How to Insert document to MongoDB and return the same document or it's ID back using InsertOneAsync in C# .Net

Question

I'm writing a generic method to act like data access layer to insert document to MongoDB using C# .Net. My method looks like below. Collection here is a MongoCollection retrieved from MongoDB.

public async T Create(T entity){
await Collection.InsertOneAsync(entity);
}

I want to return the entity inserted or it's ID which is auto generated by MongoDB back. InsertOneAsync method is returning a task. I tried changing it as below. But its's return type is void.

Collection.InsertOneAsync(entity).GetAwaiter().GetResult();

Is there a way to get the id or entity back using InsertOneAsync method. I'm using MongoDB driver for C#.

Answer 1

Ids in MongoDB are generated on the client side.

If the document does not specify an _id field, then MongoDB will add the _id field and assign a unique ObjectId for the document before inserting. Most drivers create an ObjectId and insert the _id field, but the mongod will create and populate the _id if the driver or application does not.

In you case you can generate ObjectIds manually and return them from your method (using ObjectId.GenerateNewId()) or return entire object since MongoDB driver will set proper _id value if you use [BsonId] attribute

public async Task<T> Create(T entity) where T:class
{
    await Collection.InsertOneAsync(entity);
    return entity;
}

And pass a type parameter like:

public class MyClass
{
    [BsonId]
    public ObjectId Id { get; set; }
    //other properties
}