C# MongoDb驱动包括子object

Question

I am new to Mongo and have a project that uses the C# MongoDb driver and Linq for retrieving data from MongoDb.

I have an object named Instance that I retrieve from the Mongo collection just fine. However, the object's Template property belongs in a separate collection and is null when querying the Instance collection. I would like to eager load the template data when querying the instance.

This would be the equivalent of Entity Framework's Include method which eager loads related entities. I have searched the net for an equivalent approach using the C# Mongo driver but no luck.

How can I accomplish this using the C# MongoDb driver and Linq?

public class Instance
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int? TemplateId { get; set; }
    public Template Template { get; set; }
}
public class Template
{
    public int Id { get; set; }
    public string Name { get; set; }
}

var collection = MongoDatabase.GetCollection<Instance>("Instances").AsQueryable()
var instance = collection.First(i => i.Id == 1);
var template = instance.Template; //always null

Answer 1

You can have Typed Lookup like this>

public class Instance
{
    public ObjectId Id { get; set; }
    public string Name { get; set; }
    public ObjectId TemplateId { get; set; }
    public Template Template { get; set; }
}

public class Template
{
    public ObjectId Id { get; set; }
    public string Name { get; set; }
}


string connectionString = "mongodb://localhost:27017";
var client = new MongoClient(connectionString);

var db = client.GetDatabase("test");
var instances = db.GetCollection<Instance>("Instances");
var resultOfJoin = instances.Aggregate()
    .Lookup("Templates", "TemplateId", "_id", @as: "Template")
    .Unwind("Template")
    .As<Instance>()
    .ToList();

EDIT: Unwind part edited by jcruz.

Answer 2

For eager load Try using

public virtual Template Template { get; set; } in instance. But still you have to use Include() if you want to retrieve the data from child table,

You can get templateid from the instance which should have PK of tempalte without include

Tested for EF Core 6.3