Got the workaround from here: https://github.com/diesel-rs/diesel/issues/89#issuecomment-429547936

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

pub struct DummyProductCategory(ProductCategory);

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