Many-to-many: preload with an order based on an additional column

[WinXaito]

Your Question

Hi guys,

I have a relation many-to-many like that:

type Group struct {
	Uuid              uuid.UUID `gorm:"primaryKey;column:uuid"`
	Name              string    `gorm:"column:name"`

	Machines []*Machine `gorm:"many2many:group_machines;foreignKey:Uuid;joinForeignKey:GroupUuid;References:Uuid;joinReferences:MachineUuid"`
}

type Machine struct {
	Uuid              uuid.UUID `gorm:"primaryKey;column:uuid"`
	Name              string    `gorm:"column:name"`

	Groups []*Group `gorm:"many2many:group_machines;foreignKey:Uuid;joinForeignKey:MachineUuid;References:Uuid;joinReferences:GroupUuid"`
}

// The GroupMachines tables was created automatically, but since I need to keep an order for 
// the machines in a group, I need to have the Rank. So I created a new struct that I included in the migration.
type GroupMachines struct {
	GroupUuid uuid.UUID `gorm:"primaryKey;column:group_uuid"`
	Group *Group
	MachineUuid      uuid.UUID `gorm:"primaryKey;column:machine_uuid"`
	Machine          *Machine
	
	Rank int `gorm:"column:rank"`
}

So, my question is how I can preload a group with the machine, ordered by Rank ?

I tried something like that, but that doesn't work:

gm := query_config.GroupMachines
g := query_config.Group
group, err := g.Where(g.Uuid.Eq(uid)).
	Preload(field.Associations).
	Preload(g.Machines.Order(gm.Rank)).
	First()

With that code, I got the following error: SQL logic error: no such column: group_machines.rank (1)

In fact, I don't necessarily need to have the Rank value, I just need the Machines list in my Group to be ordered correctly.

The document you expected this should be explained

Associations / Preload : indicate how to manage supplementary column in a many-to-many relation.

Expected answer

How to make that work ? :)