Gorm can't preload field - Nineya|java/go/python

I've just started getting this error for multiple entities, but I'm having a nightmare pinning down how to fix it.

Last night, it happened after I added a 'has many' relationship to an existing entity. After 2 hours, I was able to solve it simply by reordering the order in which migrations were specified with db.Automigrate()`, but I'll admit theres a chance that's was an illusory fix.

Everything was fine until I just added another has many relationship to another entity and I'm getting the message again, but no amount of moving migrations around will fix it.

In both cases, the entity I added the has many relation to is NOT the one mentioned in the error message. (E.g. I added Orders[] to Customer, and the message I got was can't preload field Item for core.PurchaseOrderLine) - seemingly unrelated.

I should stress that it is DEFINITELY the addition of the has many field that causes the error - if I remove it, the error stops.

Any suggestions highly appreciated.

Comment From: jpincas

I got to the root source of this problem. Let me try to explain:

In both cases, the preload field that was failing to load was not directly inside the entity, but wrapped inside an anonymous struct, e.g.

type Line struct {
  LineCommon
}

type LineCommon struct {
  gorm.Model
  ItemID uint
  Item Item
}

This was causing the failure. Denesting fixes it:

type Line struct {
  gorm.Model
  ItemID uint
  Item Item
}

Im not sure if thats intended behaviour - i guess not. The workaround is annoying, but not the end of the world for me.

Comment From: inliquid

It's not "anonymous" it's "embedded". You embed LineCommon into Line structure. Same behavior with GORM will be achieved by using gorm:"embedded" struct tag.

Just use this instead:

type Line struct {
  LineCommon LineCommon
}

in this case you are not embedding LineCommon struct. This is normal, idiomatic, and expected result with both Go and GORM.

Comment From: jpincas

Using a named struct field wouldn't give me the functionality I want - I'll use the gorm:"embedded" tag. Thanks.

Comment From: jpincas

type Line struct {
  LineCommon gorm:"embedded"
}

didn't work for me. I'm back to unnesting the fields.

Comment From: inliquid

Of course it didn't work, it's same as embedding. Read carefully.

Comment From: jpincas

I did read carefully. I'm not sure what advice you are giving me. I can't use a named field struct as I need to be able to access LineCommon's methods. I can't find any documentation for gorm:embeddedso I'm not sure what it does or where to use it.

Comment From: inliquid

The point was to explain you what is happening and how it works. This struct tag not documented, but it's easy to find in the code: https://github.com/jinzhu/gorm/blob/0fd395ab37aefd2d50854f0556a4311dccc6f45a/embedded_struct_test.go#L24

Comment From: jpincas

OK - lets try to simplify, as I think we are not understanding each other. Look at my second comment (Line/LineCommon). I have about 20 types, all structured like this - with the gorm.Model and other related fields embedded. Up until a few days ago, no problems at all. So from that, I undestand that it should 'just work', because, well, it does.

A few days ago, I added a has many relationship to one of those types, and I started getting preload errors ON ANOTHER type. I solved it by shifting around the order of the migrations. A few days later, some thing. The only way I've managed to completely solve it is by de-embedding (like the 3rd example in my comment).

I'm just trying to alert that there might be a subtle bug here, although I can completely understand that I haven't provided nearly enough information to track it down.

Comment From: Bipus

type Line struct {
  LineCommon []LineCommon `gorm:"ForeignKey:ItemID"`
}

type LineCommon struct {
  gorm.Model
  ItemID uint
  Item Item
}

Comment From: github-actions[bot]

This issue will be automatically closed because it is marked as GORM V1 issue, we have released the public testing GORM V2 release and its documents https://v2.gorm.io/docs/ already, the testing release has been used in some production services for a while, and going to release the final version in following weeks, we are still actively collecting feedback before it, please open a new issue for any suggestion or problem, thank you

Also check out https://github.com/go-gorm/gorm/wiki/GORM-V2-Release-Note-Draft for how to use the public testing version and its changelog

Comment From: Hao-Wu

@jpincas I am confronted with the exactly same trouble, do you find a solution? or else I have to DeNest the model struct, which I am trying to avoid.