Document Projections Edit on GitHub

Marten has now the capacity to run projection queries, where only specific document properties are retrieved. The projection queries are executed by using the linq IQueryable.Select() method.

Projection Queries

When you wish to retrieve an IEnumerable of a certain document property for example:

[Fact]
public void use_select_in_query_for_one_field()
{
    theSession.Store(new User { FirstName = "Hank" });
    theSession.Store(new User { FirstName = "Bill" });
    theSession.Store(new User { FirstName = "Sam" });
    theSession.Store(new User { FirstName = "Tom" });

    theSession.SaveChanges();

    theSession.Query<User>().OrderBy(x => x.FirstName).Select(x => x.FirstName)
        .ShouldHaveTheSameElementsAs("Bill", "Hank", "Sam", "Tom");
}

When you wish to retrieve certain properties and transform them into another type:

[Fact]
public void use_select_with_multiple_fields_to_other_type()
{
    theSession.Store(new User { FirstName = "Hank", LastName = "Aaron" });
    theSession.Store(new User { FirstName = "Bill", LastName = "Laimbeer" });
    theSession.Store(new User { FirstName = "Sam", LastName = "Mitchell" });
    theSession.Store(new User { FirstName = "Tom", LastName = "Chambers" });

    theSession.SaveChanges();

    var users = theSession.Query<User>().Select(x => new User2 { First = x.FirstName, Last = x.LastName }).ToList();

    users.Count.ShouldBe(4);

    users.Each(x =>
    {
        SpecificationExtensions.ShouldNotBeNull(x.First);
        SpecificationExtensions.ShouldNotBeNull(x.Last);
    });
}

When you wish to retrieve certain properties and transform them into an anonymous type:

[Fact]
public void use_select_to_transform_to_an_anonymous_type()
{
    theSession.Store(new User { FirstName = "Hank" });
    theSession.Store(new User { FirstName = "Bill" });
    theSession.Store(new User { FirstName = "Sam" });
    theSession.Store(new User { FirstName = "Tom" });

    theSession.SaveChanges();

    theSession.Query<User>().OrderBy(x => x.FirstName).Select(x => new { Name = x.FirstName })
        .ToArray()
        .Select(x => x.Name)
        .ShouldHaveTheSameElementsAs("Bill", "Hank", "Sam", "Tom");
}

Marten also allows you to run projection queries on deep (nested) properties:

[Fact]
public void transform_with_deep_properties()
{
    var targets = Target.GenerateRandomData(100).ToArray();

    theStore.BulkInsert(targets);

    var actual = theSession.Query<Target>().Where(x => x.Number == targets[0].Number).Select(x => x.Inner.Number).ToList().Distinct();

    var expected = targets.Where(x => x.Number == targets[0].Number).Select(x => x.Inner.Number).Distinct();

    actual.ShouldHaveTheSameElementsAs(expected);
}

Chaining other Linq Methods

After calling Select, you'd be able to chain other linq methods such as First(), FirstOrDefault(), Single() and so on, like so:

[Fact]
public void use_select_to_another_type_with_first()
{
    theSession.Store(new User { FirstName = "Hank" });
    theSession.Store(new User { FirstName = "Bill" });
    theSession.Store(new User { FirstName = "Sam" });
    theSession.Store(new User { FirstName = "Tom" });

    theSession.SaveChanges();

    theSession.Query<User>().OrderBy(x => x.FirstName).Select(x => new UserName { Name = x.FirstName })
        .FirstOrDefault()
        .Name.ShouldBe("Bill");
}

Async Projections

Marten also supports asynchronously running projection queries. You'd be able to achieve this by simply chaining the asynchronous resolving method you are after. For example:

• ToListAsync()
• FirstAsync()
• SingleOrDefaultAsync()

And so on...