Gorm Preload / load Polymorphism Association

Your Question

I have a Polymorphism Association described as follow:

type UserTypeA struct {
    Id uint64 `json:"-"`
    Firstname   string         `json:"firstname" gorm:"size:255;not null"`
    Lastname    string         `json:"lastname" gorm:"size:255;not null"`
    Message []Message  `json:"-" gorm:"polymorphic:User;polymorphicValue:UserTypeA"`
}

type UserTypeB struct {
    Id uint64 `json:"-"`
    Firstname   string         `json:"firstname" gorm:"size:255;not null"`
    Lastname    string         `json:"lastname" gorm:"size:255;not null"`
    Message []Message  `json:"-" gorm:"polymorphic:User;polymorphicValue:UserTypeB"`
}

type Message struct {
    Id uint64 `json:"-"`
    UserID    uint64 `json:"userId" gorm:"not null"`
    UserType  string `json:"userType" gorm:"size:255;not null"`
    Message   string `json:"message" gorm:"type:text;not null"`
}

Now I want to fetch all Messages and want to have the user in the json response in it. Is this possible in gorm? So far I tried the following:

var messages []Message
DB.Preload("UserTypeB").Find(&messages, Message{})

The relation "UserTypeB" can not be found. How can I get the following output? Is there another way to get the wanted result?

{
  "message": "Lorem ipsum...",
  "user": { // Gorm should either select UserTypeA or UserTypeB automatically by column "UserType"
    "id": 1234,
    "firstname": "John",
    "Lastname": "Doe"
  }
}

The document you expected this should be explained

https://gorm.io/docs/has_many.html#Polymorphism-Association

Expected answer

The documentation only describes how to create new entries. But the part of getting entries with polymorphism associations is still missing. I also searched on google & co. But I only found a solution when I would fetch UserTypeA or UserTypeB, but not Messages .

Comment From: github-actions[bot]

This issue has been automatically marked as stale because it has been open 360 days with no activity. Remove stale label or comment or this will be closed in 180 days

Comment From: timmyhoa

Any solution?

Comment From: Br3nda

does Preload of polymorphic work at all?

Comment From: nghiadanggeocomply

Any idea for this?

Comment From: duxphp

Unsolvable, feel free to try other orm or manual splicing

Comment From: alkawero

you can try with raw query like this

type MessageWithUser struct {
    MessageID     uint64
    MessageContent string
    UserType      string
    Firstname     string
    Lastname      string
}

var messages []MessageWithUser

query := `
    SELECT 
        messages.id AS message_id,
        messages.message AS message_content,
        messages.user_type,
        CASE 
            WHEN messages.user_type = 'UserTypeA' THEN user_type_as.firstname
            WHEN messages.user_type = 'UserTypeB' THEN user_type_bs.firstname
        END as firstname,
        CASE 
            WHEN messages.user_type = 'UserTypeA' THEN user_type_as.lastname
            WHEN messages.user_type = 'UserTypeB' THEN user_type_bs.lastname
        END as lastname
    FROM messages
    LEFT JOIN user_type_as ON messages.user_id = user_type_as.id AND messages.user_type = 'UserTypeA'
    LEFT JOIN user_type_bs ON messages.user_id = user_type_bs.id AND messages.user_type = 'UserTypeB'
`

db.Raw(query).Scan(&messages)