Entity Framework Core Query Types

This feature was added in EF Core 2.1

Query types are non-entity types (classes) that form part of the conceptual model and can be mapped to tables and views that don't have an identity column specified, or to a DbQuery type. As such, they can be used in a number of scenarios:

  • They can represent ad hoc types returned from FromSql method calls
  • They enable mapping to views
  • They enable mapping to tables that do not have an Identity column

Importantly, since they do not need to have a key value specified, query types do not participate in change tracking and cannot take part in Add, Update or Delete operations. They essentially represent read-only objects.

Mapping to DbQuery

The DbQuery type was introduced in EF Core 2.1. along with query types. A DbQuery is a property on the DbContext that acts in a similar way to a DbSet, providing a root for LINQ queries. However, it doesn't enable operations that write to the database e.g. Add. The DbQuery maps to a table or view. To illustrate how the DbQuery works, here is a simple model representing customers and their orders:

public class Customer
{
    public int CustomerId { get; set; }
    public string Name { get; set; }
    public ICollection<Order> Orders { get; set; } = new HashSet<Order>();
}

public class Order
{
    public int OrderId { get; set; }
    public DateTime DateCreated { get; set; }
    public int CustomerId { get; set; }
    public Customer Customer { get; set; }
    public ICollection<OrderItem> OrderItems { get; set; } = new HashSet<OrderItem>();
}

public class OrderItem
{
    public int OrderItemId { get; set; }
    public string Item { get; set; }
    public decimal Price { get; set; }
}

In the real world, the model will have a lot more properties and any queries against the respective DbSet objects will return all columns of data. There will be occasions where just a subset of columns are required, as defined in the following database view named OrderHeaders:

create view OrderHeaders as
    select      c.Name as CustomerName, 
                o.DateCreated, 
                sum(oi.Price) as TotalPrice, 
                count(oi.Price) as TotalItems
    from        OrderItems  oi 
                inner join Orders o on oi.OrderId = o.OrderId
                inner join Customers c on o.CustomerId = c.CustomerId
    group by oi.OrderId, c.Name, o.DateCreated

The data returned from calling the view is represented by the following query type:

public class OrderHeader
{
    public string CustomerName { get; set; }
    public DateTime DateCreated { get; set; }
    public int TotalItems { get; set; }
    public decimal TotalPrice { get; set; }
}

The OrderHeader class is mapped to the view via the DbQuery<OrderHeader> property that's added to the DbContext along with the DbSet properties for the orders, orderitems and the customers:

public class SampleContext : DbContext
{
    public DbSet<Order> Orders { get; set; }
    public DbSet<OrderItem> OrderItems { get; set; }
    public DbSet<Customer> Customers { get; set; }

    public DbQuery<OrderHeader> OrderHeaders { get; set; }

    ...
}

This is all that is required to enable querying of data in the same way as if the OrderHeaders property was a DbSet:

var orderHeaders = db.OrderHeaders.ToList();

As with normal DbSet queries, you can also specify filter criteria:

var orderHeaders = db.OrderHeaders.Where(x => x.TotalItems > 15).ToList();

Configuration

If you don't want to clutter up your DbContext with multiple DbQuery properties as well as the DbSet declarations, you have two options. One is to make your DbContext class a partial class and then split the DbQuery declarations into a separate file:

[SampleContext.cs]
public partial class SampleContext : DbContext
{
    public DbSet<Order> Orders { get; set; }
    public DbSet<OrderItem> OrderItems { get; set; }
    public DbSet<Customer> Customers { get; set; }

    ...
}
[SampleContextDbQuery.cs]
public partial class SampleContext : DbContext
{
    public DbQuery<OrderHeader> OrderHeaders { get; set; }
    public DbQuery<OrderTotal> OrderTotals { get; set; }
    ...
}

The other option is to use configuration to include the query objects in the conceptual model using the ModelBuilder.Query method:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Query<OrderHeader>().ToView("OrderHeaders");
}

Now there is no DbQuery type object to base your query from, so you use the DbContext.Query<TEntity> method instead:

var orderHeaders = db.Query<OrderHeader>().ToList();

Relationships

It is possible for query types to take part in relationships as a navigational property, but the query type cannot form the principal in the relationship. The key property of the principal end of the relationship needs to be included in the table or view represented by the query type so that a SQL join can be made:

select      c.Name as CustomerName, 
            c.CustomerId,
            o.DateCreated, 
            sum(oi.Price) as TotalPrice, 
            count(oi.Price) as TotalItems
from        OrderItems  oi 
            inner join Orders o on oi.OrderId = o.OrderId
            inner join Customers c on o.CustomerId = c.CustomerId
group by oi.OrderId, c.Name, o.DateCreated

Then the Customer entity must be included as a navigational property:

public class OrderHeader
{
    public string CustomerName { get; set; }
    public DateTime DateCreated { get; set; }
    public int TotalItems { get; set; }
    public decimal TotalPrice { get; set; }
    public Customer Customer { get; set; }
}

This is enough to enable the Include method to eager load the principal entity:

var orderHeaders = db.Query<OrderHeader>().Include(o => o.Customer).ToList();

FromSql

You can use a query type to return non-entity types from FromSql method calls more efficiently than the previous approach that required you to query an entity type and then project a subset of the properties to a different type.

The query type must be registered as a DbQuery and you must include all the columns in the table or view:

var orderHeaders = db.OrderHeaders.FromSql("select * from OrderHeaders");

The is no particular benefit to taking this approach with such as simple query. However, it is likely to be more useful if the SQL generated by EF Core is not efficient enough for your needs.

Last updated: 19/08/2018 08:48:01

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