Can SelectThree be extended to implement SelectNine through macros?

[cnwzhu]

A similar approach with the following code can better extend associative queries.
#2518

macro_rules! generate_select_structs {
    ($name:ident, $($generics:ident),+) => {
        /// Defines a structure to perform a SELECT operation on two Models
        #[derive(Clone, Debug)]
        pub struct $name<$($generics),*>
        where
            $($generics: EntityTrait),*
        {
            pub(crate) query: SelectStatement,
            pub(crate) entity: PhantomData<($($generics),*)>,
        }
    };

}

// Rust 宏系统不支持 `concat_idents!` 直接在 `macro_rules!` 内使用，
// 这里使用手动展开
generate_select_structs!(SelectTwo, E, F);
generate_select_structs!(SelectThree, E, F, G);
generate_select_structs!(SelectFour, E, F, G, H);
generate_select_structs!(SelectFive, E, F, G, H, I);
generate_select_structs!(SelectSix, E, F, G, H, I, J);
generate_select_structs!(SelectSeven, E, F, G, H, I, J, K);
generate_select_structs!(SelectEight, E, F, G, H, I, J, K, L);
generate_select_structs!(SelectNine, E, F, G, H, I, J, K, L, M);


macro_rules! re_impl_query_trait {
    ($name:ident, $($generics:ident),+) => {
        impl<$($generics),*> QueryTrait for $name<$($generics),*>
        where
            $($generics: EntityTrait),*
        {
            type QueryStatement = SelectStatement;

            fn query(&mut self) -> &mut Self::QueryStatement {
                &mut self.query
            }

            fn as_query(&self) -> &Self::QueryStatement {
                &self.query
            }

            fn into_query(self) -> Self::QueryStatement {
                self.query
            }
        }
    };
}

// 生成 SelectOne ~ SelectNine
re_impl_query_trait!(SelectThree, E, F, G);
re_impl_query_trait!(SelectFour, E, F, G, H);
re_impl_query_trait!(SelectFive, E, F, G, H, I);
re_impl_query_trait!(SelectSix, E, F, G, H, I, J);
re_impl_query_trait!(SelectSeven, E, F, G, H, I, J, K);
re_impl_query_trait!(SelectEight, E, F, G, H, I, J, K, L);
re_impl_query_trait!(SelectNine, E, F, G, H, I, J, K, L, M);

[tyt2y3]

well there are some naunces with type system that's tricky with macro
so I've added it by hand in https://github.com/SeaQL/sea-orm/pull/2757/files
there's no reason why it can't be further extended!

    // join path:
    // one -> two -> three -> four -> five
    //     -> six
    let (one, two, three, four, five, six) = one::Entity::find()
        .find_also(one::Entity, two::Entity)
        .find_also(two::Entity, three::Entity)
        .find_also(three::Entity, four::Entity)
        .find_also(four::Entity, five::Entity)
        .find_also(one::Entity, six::Entity)
        .one(db)
        .await?
        .unwrap();