Gorm Many to Many关联模式下关于BeforeCreate的疑惑

Your Question

假设有a、b、c三个模型，它们之间的关联关系如下：

type c struct {
    ID uint
    Bs []*b `gorm:"many2many:bs_cs"`
}
// c 有一个 BeforeCreate（填充 ID ）
func (r *c) BeforeCreate(*gorm.DB) error {
    r.ID = uint(time.Now().Unix())
}

type b struct {
    ID uint
    Cs []*c `gorm:"many2many:bs_cs"`
}

type a struct {
    ID uint
    Bs []b
}

即 a Has Many b，b 和 c 为 Many to Many。

// 假设数据库已经有数条 c 记录
cs := make([]c, 0)
db.Find(&cs)
ptrs := make([]*c, len(cs))
for i, v := range cs {
    ptrs[i] = &c{ID: v.ID}
}

// bs
bs := []b{{Cs: ptrs}}

// 现在新增一条 a 
data := &a{
    ID: 1,
    Bs: bs,
}
db.Create(&data)

debug发现c的BeforeCreate竟然被触发。

The document you expected this should be explained

https://gorm.io/zh_CN/docs/many_to_many.html

Expected answer

预期的行为应该是：a、b表新增数据，并将b的数据与c的既有数据进行关联（即新增关联表bs_cs的数据），

而上述情形却被当作要新增c表的数据。

注释掉BeforeCreate后行为与预期相符。

很困惑，这是为何？

Comment From: black-06

Same as #6324

GORM doesn't know if the associated data exists, so Association.Append will try INSERT ... ON CONFLICT DO NOTHING.

Then the BeforeCreate hook is triggered.

Comment From: black-06

So you need to conditionally populate the id in the hook.

if r.ID != 0 {
    r.ID = uint(time.Now().Unix())
}

Comment From: congjunhua

So you need to conditionally populate the id in the hook.

go if r.ID != 0 { r.ID = uint(time.Now().Unix()) }

现在确实是通过在钩子中判断ID是否为空来决定是否执行ID填充，但本质上还是没有解决钩子函数一定会被执行的问题，而这个执行是不被期待的，感觉这徒增了心智负担。

我在考虑是不是要弃用钩子，但不使用钩子又会带来新问题，即钩子中的逻辑会被分散到多个地方，这又降低了可维护性，很纠结。

是否可以通过主键来判断数据是否存在呢？假设改表格有主键的话。

Comment From: black-06

Yes, Hook will be triggered unless use DB.Session(&gorm.Session{SkipHooks: true})

Or try to skip auto create/update, https://gorm.io/docs/associations.html#Skip-Auto-Create-x2F-Update

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