Relationships in MongoDB (using .Net)

Question

Is there anyway in mongoDB using .Net to create some sort of equivalent to the "SQL-Join"? I have read the MongoDB docs (https://docs.mongodb.com/manual/tutorial/model-referenced-one-to-many-relationships-between-documents/) regarding relationships.. and from what I understand you simply add relationships by refering to their IDs. However.. does this also mean that for each relationship you also need to do one additional query?..

Answer 1

If you don't mind using a library, MongoDB.Entities can do relationships quite easily without having to do joins manually, unless you really want to :-)

Have a look at the code below, which demonstrates a one-to-many relationship between an author and book entities. the book entity knows nothing of the authors. but you can still get reverse relationship access by supplying either a book ID, an array of book IDs or even an IQueryable of books. [disclaimer: I'm the author of the library]

using System;
using System.Linq;
using MongoDB.Entities;
using MongoDB.Driver.Linq;

namespace StackOverflow
{
    public class Program
    {
        public class Author : Entity
        {
            public string Name { get; set; }
            public Many<Book> Books { get; set; }

            public Author() => this.InitOneToMany(() => Books);
        }

        public class Book : Entity
        {
            public string Title { get; set; }
        }

        static void Main(string[] args)
        {
            new DB("test");

            var book = new Book { Title = "The Power Of Now" };
            book.Save();

            var author = new Author { Name = "Eckhart Tolle" };
            author.Save();

            author.Books.Add(book);

            // Build a query for finding all books that have Power in the title.
            var bookQuery = DB.Queryable<Book>()
                              .Where(b => b.Title.Contains("Power"));

            // Find all the authors of books that have a title with Power in them.
            var authors = author.Books
                                .ParentsQueryable<Author>(bookQuery);

            // Get the result
            var result = authors.ToArray();

            // Output the aggregation pipeline
            Console.WriteLine(authors.ToString());

            Console.ReadKey();
        }
    }
}

Answer 2

Considering simplest relationship like below:

db.publishers.save({
    _id: "oreilly",
    name: "O'Reilly Media",
    founded: 1980,
    location: "CA"
})

db.books.save({
    _id: 123456789,
    title: "MongoDB: The Definitive Guide",
    author: [ "Kristina Chodorow", "Mike Dirolf" ],
    published_date: ISODate("2010-09-24"),
    pages: 216,
    language: "English",
    publisher_id: "oreilly"
})

In MongoDB you can use $lookup operator to get the data from both collections in one query:

db.books.aggregate([
    {
        $lookup: {
            from: "publishers",
            localField: "publisher_id",
            foreignField: "_id",
            as: "publisher"
        }
    }
])

which returns:

{ 
    "_id" : 123456789, 
    "title" : "MongoDB: The Definitive Guide", 
    "author" : [ "Kristina Chodorow", "Mike Dirolf" ], 
    "published_date" : ISODate("2010-09-24T00:00:00Z"), 
    "pages" : 216, 
    "language" : "English", 
    "publisher_id" : "oreilly", 
    "publisher" : [ { "_id" : "oreilly", "name" : "O'Reilly Media", "founded" : 1980, "location" : "CA" } ] 
}

Using MongoDB .NET Driver you can use LINQ syntax and join operator which will get translated into $lookup:

var books = db.GetCollection<Book>("books");
var publishers = db.GetCollection<Publisher>("publishers");

var q = from book in books.AsQueryable()
        join publisher in publishers.AsQueryable() on
            book.publisher_id equals publisher._id
        select new
        {
            book,
            publisher = publisher
        };

var result = q.ToList();

which is translated into $lookup with $unwind so that you get a single publisher object instead of array