EntityFramework Core：渴望加載派生類型的導航屬性

Question

我正在使用EntityFramework Core，並試圖急切加載僅存在於某些派生類型（全部都在單個查詢中）的導航屬性。 最好用一個簡單的例子來演示。

假設您有一些數據結構，例如

class Transaction
{
    public Product product { get; set; }
    public DateTime date { get; set; }
}

abstract class Product
{
    public string Name { get; set; }
}

class PhysicalProduct : Product
{
    public Photo photo { get; set; }
}

class Service : Product
{
    public Person provider { get; set; }
}

還有一些DbContext

class MyContext : DbContext
{
    public DbSet<Transaction> Transactions;
}

如何查詢MyContext.Transactions以返回所有事務，並包括（急於加載）Transaction.product.photo（如果產品為PhysicalProduct）和Transaction.product.provider（如果產品為Service）？ 如前所述，嘗試僅通過一個查詢來實現這一目標。

我嘗試了以下方法：

// This is conceptually what I want to achieve.
// Not very surprisingly, this will throw an InvalidCastException
Transactions
  .Include(x => ((PhysicalProduct)x.product).photo)
  .Include(x => ((Service)x.product).provider)
  .ToList();

// Based on http://stackoverflow.com/questions/7635152/entity-framework-eager-loading-of-subclass-related-objects
// Projection into an anonymous type, then transform back.
// doesn't work though, throws an InvalidOperationException, e.g.
// The property "photo" on entity type "Product" could not be found. Ensure that the property exists and has been included in the model.
// i.e. even though I wrapped this in a condition (x.product is PhysicalProduct), seems like EntityFramework still tries to execute or parse the statement thereafter even if the condition is not true.
var query = Transactions.Select(x => new
{
  _transaction = x,
  _physicalProductPhoto = (x.product is PhysicalProduct) ? ((PhysicalProduct)x.product).photo : null;
  _serviceProvider = (x.product is Service) ? ((Service)x.product).provider : null;
})
.ToList()  // Execute query. Exception will be thrown at this step.
.Select(x =>
{
  var result = x._transaction;

  if (x.product is PhysicalProduct)
    ((PhysicalProduct)x.product).photo = x._physicalProductPhoto;
  else if(x.product is Service)
    ((Service)x.product).provider = x._serviceProvider;

  return result;
})
.ToList();

誰能想到實現這一目標的方法？ 謝謝！

Answer 1

昨天，我在EF6- EF Eager獲取派生類中遇到了類似問題。 目前，EF Core在這方面並不更好-實際上，情況更糟，因為從3種EF6解決方法中，只有＃2可以在這里工作。

解決方法是：

絕對不能通過單個查詢來完成。 您需要執行主查詢並在內存中具體化結果。 然后，對於每種派生的導航類型，收集PK並執行由這些鍵過濾的查詢。 最后，由於EF導航屬性修復，您將最終加載了所有導航屬性。

var transactions = db.Transactions.Include(e => e.product).ToList();

var productIds = transactions.Where(e => e.product is PhysicalProduct)
    .Select(e => e.product.Id).Distinct();
db.BaseProducts.OfType<PhysicalProduct>().Include(e => e.photo)
    .Where(e => productIds.Contains(e.Id)).Load();

var serviceIds = transactions.Where(e => e.product is Service)
    .Select(e => e.product.Id).Distinct();
db.BaseProducts.OfType<Service>().Include(e => e.provider)
    .Where(e => serviceIds.Contains(e.Id)).Load();

Answer 2

EF Core尚不支持此功能。 請參閱https://github.com/aspnet/EntityFramework/issues/3910了解跟蹤它的問題。

我相信唯一的解決方法是執行多個查詢，然后讓EF上下文為您完成修復。

Answer 3

我相信從鏈接的文檔中可以得知，通過使用“ ThenInclude”。 您能夠做到這一點。 抱歉，但我自己沒有嘗試過，因此無法驗證它是否有效。

var blogs = context.Blogs
.Include(blog => blog.Posts)
    .ThenInclude(post => post.Author)
    .ThenInclude(author => author.Photo)
.Include(blog => blog.Owner)
    .ThenInclude(owner => owner.Photo)
.ToList();

https://docs.efproject.net/en/latest/querying/related-data.html