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（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个实体的一对多关系：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 对象是 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 Include() ProductCustomer 实体，实体框架不仅包括所有递归引用，还包括 Product 对象而不是 Customer!

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特别要求 ProductCustomer 一个。

第二件事：为什么是 Product 而不是 Customer？！

为了完整性，我包括上下文模型：

 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，答案由 EF Core 加载相关数据 - 预加载 部分中的以下提示提供documentation（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.

多对多关系中的 EF Core Include()

EF Core Include() in Many to Many relation

c#

entity-framework

lazy-loading

ef-code-first

entity-framework-core