GORM select certain column with preload - go

Question

Let's say I have these struct that represent my models
type QuestionHeader struct {
QuestionHeaderId int `gorm:"primaryKey;column:question_id" json:"question_id"`
LevelId int `gorm:"column:level_id" json:"level_id"`
SubjectId int `gorm:"column:subject_id" json:"subject_id"`
QuestionBody []QuestionBody
QuestionSolution []QuestionSolution
OtherColumn1A string
OtherColumn2A string
}
type QuestionBody struct {
QuestionBody int `gorm:"primaryKey;column:question_id" json:"question_id"`
Value string `gorm:"column:value" json:"value"`
Type string `gorm:"column:type" json:"type"`
OtherColumn1B string
OtherColumn2B string
}
type QuestionSolution struct {
QuestionSolutionId int `gorm:"primaryKey;column:question_id" json:"question_id"`
Value string `gorm:"column:value" json:"value"`
Type string `gorm:"column:type" json:"type"`
OtherColumn1C string
OtherColumn2c string
}
Then I want to do a query with preload like this
qs.db().
Preload("QuestionHeader.QuestionBody", func(db *gorm.DB) *gorm.DB {
return db.Where("question_body.status = ?", 1)
// I only want to select certain column here
}).
Preload("QuestionHeader.QuestionSolution", func(db *gorm.DB) *gorm.DB {
return db.Where("question_solution.status = ?", 1)
// I only want to select certain column here
}).
Where("level_id = ?", 2).
Find(&questionHeaders) // I only want to select certain column here
But the catch with that code is, I will select all the columns based on my model property
What if I want to select only a certain column with preload?
For example, I want to select only type for QuestionBody
I know that you can create another struct for your select statement as described in the docs, but are there any other ways? I don't want to create a struct for every select statement
In Laravel Eloquent, you can do something like this

Answer 1

There are few issue that i would like to point out
You don't have the many2many association defined in QuestionHeader for QuestionBody or QuestionSolution
In query you are using status column but I don't seem them defined in QuestionBody or QuestionSolution
Rename QuestionBody id column to QuestionBodyId in QuestionBody
As per the points changes would be as below:
type QuestionHeader struct {
QuestionHeaderId int `gorm:"primaryKey;column:question_id" json:"question_id"`
LevelId int `gorm:"column:level_id" json:"level_id"`
SubjectId int `gorm:"column:subject_id" json:"subject_id"`
QuestionBody []QuestionBody `gorm:"many2many:question_header_question_bodies;"`
QuestionSolution []QuestionSolution `gorm:"many2many:question_header_question_solutions;"`
OtherColumn1A string
OtherColumn2A string
}
type QuestionBody struct {
QuestionBodyId int `gorm:"primaryKey;column:question_id" json:"question_id"`
Value string `gorm:"column:value" json:"value"`
Type string `gorm:"column:type" json:"type"`
Status uint8 `gorm:"column:status;default:0" json:"status"`
OtherColumn1B string
OtherColumn2B string
}
type QuestionSolution struct {
QuestionSolutionId int `gorm:"primaryKey;column:question_id" json:"question_id"`
Value string `gorm:"column:value" json:"value"`
Type string `gorm:"column:type" json:"type"`
Status uint8 `gorm:"column:status;default:0" json:"status"`
OtherColumn1C string
OtherColumn2c string
}
var questionHeaders QuestionHeader
db.
Preload("QuestionBody", func(db *gorm.DB) *gorm.DB {
return db.Where("Status = ?", 1).Select("QuestionBodyId", "Value")
}).
Preload("QuestionSolution", func(db *gorm.DB) *gorm.DB {
return db.Where("Status = ?", 1).Select("QuestionSolutionId", "Value")
}).
Where("level_id = ?", 2).
Select("SubjectId").
Find(&questionHeaders)
}
In Preload Select we would have to include the unique primary key for gorm to uniquely identify the associated slice of structs