Create a generic find function for mongodb in Rust

Question

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;
        }
    };
}

piojo · Accepted Answer · 2022-01-02 00:20:43Z

1

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 think Send and Sync 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 Unpin and DeserializeOwned. (T automatically implements Send and Sync if all T's members are Send and Sync.)

edited Jan 2, 2022 at 0:20

answered Jan 1, 2022 at 10:58

piojo

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2 Comments

iuer ui Over a year ago

I think I have same issue, can you give example ?? ref:stackoverflow.com/questions/77206788/…

piojo Over a year ago

@iuerui I'm not currently thinking in Rust, so writing up an example would take more time than I have at the moment. But on your question, someone suggested you post a minimum reproducible example. That's a good idea, that way people can run your code and play around with it even if they don't understand the problem/solution at first glance. (If your question happens to get closed before that, feel free to post it again with the full example code that shows the issue--there's no harm in a do-over.)

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