You could use multiple relation calls, Relation("Relation.a").Relation("Relation.b").....

uhm, i think you misunderstood the problem, so i have table
order that has relation with voucher that has relation with package that has relation with service that has relation with service type
which makes
o -> v -> p -> s -> st

and I only have the order data which makes me can't do the suggestion you give

Oh, you need to also select the relation column, i guess go-pg is that by design.

for example:

type struct Roles {
id uint
name string
Users []Users
}

type struct Users {
id uint
name string
RoleID uint
}

in order to select get users from role, you also need to select relation column if you have explicit column selection.
example:

// will not work
Model(&Role{})Relation("Users.name")

// will work
Model(&Role{}).Relation("Users.name").Relation("Users.role_id")