rust 具有不同泛型类型的新动态特征变量？

Question

I'm quite new to Rust. Trying to create database connection with diesel-rs.

Here's part of my code:

use diesel::Connecction;
use diesel::mysql::MysqlConnection;
use diesel::sqlite::SqliteConnection;

let engine = "mysql";
let mysql_url = "mysql://username:password@localhost:3306/test";
let sqlite_url = "sqlite://sqlite.db";
let connection : Box<dyn Connection> = if engine == "mysql" {
  Box::new(MysqlConnection::establish(mysql_url).unwrap())
} else {
  Box::new(SqliteConnection::establish(sqlite_url).unwrap())
}

Here's the compiler error:

error[E0191]: the value of the associated types `Backend` (from trait `Connection`), `TransactionManager` (from trait `Connection`) must be specified
  --> src/quant/common/persistence/database.rs:11:25
   |
11 |     connection: Box<dyn Connection>,
   |                         ^^^^^^^^^^ help: specify the associated types: `Connection<Backend = Type, TransactionManager = Type>`

Is it possible to create different connection when program starts with different parameters?

Answer 1

This use case is not supported. See this issue . If you manage to do this with connection, you'll have to wrap the transaction and queries, and then likely some of the generated table! types, and down to a rabbit hole.