How can NodaTime be used with EF Code First?

问题:

I really want to be able to use NodaTime in my Entity Framework Code First database projects but haven't found a "clean" way to do it. What I really want to do is this:

public class Photoshoot
{
    public Guid PhotoshootId{get; set;}
    public LocalDate ShootDate{get; set;} //ef ignores this property
}

Is there any supported or recommended approach to using NodaTime with EF Code First?

回答1:

Until custom primitive type persistence is natively supported in Entity Framework, a common work around is to use buddy properties.

For each custom primitive within your domain model, you create an associated mapped primitive to hold the value in a format supported by Entity Framework. The custom primitive properties are then calculated from the value of their corresponding buddy property.

For example:

public class Photoshoot
{
    // mapped
    public Guid PhotoshootId{get; set;}

    // mapped buddy property to ShootDate
    public DateTime ShootDateValue { get; set; }

    // non-mapped domain properties
    public LocalDate ShootDate 
    {
        get { // calculate from buddy property }
        set { // set the buddy property }
    }
}

We use NodaTime in our code first POCO's using exactly this approach.

Obviously this leaves you with a single type acting as both a code first POCO and a domain type. This can be improved at the expense of complexity by separating out the different responsibilities into two types and mapping between them. A half-way alternative is to push the domain properties into a subtype and make all mapped buddy properties protected. With a certain amount of wanging Entity Framework can be made to map to protected properties.

This rather splendid blog post evaluates Entity Framework support for various domain modelling constructs including encapsulated primitives. This is where I initially found the concept of buddy properties when setting up our POCO's: http://lostechies.com/jimmybogard/2014/04/29/domain-modeling-with-entity-framework-scorecard/

A further blog post in that series discusses mapping to protected properties: http://lostechies.com/jimmybogard/2014/05/09/missing-ef-feature-workarounds-encapsulated-collections/

回答2:

No "clean" way that I'm aware of because EF, as of this writing, doesn't have a mechanism for simple type conversion like you see in NHibernate (IUserType). A real limitation in EF as an ORM which causes me to change my domain to suit my ORM.

回答3:

EF Core 2.1 has a new feature Value Conversions, which is exactly for this scenario.

//OnModelCreating
builder.Entity<MyEntity>
       .Property(e => e.SomeInstant)
       .HasConversion(v => v.ToDateTimeOffset(), v => Instant.FromDateTimeOffset(v));

.HasConversion has some other overloads to make this logic re-useable, for example you can define your own ValueConverter.