Implementing relationships with value arrays in Gorm

In Gorm, implementing relationships with arrays of values ​​is a very useful feature. Gorm is a popular Go language ORM (Object Relational Mapping) library that allows developers to use the Go language to operate databases. Unlike traditional relational databases, a value array is a special data structure that allows multiple values ​​to be stored as a whole in a field in the database. This is very helpful for storing some complex data structures or improving query efficiency. In this article, I will introduce you to how to implement relationships with value arrays in Gorm and how to use it to improve development efficiency.

Question content

I am trying to implement a model for an invoice application using go and gorm. I have defined an invoice structure and want to include invoice line items from a separate structure.

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 structure.

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 following 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 line item that can choose from a fixed rate and description to limit duplication.

I'm not sure if I'm going about it the right way.

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

Workaround

Based on your column names, I can think of three ways to accomplish this:

1. Use default syntax (none 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. Specify a custom foreign key (assuming the second struct has a different column name that you want to use as a foreign key)

   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 the two structures 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
    }

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