Define the order() argument from a variable

[agateau]

I am new to Diesel, and I want to create a method returning a vector of models sorted by a method parameter. My current code looks like this:

pub enum Order {
    DATE,
    LENGTH,
    DURATION
}
// ...
    pub fn get_tracks(&self, order: Order) -> Vec<Track> {
        let order_arg = match order {
            DATE => {
                start_date_col.asc()
            },
            LENGTH => {
                length_col.asc()
            },
            DURATION => {
                duration_col.asc()
            }
        };

        tracks_table
            .order(order_arg)
            .load::<Track>(&self.conn)
            .unwrap()
    }

The *_col identifiers are my column names, which I use with an alias to avoid collisions, so start_date_col is use crate::schema::tracks::columns::start_date as start_date_col)

But this fails to compile because Rust says the matches are not the same type:

error[E0308]: `match` arms have incompatible types
  --> src/database.rs:82:17
   |
77 |           let order_arg = match order {
   |  _________________________-
78 | |             DATE => {
79 | |                 start_date_col.asc()
   | |                 -------------------- this is found to be of type `Asc<columns::start_date>`
80 | |             },
81 | |             LENGTH => {
82 | |                 length_col.asc()
   | |                 ^^^^^^^^^^^^^^^^ expected struct `columns::start_date`, found struct `columns::length`
...  |
86 | |             }
87 | |         };
   | |_________- `match` arms have incompatible types
   |
   = note: expected type `Asc<columns::start_date>`
            found struct `Asc<columns::length>`

[weiznich]

As the compiler message says each match arm returns a different type. That's expected as each column type is it's own distinct type. See the Schema in depth guide for details about what is produced by diesel's table! macro internally.

Now there are multiple ways to "solve" your problem:

• Construct the query before the match statement, use .into_boxed() to produce a stable query type and then apply the order in each match arm
• Use BoxableExpression to construct a compatible common trait object based on your expressions.

[agateau]

Using .into_boxed() works fine, thanks!