按关联计数转到gorm订单

Question

I have a following data structure:

type Collection struct {
    gorm.Model
    Name            string
    CollectionItems []CollectionItem
}

type CollectionItem struct {
    CollectionID uint
    ItemID     uint
    Item
}

How can I query collections table ordering results by a number of collection_item associations i.e. collections with most items come first.

Thanks.

Answer 1

There is no such feature like rails counter cache in the gorm, but gorm has callbacks before* & after* so it is easy to implement order by collection count feature.

For example:

// Updating data in same transaction
func (c *Collection) AfterUpdate(tx *gorm.DB) (err error) {
  tx.Model(&Collection{}).Where("id = ?", c.ID).Update("items_count", gorm.Expr("items_count + ?", 1))
  return
}