Preload with additional column - go

Question

So I have a table of users where each user is associated with a main tool and tools in their backpacks. Each tool also have a "quality". But I have no idea of how to fetch this value so I appreciate any help on how to do this.
Databases
users
+----+------+--------------+
| id | name | main_tool_id |
+----+------+--------------+
| 1 | adam | 1 |
+----+------+--------------+
tools
+----+-------------+
| id | name |
+----+-------------+
| 1 | hammer |
| 2 | screwdriver |
+----+-------------+
user_tools
+----+---------+---------+---------+
| id | user_id | tool_id | quality |
+----+---------+---------+---------+
| 1 | 1 | 1 | LOW |
| 2 | 1 | 2 | HIGH |
+----+---------+---------+---------+
Models
type User struct {
Id int64 `json:"-"`
Name string `json:"name"`
MainToolId int64 `json:"-"`
MainTool Tool `json:"main_tool"`
ToolsInBackpack []Tool `json:"tools_in_backpack" gorm:"many2many:user_tools"`
}
type Tool struct {
Id int64 `json:"-"`
Name string `json:"name"`
Quality string `json:"quality"`
}
Code
var users []User
DB.Preload("MainTool").Preload("ToolsInBackpack").Find(&users) // <-- Modify this (I guess)
Real result
{
"name": "adam",
"main_tool": {
"name": "hammer",
"quality": ""
},
"tools_in_backpack": [
{
"name": "hammer",
"quality": ""
},
{
"name": "screwdriver",
"quality": ""
},
]
}
Desired result
{
"name": "adam",
"main_tool": {
"name": "hammer",
"quality": "LOW"
},
"tools_in_backpack": [
{
"name": "hammer",
"quality": "LOW"
},
{
"name": "screwdriver",
"quality": "HIGH"
},
]
}
Thanks a lot for your help!

Answer 1

If you do not require MainTool to return an object.
maybe you uses it
type User struct {
ID uint `json:"-"`
Name string
MainTool int64
ToolsInBackpack string
Tools []*Tool
}
type Tool struct {
ID uint`json:"-"`
Name string
UserID uint
}
User{MainTool: 1, ToolsInBackpack: "[1,2]", Tools: []*Tool{&t1, &t2}}
if you don't wana to use string at ToolsInBackpack string you can't use datatypes.JSON at gorm.io/datatypes
for example
ToolsInBackpack datatypes.JSON

Answer 2

This isn't supported in Gorm so there won't be an easy way to just Preload and it's done.
Here are a few techniques you could look at:
Use a Custom Join Table Model that contains the fields that you need
type UserTool struct {
UserID int64 `gorm:"primaryKey"`
ToolID int64 `gorm:"primaryKey"`
Quality string
}
err := db.SetupJoinTable(&User{}, "ToolsInBackpack", &UserTool{})
// handle error
(I left out the UserTools.ID field, Gorm doesn't need it, and you only need it if a user can have the same tool twice, in which case put it back on as part of the primaryKey. I don't know whether Gorm will be happy with it but try it out).
Then you can use this model to query the field as any other model:
userTools := []UserTool
err := db.Where("user_id = ?", user.ID).Find(&userTools).Error
// handle error
// now userTools[i].Quality is filled, you could use it to update the corresponding users[j].ToolsInBackpack[k].Quality
It's a pain because you need to match IDs as a post-processing step.
Use a Has-Many/Belongs-To hybrid relation to model the Many-To-Many Join Table:
Here the User Has Many UserTools and a Tool belongs to one or more UserTools. This effectively models the meaning of Many-to-Many (in ERD, a relationship like [User]>--<[Tool] can be decomposed into [User]--<[UserTool]>--[Tool]).
type User struct {
ID int64
Name string
MainToolId int64
MainTool Tool
ToolsInBackpack []UserTool
}
type UserTool struct {
UserID int64 `gorm:"primaryKey"`
ToolID int64 `gorm:"primaryKey"`
Tool Tool
Quality string
}
type Tool struct {
ID int64
Name string
}
Now you can Preload this association like this:
err := db.Model(&User{}).Preload("ToolsInBackpack.Tool").Find(&users).Error
// handle error
// now users[i].ToolsInBackpack[j].Quality and
// users[i].ToolsInBackpack[j].Tool.Name will be set correctly.
Only problem now is that you've got a weird shape in the model that you're then trying to marshal into JSON (most likely to be used in an API). My advice here is to split the DB model from the JSON API model, and have a mapping layer in your code. The DB model and the API Messages invariably diverge in important ways and trying to reuse one model for both soon leads to pain.