LINQ queries

See the mongodb c# driver linq documentation to see which LINQ operations are available. Also see the c# driver expressions documentation to see all supported expressions.

Tip

Don't forget to first import the mongodb linq extensions with using MongoDB.Driver.Linq;

Query collections

var author = await (from a in db.Queryable<Author>()
                    where a.Name.Contains("Eckhart")
                    select a).FirstOrDefaultAsync();

Forward relationship access

Every Many<T> property gives you access to an IQueryable of child entities.

var authors = from a in book.Authors.ChildrenQueryable()
              select a;

This .ChildrenQueryable() is an already filtered IQueryable of child entities. For ex: the above .ChildrenQueryable() is limited to only the Authors of that particular Book entity. It does not give you access to all of the Author entities in the Authors collection.

Reverse relationship access

For example, if you'd like to get all the books belonging to a genre, you can do it with the help of .ParentsQueryable() like so:

var books = book.Genres.ParentsQueryable("GenreID");

It is basically a convenience method instead of having to do a manual join like the one shown below in order to access parents of one-to-many or many-to-many relationships.

Relationship joins

Many<T>.JoinQueryable() gives you access to all the join records of that particular entity relationship. A join record has two properties ParentID and ChildID that you can use to gain access to parent Entities like so:

//LINQ syntax
var allBooksOfAnAuthor = await (from j in book.Authors.JoinQueryable()
                                where j.ChildID == "AuthorID"
                                join b in db.Collection<Book>() on j.ParentID equals b.ID
                                select b
                               ).ToListAsync();

//Lambda syntax
var allBooksOfAnAuthor = await book.Authors
                                   .JoinQueryable()
                                   .Where(j => j.ChildID == "AuthorID")
                                   .Join(
                                       db.Collection<Book>(), //foreign collection
                                       j => j.ParentID,       //local ID
                                       b => b.ID,             //foreign ID
                                       (j, b) => b)           //result selector expression
                                   .ToListAsync();

In cases where you don't have access to an instance of the parent entity and only have the ID of the parent, you can access the join records like so:

DB.Entity<Book>("Book ID").Authors.JoinQueryable();

And when you need to access all the join records for a given entity relationship, you can access the join records like so:

DB.Entity<Book>().Authors.JoinQueryable();

Counting children

You can get how many entities are there in the opposite side of any relationship as shown below:

var authorCount = await book.Authors.ChildrenCountAsync();
var bookCount = await author.Books.ChildrenCountAsync();