违反UNIQUE约束时，如何防止gorm无法保存关联？

I have the following struct

type Category struct {
  gorm.Model
  Name string `gorm:"type:varchar(128);not null;unique"`
}

which is associated to Item struct:

// Item is a type representing a job listing on stackoverflow jobs
type Item struct {
  GUID   string `gorm:"primary_key"`
  Link   string

  Categories []Category `gorm:"many2many:item_categories"`
}

I'd like to save Items in db using gorm together with associations in table item_categories

The problem arises when I INSERT an Item which has at least on Category that is already in the DB (because of the UNIQUE constraint on Category.Name) then I get an error while doing:

var item Item
// fill item...
db.Create(&item)

and it propagates so that all INSERTs (of Item, Category and Item_Categories) are not performed.

I've tried using BeforeCreate/AfterCreate but the result is the same

func (c *Category) BeforeCreate(scope *gorm.Scope) (err error) {
  if err = scope.DB().Where(&Category{Name: c.Name}).First(c).Error; err == nil {
    return fmt.Errorf("Category %s already found in DB", c.Name)
  }
  return nil
}

What can I do to make it insert Items (and related new categories) with categories that are already in the DB?

Example of log I receive when adding a category (inside an Item) that already exists in the DB:

(/home/USER/.gvm/pkgsets/go1.9.4/global/src/github.com/USER/project/model.go:33)
[2018-02-19 13:42:46]  [0.17ms]  SELECT * FROM "categories"  WHERE "categories"."deleted_at" IS NULL AND (("categories"."name" = 'go')) ORDER BY "categories"."id" ASC LIMIT 1
[1 rows affected or returned ]
BeforeCreate First
Category &{{4 2018-02-19 13:42:46.164188984 +0100 +0100 2018-02-19 13:42:46.164188984 +0100 +0100 <nil>} go}

(/home/USER/.gvm/pkgsets/go1.9.4/global/src/github.com/USER/project/storage.go:20)
[2018-02-19 13:42:46]  Category go already found in DB

(/home/USER/.gvm/pkgsets/go1.9.4/global/src/github.com/USER/project/storage.go:20)
[2018-02-19 13:42:46]  Category go already found in DB

(<autogenerated>:1)
[2018-02-19 13:42:46]  [0.12ms]  INSERT INTO "item_categories" ("item_guid","category_id") SELECT '157652','4'  WHERE NOT EXISTS (SELECT * FROM "item_categories" WHERE "item_guid" = '157652' AND "category_id" = '4')
[1 rows affected or returned ]

(/home/USER/.gvm/pkgsets/go1.9.4/global/src/github.com/USER/project/storage.go:20)
[2018-02-19 13:42:46]  Category go already found in DB

(/home/USER/.gvm/pkgsets/go1.9.4/global/src/github.com/USER/project/storage.go:20)
[2018-02-19 13:42:46]  Category go already found in DB
2018/02/19 13:42:46 Couldn't save job listing 157652 in db: Category go already found in DB