Styjun

I have an application that has two similar but different objects and I want to store those objects in the same collection. What is the best way to do this? And how can I query this collection?

Today my collections is represented by:

public IMongoCollection<Post> Posts

 get
 
 return _database.GetCollection<Post>("posts");
 

And I have this class:

public class Post

 public string Id get; set; 
 public string Message get; set; 


public class NewTypePost

 public string Id get; set; 
 public string Image get; set; 

So, today I just can save and query using Post class. Now I want to store and retrive the both classes, Post and NewTypePost.

I tried to change the class type from Post to dynamic. But when I did this, I could not query the collections.

Suppose I have the next conn to a test database:

var mongoClient = new MongoClient(new MongoClientSettings

 Server = new MongoServerAddress("localhost"),
);
var database = mongoClient.GetDatabase("TestDb");

Then I can do something like:

var col = database.GetCollection<Post>("posts");
var col2 = database.GetCollection<NewTypePost>("posts");

To get two different instances of IMongoCollection but pointing to the same collection in the database. Further I am able to save to each collection in the usual way:

col.InsertOne(new Post Message = "m1" );
col2.InsertOne(new NewTypePost Image = "im1" );

Then, I'm also able to query from those collection base on the specific fields:

var p1= col.Find(Builders<Post>.Filter.Eq(x=>x.Message, "m1")).FirstOrDefault();
var p2 =col2.Find(Builders<NewTypePost>.Filter.Eq(x=>x.Image, "im1")).FirstOrDefault();

Console.WriteLine(p1?.Message); // m1
Console.WriteLine(p2?.Image); // im1

I don't know if that's what you want but it uses the same collection. BTW, change the Id properties to be decorated with [BsonId, BsonRepresentation(BsonType.ObjectId)]. Hope it helps.

MongoDB .NET driver offers few possibilites in such cases:

Polymorphism

You can build a hierarchy of classes and MongoDB driver will be able to determine a type of an object it gets retrieved from the database:

[BsonKnownTypes(typeof(Post), typeof(NewTypePost))]
public abstract class PostBase

 [BsonId]
 public string Id get; set; 



public class Post: PostBase
 
 public string Message get; set; 


public class NewTypePost: PostBase

 public string Image get; set; 

MongoDB driver will create additional field _t in every document which will represent corresponding class.

Single Class

You can still have Post class and use BsonIgnoreIfNull attribute to avoid serialization exception. MongoDB .NET driver will set those properties to null if they don't exist in your database.

public class Post
 
 [BsonId]
 public string Id get; set; 
 [BsonIgnoreIfNull] 
 public string Message get; set; 
 [BsonIgnoreIfNull]
 public string Image get; set; 

BsonDocument

You can also drop strongly-typed approach and use BsonDocument class which is dynamic dictionary-like structure that represents your Mongo documents

var collection = db.GetCollection<BsonDocument>("posts");

More details here

dynamic

Specifying dynamic as generic parameter of ICollection you should get a list of ExpandoObject that will hold all the values you have in your database.

var collection = db.GetCollection<dynamic>("posts");
var data = collection.Find(Builders<dynamic>.Filter.Empty).ToList();
var firstMessage = data[0].Message; // dynamically typed code