Implementing a relationship with an array of values in Gorm

I'm trying to implement a model for an invoicing application using Go and Gorm. I have defined my Invoice struct and want to include the invoice line items from a separate struct.

type Invoice struct {
	Base
	CompanyID  string `gorm:"not null"`
	Company    Company
	InvoiceNo  string     `gorm:"not null"`
	Currency   string     `gorm:"not null;default:'GBP'"`
	Total      float64    `gorm:"not null"`
	Terms      int        `gorm:"not null;default:30"`
	IssuedDate time.Time  `gorm:"not null"`
	DueDate    time.Time  `gorm:"not null"`
	Paid       bool       `gorm:"not null"`
	LineItems  []LineItem `gorm:"foreignKey:InvoiceID"`
	Void       bool       `gorm:"default:false"`
}

This is my LineItem struct.

type LineItem struct {
	Base
	Service     string `gorm:"not null"`
	Description string `gorm:"not null;"`
	Amount      float64
	Count       int
	UnitBase    string  `gorm:"not null;default:'Days'"` // days or hours
	Total       float64 `gorm:"not null"`
}

When I try to build the application I get the below error.

... got error invalid field found for struct github.com/repo/API/database/models.Invoice's field LineItems: define a valid foreign key for relations or implement the Valuer/Scanner interface

The idea is that I can define a limited set up line items which can be chosen from with fixed rates and descriptions to limit repetition.

I'm unsure if I'm going about this the right/correct way.

So my question is, is it possible to include an array of items from a relational model in this way?

Depending on your column names, there are three ways I can think of how you can achieve this:

1. Use default syntax (No gorm:foreignKey)

    type Invoice struct {
        ID     //this is the primary key, may also be in your base model
        Lineitems []LineItem
        ..other fields
    }
   
    type LineItem struct {
        ID          //primary key of LineItem
        InvoiceID   //automatically this will be your foreign key
        ..other fields
    }
   

2. Specifying custom foreign key (lets say second struct has a different column name which you want as the foreignKey)

    type Invoice struct {
        ID     //this is the primary key, may also be in your base model
        Lineitems []LineItem `gorm:"foreignKey:ParentID"`
        ..other fields
    }
   
    type LineItem struct {
        ID          //primary key of LineItem
        ParentID   //this is the custom column referencing Invoice.ID
        ..other fields
    }
   

3. Or both structs have different column names

    type Invoice struct {
        ID     //this is the primary key, may also be in your base model
        InvoiceNum 
        Lineitems []LineItem `gorm:"foreignKey:ParentNum;references:InvoiceNum"`
        ..other fields
    }
   
    type LineItem struct {
        ID          //primary key of LineItem
        ParentNum   //this is now referencing Invoice.InvoiceNum
        ..other fields
    }