Projection Operators

Select()

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

[Fact]
public async Task 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" });

    await theSession.SaveChangesAsync();

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

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

[SerializerTypeTargetedFact(RunFor = SerializerType.Newtonsoft)]
public async Task 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" });

    await theSession.SaveChangesAsync();

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

    users.Count.ShouldBe(4);

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

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

[Fact]
public async Task 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" });

    await theSession.SaveChangesAsync();

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

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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);
}

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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 async Task 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" });

    await theSession.SaveChangesAsync();

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

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SelectMany()

::: top As of Marten V4, you can chain SelectMany() operators N-deep with any possible Where() / OrderBy / Distinct() / etc operators :::

Marten has the ability to use the SelectMany() operator to issue queries against child collections. You can use SelectMany() against primitive collections like so:

[Fact]
public async Task can_do_simple_select_many_against_simple_array()
{
    var product1 = new Product {Tags = new[] {"a", "b", "c"}};
    var product2 = new Product {Tags = new[] {"b", "c", "d"}};
    var product3 = new Product {Tags = new[] {"d", "e", "f"}};

    using (var session = theStore.LightweightSession())
    {
        session.Store(product1, product2, product3);
        await session.SaveChangesAsync();
    }

    using (var query = theStore.QuerySession())
    {
        var distinct = query.Query<Product>().SelectMany(x => x.Tags).Distinct().ToList();

        distinct.OrderBy(x => x).ShouldHaveTheSameElementsAs("a", "b", "c", "d", "e", "f");

        var names = query.Query<Product>().SelectMany(x => x.Tags).ToList();
        names
            .Count().ShouldBe(9);
    }
}

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Or against collections of child documents:

var results = query.Query<Target>()
    .SelectMany(x => x.Children)
    .Where(x => x.Flag)
    .OrderBy(x => x.Id)
    .Skip(20)
    .Take(15)
    .ToList();

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A few notes on the SelectMany() usage and limitations:

• As of 1.2, you are only able to use a single SelectMany() operator in a single Linq query. That limitation will be removed in 1.3.
• You can use any other Linq operator that Marten supports after the SelectMany() in a Linq query, including the Stats() and Include() operators
• Take() and Skip() operators in a Linq query that contains a SelectMany() operator will always apply to the child collection database rather than the parent document regardless of the order in which the operators appear in the Linq query
• You cannot use SelectMany() with both a Distinct() and a Count() operator at this point.