Golang cannot range over pointer to slice

I keep getting this error when trying to range over a slice pointer.

app/domain/repositories/class_repository.go:24: cannot range over classes (type *[]entities.Class)

What am I doing wrong?

Here is the struct:

 package repositories
    
import (
	"mobifit/app/domain/entities"
)

type ClassRepository struct {
	*Repository
}

func (c *ClassRepository) ClassesForLastNDays(days int) *[]entities.Class {
	classes := new([]entities.Class)
	query := Select("*").
		From("Class").
		Where("VisibleAt > CURRENT_TIMESTAMP() - INTERVAL ? DAY").
		OrderBy("ClassTypeId").
		Sql()
	c.Repository.Select(classes, query, days)
	c.populateClassRelationships(classes)
	return classes
}

func (c *ClassRepository) populateClassRelationships(classes *[]entities.Class) {
	for i := range classes {  <<<<<<<<<<< Here is the problem
		class := classes[i]

		// ClassType
		c.Repository.GetById(class.ClassType, class.ClassTypeId)

		//Instructor
		c.Repository.GetById(class.Instructor, class.ClassType.InstructorId)

		// Equipment
		query := Select("E.*").
			From("Equipment E").
			Join("ClassEquipment CE on E.Id = CE.EquipmentId").
			Where("CE.ClassId = ?").
			Sql()
		c.Repository.Select(class.Equipment, query, class.Id)
	}
}

Here is the Class struct:

package entities

import (
	"time"
)

    type Class struct {
    	Id                int
    	ClassTypeId       int
    	VideoPath         string
    	VideoSize         int
    	Duration          float64
    	CreatedAt         time.Time
    	VisibleAt         time.Time
    	NoLongerVisibleAt time.Time
    
    	// Relationships
    	ClassType  ClassType
    	Instructor User
    	Equipment  []Equipment
    }

You're assuming the pointer to a slice will be automatically dereferenced for the iteration.

That's not the case and there's no reason for that because a slice is already a kind of pointer, rendering a pointer to a slice totally useless.

From Effective Go :

> If a function takes a slice argument, changes it makes to the elements
> of the slice will be visible to the caller, analogous to passing a
> pointer to the underlying array.

Internally, a slice is made of

• a pointer to the first element of the slice in the underlying array
• the length of the slice
• the capacity of the slice (the slice can usually be extended until the end of the array)

This structure is very small, rendering a pointer useless.

if you need to pull an individual element from the *slice, you have to dereference it first like this: (*slice)[0]. I pounded my head against *slice[0] for about 6 hours before I realized this. It has to do with the order of operations, and is not, IMO, a very elegant result.

I ended up writing some pointer receiver methods to do in-place modifications like append and pop in a more, to my mind, reasonable way - an example can be found here: https://play.golang.org/p/qZEYMcPHl4

From Effective Go:
> If you're looping over an array, slice, string, or map, or reading
> from a channel, a range clause can manage the loop.

You are attempting to iterate over a pointer to a slice which is a single value, not a collection therefore is not possible.

Change the argument to populateClassRelationships to be an slice, not a pointer to a slice. Or you could dereference the pointer:

func (c *ClassRepository) populateClassRelationships(classes *[]entities.Class) {
	for i := range *classes { // dereferencing the pointer to get the actual slice
		class := classes[i]

		// ClassType
		c.Repository.GetById(class.ClassType, class.ClassTypeId)

		//Instructor
		c.Repository.GetById(class.Instructor, class.ClassType.InstructorId)

		// Equipment
		query := Select("E.*").
			From("Equipment E").
			Join("ClassEquipment CE on E.Id = CE.EquipmentId").
			Where("CE.ClassId = ?").
			Sql()
		c.Repository.Select(class.Equipment, query, class.Id)
	}
}

You could dereference the pointer:

func (c *ClassRepository) populateClassRelationships(classes *[]entities.Class) {
    for _, class := range *classes { // NOTE the * dereference
	// ClassType
	c.Repository.GetById(class.ClassType, class.ClassTypeId)

	//Instructor
	c.Repository.GetById(class.Instructor, class.ClassType.InstructorId)

	// Equipment
	query := Select("E.*").
	    From("Equipment E").
	    Join("ClassEquipment CE on E.Id = CE.EquipmentId").
	    Where("CE.ClassId = ?").
	    Sql()
	c.Repository.Select(class.Equipment, query, class.Id)
    }
}

I also changed the range clause as I don't think you're modifying classes.