我有这段代码来连接MongoDB，当我试图运行cargo run时，代码在if语句中返回一个错误，我不知道为什么。我试着调试了好几天。这就是代码：

use std::iter::FilterMap;
use rocket::serde::json::{json, Value};
use futures::{StreamExt};
use mongodb::{bson::doc, options::ClientOptions, Client};
use bson::{Document};
use warp::body::json;

/**
 * MongoDB client
 */
#[tokio::main]
pub async fn database_connection(key: &str, value: &str) -> Result<Option<Value>, mongodb::error::Error> {

    // uri to connect to the database
    let uri = "mongodb://admin:adminpassword@localhost:27017";
    
    // options to connect to the database
    let mut client_options =
        ClientOptions::parse(uri)
        .await?;

    // Create a new client and connect to the server with the given options
    let client = Client::with_options(client_options)?;

    //Connect to the BinGo! database
    let material = client.database("BinGo!");
    
    // Search thrue a collection
    let collection = material.collection::<Document>("bingo_materials");
    
    let filter = doc! {
        key: value
    };
    
    print!("Making the query to the database...\n");
    
    println!("Filter: {:#?}", filter);
    print!("-------------------------\n");

    let mut cursor = collection.
                find(filter, None)
                .await?;
    
    if let  Some(result) = cursor.next().await {
        println!("{:#?}", result);
        let result_json: &Result<Document, mongodb::error::Error> = &result;
        return Ok(Some(result_json));
    }else{
        return Ok(None);
    }

}

字符串
错误如下：

error[E0308]: mismatched types
  --> src/databases.rs:57:5
   |
25 |   pub async fn database_connection(key: &str, value: &str) -> Result<Option<Value>, mongodb::error::Error> {
   |                                                               -------------------------------------------- expected `Result<std::option::Option<rocket_dyn_templates::tera::Value>, mongodb::error::Error>` because of return type
...
57 | /     if let  Some(result) = cursor.next().await {
58 | |         println!("{:#?}", result);
59 | |         let result_json: &Result<Document, mongodb::error::Error> = &result;
60 | |         return Ok(Some(result_json));
61 | |     }else{
62 | |         return Ok(None);
63 | |     }
   | |_____^ expected `Result<Option<Value>, Error>`, found `Result<Option<&...>, ...>`
   |
   = note: expected enum `Result<std::option::Option<rocket_dyn_templates::tera::Value>, mongodb::error::Error>`
              found enum `Result<std::option::Option<&Result<bson::Document, mongodb::error::Error>>, _>`

型
请你客气一点，我是新手，还在学习。
我期望返回result_json的值，因为database_connection()是从rocket.rs中的路由调用的。