Gorm Preload as deep as necessary

I'm using Gorm and have some questions as to how to retrieve nested SubComments from the model. The problem I'm getting is comments nested two levels deep, i.e. the Comment.SubComments are not loading. Am I missing something with the Preload?

I also think I need a composite foreign key on Comment of foreignKey:parent_id,parent_type but thats not working.

https://goplay.tools/snippet/kOhjUs7X6NQ

package main

import (
    "gorm.io/gorm"
    "gorm.io/gorm/clause"
)

type BlogPost struct {
    ID       uint `gorm:"primary_key"`
    Content  string
    Comments []Comment `gorm:"foreignKey:parent_id;references:id"`
}

type ParentType int

const (
    PT_BlogPost ParentType = 1
    PT_Comment             = 2
)

type Comment struct {
    ID          uint `gorm:"primary_key"`
    ParentId    uint
    ParentType  ParentType
    Comment     string
    // TODO composite foreign key not working
    SubComments []Comment `gorm:"foreignKey:parent_id,parent_type;references:id"`
}

func createComment(parentId uint, parentType ParentType) {
    switch parentType {
    case PT_BlogPost:
        var blogPost BlogPost
        // lookup blog post
        if err := models.DB.Where("id = ?", parentId).First(&blogPost).Error; err != nil {
            return
        }
        comment := Comment{
            ParentId:    parentId,
            ParentType:  PT_BlogPost,
            Comment:     "",
            SubComments: nil,
        }
        models.DB.Create(&comment)

        models.DB.Model(&blogPost).Updates(&BlogPost{
            Comments: append(blogPost.Comments, comment),
        })

    case models.PT_Comment:
        var parentComment Comment
        // lookup comment
        if err := models.DB.Where("id = ?", parentId).First(&parentComment).Error; err != nil {
            return
        }
        // Create comment and add comment to db
        comment := Comment{
            ParentId:    parentComment.ID,
            ParentType:  models.PT_Comment,
            Comment:     "",
            SubComments: nil,
        }
        models.DB.Create(&comment)
        // Append to Parent Comment and persist Parent Comment
        models.DB.Session(&gorm.Session{FullSaveAssociations: true}).Model(&parentComment).Updates(&Comment{
            SubComments: append(parentComment.SubComments, comment),
        })
    }
}

func GetCommentsForBlogPost(blogPostId uint) {
    var comments []Comment

    // Lookup Comments by BlogPostId
    **// TODO Problem is it is not returning all nested comments**
    **// i.e. The Comments.SubComments**
    if err := models.DB.Preload(clause.Associations).
        Where(&Comment{ParentType: PT_BlogPost, ParentId: blogPostId}).
        Find(&comments).Error; err != nil {
        return
    }
}

Trying to create an index on ParentId and ParentType and setting that as the foreign key does not work either:

type Comment struct {
    ID          uint `gorm:"primary_key"`
    ParentId    uint `gorm:"index:idx_parent"`
    ParentType  ParentType `gorm:"index:idx_parent"`
    Comment     string
    // TODO composite foreign key not working
    SubComments []Comment `gorm:"foreignKey:idx_parent"`
}

I get an error on the commented line below: invalid field found for struct Comment's field SubComments: define a valid foreign key for relations or implement the Valuer/Scanner interface

type CreateBlogPostInput struct {
    Title   string `json:"title" binding:"required"`
    Content string `json:"content" binding:"required"`
}

func CreateBlogPost(input CreateBlogPostInput) {
    var input CreateBlogPostInput

    // Create blog post
    blogPost := models.BlogPost{
        Title:    input.Title,
        Content:  input.Content,
        Comments: []models.Comment{},
    }

    // ***Foreign key error here***
    models.DB.Create(&blogPost)
}

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