bigint',
    })
    readonly id: number
  
    @Column({
      name: 'category',
      type: 'enum',
      enum: ActionCategory,
      default: ActionCategory.USER
    })
    readonly category: ActionCategory
  }
  </code></pre>
  <p>but I am running into an error:</p>
  <pre><code>[Nest] 52222   - 27/01/2021, 10:30:25   [TypeOrmModule] Unable to connect to the database. Retrying (2)... +1ms
  QueryFailedError: column &quot;action_category&quot; contains null values
      at new QueryFailedError (/Users/rai/dev/lexstep/lexstep-nest/node_modules/.pnpm/typeorm@0.2.29/node_modules/typeorm/error/QueryFailedError.js:11:28)
  I can avoid this issue by changing the name of one of the columns (either the @TableInheritance 'action_category' or the @Column 'action_category' but this results in an extra column being created
  </code></pre>
  <p>If I use a default value then I get the error</p>
  <pre><code>QueryFailedError: column &quot;action_category&quot; of relation &quot;action&quot; already exists
  </code></pre>