Golang cannot range over pointer to slice - go

Question

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
}

Answer 1

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.

Answer 2

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

Answer 3

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)
}
}

Answer 4

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.