Diesel 中多个外键引用同一个表

我正在尝试创建一个引用同一个表两次的结构。这样做的目的是创建一种类别层次结构。这是我正在尝试对下表执行的操作：

create table product_category_rollup(
    id serial primary key,
    upper_category_id integer not null,
    lower_category_id integer not null,
    foreign key (upper_category_id) references product_category(id),
    foreign key (lower_category_id) references product_category(id)
);

create table product_category(
    id serial primary key,
    name varchar unique not null
);

我正在尝试创建匹配的结构，如下所示：

#[derive(Identifiable, Queryable)]
#[table_name = "product_category"]
pub struct ProductCategory {
    id: i32,
    name: String,
}

#[derive(Queryable, Identifiable, Associations)]
#[belongs_to(ProductCategory, foreign_key="upper_category_id")]
#[belongs_to(ProductCategory, foreign_key="lower_category_id")]
#[table_name = "product_category_rollup"]
pub struct ProductCategoryRollup {
    id: i32,
    upper_category_id: i32,
    lower_category_id: i32,
}

我收到一条错误消息：

error[E0119]: conflicting implementations of trait `diesel::associations::BelongsTo<entities::ProductCategory>` for type `entities::ProductCategoryRollup`:
  --> src/entities.rs:29:35
   |
29 | #[derive(Queryable, Identifiable, Associations)]
   |                                   ^^^^^^^^^^^^
   |                                   |
   |                                   first implementation here
   |                                   conflicting implementation for `entities::ProductCategoryRollup`
   |
   = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

让多个外键引用同一个表的正确方法是什么？这是 Diesel 的一些尚未解决的固有限制吗？

The BelongsTo https://docs.diesel.rs/diesel/associations/trait.BelongsTo.html特质定义为：

pub trait BelongsTo<Parent> {
    type ForeignKey: Hash + Eq;
    type ForeignKeyColumn: Column;
    fn foreign_key(&self) -> Option<&Self::ForeignKey>;
    fn foreign_key_column() -> Self::ForeignKeyColumn;
}

后果ForeignKey (and ForeignKeyColumn）是关联类型，而不是泛型参数，给定Child只能有一个实现BelongsTo<Parent>.

总的来说，似乎BelongsTo相当有限；请注意，它也仅限于单个列。