Seed entity with owned property

Thank Ivan Stoev's answer. i add some more code to easy to imagine. this is code of seed data function base on example.

• First adding data of User.
• After that add data of owned object.
• Data of owned object have to be anonymous because PK will request. This PK will not appear in database. Name should be entity name + Id

Example: Entity XXX => PK will be XXXId

private void SeedUser(ModelBuilder builder)
{
    builder.Entity<User>(b =>
    {
        b.HasData(new User
        {
            Id = "37846734-172e-4149-8cec-6f43d1eb3f60",
            Email = "[email protected]",
            UserName = "[email protected]",
            // more properties of User
        });
        b.OwnsOne(e => e.EmailPermissions).HasData(new 
        {
                UserId = "37846734-172e-4149-8cec-6f43d1eb3f60",
                Newsletter = true,
                PromotionalOffers = true,
                PrestationReminders = true,
                PrestationOffers = true
        });
    });
}

If you want to avoid using an anonymous type to specify the shadow property keys, you can declare them explicitly in your model class and configure them with the Fluent API as keys. This way you don't have to guess the property names and it's less error-prone.

If the name supplied to the Property method matches the name of an existing property (a shadow property or one defined on the entity class), then the code will configure that existing property rather than introducing a new shadow property. Source

Currently this information is missing from the documentation (tracked by #710: Document how to seed owned types). It's explained by EF Core team (with example) in the #12004: Problem seeding data that contains owned type thread:

Owned types must be seeded with a HasData call after the OwnsOne call. Also, since owned types by convention have a primary key generated in shadow state, and since seed data requires keys to be defined, then this requires use of an anonymous type and setting the key.

which is basically what the exception message is telling you.

Following the advice, you should remove the instantiation of the EmailPermissions property from the constructor and add a seeding code like this:

builder.Entity<User>().OwnsOne(e => e.EmailPermissions).HasData(
    new
    {
        UserId = "37846734-172e-4149-8cec-6f43d1eb3f60",
        // other properties ...
    }
);

Quite annoying and error prone due to the need to know the shadow PK name and the usage of an anonymous type. As the same member mentioned

Note that this would become easier if navigations were supported for seeding, which is tracked by #10000: Data Seeding: Add support for navigations