如何在GORM中建立一对一关系？

Question

I have a series of categories of a type of Product. There are only 3 of them so I can create a table for each. Lets call them SubProductA, SubProductB and SubProductC. They are distinct enough that storing them in the parent Product table would make a mess.

So naturally, I just allow the primary key to be the foreign key linking back to product for each of these. However, GORM seems to not be able to preload any field like this. Here is what I have (as an example) for one of these:

type SubProductA struct {
  ProductID uint64 `gorm:"primary_key;ForeignKey:ProductID"`
  Product Product
  CreatedAt           time.Time
  UpdatedAt           time.Time
  DeletedAt           *time.Time
}

However, when I run a query like

db.Where("product_id=?", p.ID).Preload("Product").First(&subProduct).Error

The result is:

Can't preload field Product for models.SubProductA.

I suspect this is because I'm not properly identifying the primary key as the foreign key as well. However, I have tried a bunch of different things and I get the same error.

Is there a way to make this work?

Answer 1

To answer my own question for people passing through...

It seems that GORM has problems with multiple-level one-to-one relationships. For a single level, the example I gave, it works without needing anything more. However, to do a preload on a field that is also a one-to-one with the same key farther up the tree, I needed to do the following:

type SubSubProductA struct {
  SubProductID uint64 `gorm:"primary_key"`
  SubProductA SubProductA `gorm:"ForeignKey:SubProductID`"
  CreatedAt           time.Time
  UpdatedAt           time.Time
  DeletedAt           *time.Time
}

In other words, the ForeignKey part goes on the object itself. This fixed the preloading for me. Hope this helps someone. The GORM documentation is very sparse for anything but the common cases.