I'm trying to create a method to which I can pass a mongodb connection pool, an objectId and the collection name to retrieve data.

I came up with the code below but which doesn't compile because of the following error:

error: the method find_one exists for struct mongodb::Collection<T>, but its trait bounds were not satisfied label: method cannot be called on mongodb::Collection<T> due to unsatisfied trait bounds note: the following trait bounds were not satisfied: T: DeserializeOwned T: Unpin T: std::marker::Send T: Sync label: method cannot be called on mongodb::Collection<T> due to unsatisfied trait bounds

What I'm I doing wrong?

pub async fn generic_find_by_id<T>(db: &AppContext, object_id: String, collection_name: &str) -> Option<T> {
    let collection = db.mongodb_pool.collection::<T>(collection_name);
    
    let id_obj = ObjectId::parse_str(object_id);

    let found = match id_obj {
        Ok(id) => {
            let filter = doc! {"_id": id};
            let result = collection.find_one(filter, None).await;
            match result {
                Ok(result) => {
                    match result {
                        Some(result) => {
                            return Some(result);
                        }
                        None => {
                            return None;
                        }
                    }
                }
                Err(_) => {
                    return None;
                }
            }
        }
        Err(_) => {
            return None;
        }
    };
}    

CodePudding user response：

It seems Collection<T>::find_one() is only implemented if T: DeserializeOwned Unpin Send Sync. (See it in the source here: https://docs.rs/mongodb/latest/src/mongodb/coll/mod.rs.html#795). I gather some of those traits have to do with the collection being potentially sent across threads during async function calls. To solve this, you can make your T type implement those traits. T automatically implements Send and Sync if all T's members are Send and Sync.