static updated (model, record) {
     ...
  }

import type { Element } from 'pinia-orm'
import { BelongsTo as BelongsToClass, Model, useRepo } from 'pinia-orm'
import { Attr, BelongsTo, Num, Str } from 'pinia-orm/dist/decorators'
import Post from '~/models/Post'

export default class User extends Model {
  static entity = 'users'

  @Num(0) declare id: number
  @Attr() declare postId: number | null
  @Str('') declare name: string
  @Num(0) declare age: number
  @BelongsTo(() => Post, 'postId') declare post: Post | null

  static saving(model: Model, record?: Element) {
    model.name = 'John'
    const fields = model.$fields()
    for (const name in fields) {
      if (fields[name] instanceof BelongsToClass && record && record[name] === null)
        model[(fields[name] as BelongsToClass).foreignKey] = null
    }
  }
}