EF Core Include（）在许多关系中

本文介绍了EF Core Include（）在许多关系中的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

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使用EF Core，我们将这个关系与第三个实体的两个一对多关系分开： ProductCustomer

public partial class ProductCustomer { public long ProductId {get;组; } public long CustomerId {get;组; } public virtual Customer Customer {get;组; } public virtual Product Product {get;组; } public virtual ICollection< UsageRecord> UsageRecord {get;组; } }

UsageRecord 是包含某个客户在使用产品时使用的数据量

public partial class UsageRecord { public long Id {get;组; } public long ProductId {get;组; } public long CustomerId {get;组; } public decimal数量{get;组; } public virtual ProductCustomer ProductCustomer {get;组; } }

现在，如果我尝试阅读具体的 UsageRecord ， ProductCustomer 对象是 null （完美，我正在使用热切的加载方式）

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

但是，如果我特意要求Include（）产品客户端实体，实体框架不仅包括所有递归引用，还包括 Product 对象和不是客户！

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，如果我专门要求 ProductCustomer 一个。

第二件事：为什么产品而不是客户？！

我将包含完整性的上下文模型：

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。 .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）; }）; }

解决方案

EF Core 部分中的提示 / core / querying / related-datarel =nofollow noreferrer>文档（突出显示是我的）：

实体Framework Core将自动将导航属性修复到之前加载到上下文实例中的任何其他实体。 因此，即使您没有明确地包含导航属性的数据，如果先前加载了一些或所有相关实体，则该属性仍可能被填充。

The relation between Product and Customer is of type many-to-many (from a design point a view).

Using EF Core, we split this relation in two one-to-many relations with a third entity: 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 is a list of Records containing the quantity of data used by a certain customer while he is using a product

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; } }

Now, if i try to read a specific UsageRecord, the ProductCustomer object is null (perfect, i am using an eager loading approach)

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

But if i specifically ask to Include() the ProductCustomer entity, the entity framwork, not only includes all the recursive references but also includes the Product object and NOT the Customer!

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

First thing: I don't understand why it is including the whole chain of objects if i specifically ask just for the ProductCustomer one.

Second thing: Why the Product and NOT the Customer?!

I include for completeness the Context model:

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"); }); }

解决方案

Basically the answer is provided by the following Tip in the Loading Related Data - Eager loading section of the EF Core documentation (highlight is mine):

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.