c# - 多对多关系中的 EF Core Include()

Question

Product 和Customer 之间的关系是多对多类型的(从设计的角度来看)。

使用 EF Core，我们将此关系拆分为两个与第三方实体的一对多关系:ProductCustomer

public partial class ProductCustomer
{
    public long ProductId { get; set; }
    public long CustomerId { get; set; }   
    public virtual Customer Customer { get; set; }
    public virtual Product Product { get; set; }

    public virtual ICollection<UsageRecord> UsageRecord { get; set; }
}

UsageRecord 是包含特定客户在使用产品时使用的数据量的记录列表

public partial class UsageRecord
{
     public long Id { get; set; }
     public long ProductId { get; set; }
     public long CustomerId { get; set; }           
     public decimal Quantity { get; set; }                
     public virtual ProductCustomer ProductCustomer { get; set; }                
}

现在，如果我尝试读取特定的 UsageRecord，ProductCustomer 对象为 null(完美，我正在使用预加载方法)

return _usageRecordEntity.Where(x => x.ProductId == productId).AsEnumerable();

但是，如果我特别要求对 ProductCustomer 实体进行 Include()， Entity Framework 不仅包括所有递归引用，还包括 Product 对象而不是客户!

return _usageRecordEntity.Where(x => x.ProductId == productId).Include(p => p.ProductCustomer).AsEnumerable();

第一件事:我不明白为什么它包括整个对象链如果我专门询问 ProductCustomer 一个。

第二件事:为什么是产品而不是客户？!

---

为了完整起见，我包括了 Context 模型:

 protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Customer>(entity =>
        {
            entity.Property(e => e.customerId)
                .IsRequired()
                .HasColumnName("CustomerId")
                .HasMaxLength(255);
        });

        modelBuilder.Entity<Product>(entity =>
        {
            entity.Property(e => e.Name)
                .IsRequired()
                .HasMaxLength(50);
        });

        modelBuilder.Entity<ProductCustomer>(entity =>
        {
            entity.HasKey(e => new { e.ProductId, e.CustomerId })
                .HasName("PK__ProductCustomerComposite");

            entity.HasOne(d => d.Customer)
                .WithMany(p => p.ProductCustomer)
                .HasForeignKey(d => d.CustomerId)
                .OnDelete(DeleteBehavior.Restrict)
                .HasConstraintName("FK__ProductCu__CustomerId");

            entity.HasOne(d => d.Product)
                .WithMany(p => p.ProductCustomer)
                .HasForeignKey(d => d.ProductId)
                .OnDelete(DeleteBehavior.Restrict)
                .HasConstraintName("FK__ProductCu__ProductId");
        });

        modelBuilder.Entity<UsageRecord>(entity =>
        {
            entity.Property(e => e.Quantity)
                .HasColumnType("decimal")
                .HasDefaultValueSql("0");

            entity.HasOne(d => d.ProductCustomer)
                .WithMany(p => p.UsageRecord)
                .HasForeignKey(d => new { d.ProductId, d.CustomerId })
                .OnDelete(DeleteBehavior.Restrict)
                .HasConstraintName("FK_UsageRecordProductcustomer");
        });
    }

Answer 1

基本上，EF Core documentation 的加载相关数据 - 预先加载部分中的以下提示提供了答案。 (突出显示是我的):

Entity Framework Core will automatically fix-up navigation properties to any other entities that were previously loaded into the context instance. So even if you don't explicitly include the data for a navigation property, the property may still be populated if some or all of the related entities were previously loaded.