Marten is a .NET document database library that uses an underlying PostgreSQL database to store objects as JSON.

In the  previous article (Enforcing Referential Integrity Between Documents with Marten and PostgreSQL) we saw how we can enable referential integrity between documents and the example showed a Customer document may have many related Order documents.

When creating LINQ queries with Marten, one feature is the idea of included documents. So for example we might want to get a Customer document and all the Order documents for that customer.

We could write some code as follows to get Customer 4001 and a list of their Orders:

// 2 trips to database, 2 queries
using (var session = store.QuerySession())
{
    Customer customer = session.Query<Customer>()
                               .Single(x => x.Id == 4001);

    Console.WriteLine(customer.Name);

    IEnumerable<Order> orders = session.Query<Order>()
                                       .Where(x => x.CustomerId == 4001);

    foreach (var order in orders)
    {
        Console.WriteLine($" Order {order.Id} for {order.Quantity} items");
    }
}

The preceding code however will result in two round trips to the database which may result in a performance problem:

select d.data, d.id, d.mt_version 
from public.mt_doc_customer as d 
where d.id = 4001 LIMIT 2

select d.data, d.id, d.mt_version
from public.mt_doc_order as d 
where d.customer_id = 4001

The .Include() extension method of Marten allows a single round trip to be made to the database that executes a SQL join and returns both the Customer data and Orders.

The following code shows how to accomplish this, notice that we need to declare an “output” variable to hold the “included” customer.

// 1 trip to database, 1 query
using (var session = store.QuerySession())
{                
    Customer customer = null;

    List<Order> orders = 
         session.Query<Order>()
                .Include<Customer>(joinOnOrder => joinOnOrder.CustomerId, includedCustomer => customer = includedCustomer)
                .Where(x => x.CustomerId == 4001)
                .ToList();


    Console.WriteLine(customer.Name);

    foreach (var order in orders)
    {
        Console.WriteLine($" Order {order.Id} for {order.Quantity} items");
    }  
}

The preceding code now only makes a single request to the database with the following SQL:

select d.data, d.id, d.mt_version, customer_id.data, customer_id.id, customer_id.mt_version 
from public.mt_doc_order as d 
INNER JOIN public.mt_doc_customer as customer_id ON d.customer_id = customer_id.id 
where d.customer_id = 4001

To learn more about the document database features of Marten check out my Pluralsight courses: Getting Started with .NET Document Databases Using Marten and Working with Data and Schemas in Marten.

You can start watching with a Pluralsight free trial.

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