Entity Framework 主键+外键约束与级连删除 Database First

在SQL中先建几张表：

设置为唯一级连删除：在keymain2与keysubse2之间

这么看有点累， 上个关系图就好理解了：

建立数据实体后代码是这样的：

//------------------------------------------------------------------------------
// <auto-generated>
//     This code was generated from a template.
//
//     Manual changes to this file may cause unexpected behavior in your application.
//     Manual changes to this file will be overwritten if the code is regenerated.
// </auto-generated>
//------------------------------------------------------------------------------

namespace WebAppNet.Models
{
    using System;
    using System.Collections.Generic;
    
    public partial class KeyMain
    {
        [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
        public KeyMain()
        {
            this.KeyMain2 = new HashSet<KeyMain2>();
            this.KeySubsets = new HashSet<KeySubset>();
        }
    
        public System.Guid KeyID { get; set; }
        public Nullable<System.Guid> KeyType { get; set; }
        public string KeyValue { get; set; }
        public string KeyName { get; set; }
    
        public virtual KeyType KeyType1 { get; set; }
        [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
        public virtual ICollection<KeyMain2> KeyMain2 { get; set; }
        [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
        public virtual ICollection<KeySubset> KeySubsets { get; set; }
    }
}

//------------------------------------------------------------------------------
// <auto-generated>
//     This code was generated from a template.
//
//     Manual changes to this file may cause unexpected behavior in your application.
//     Manual changes to this file will be overwritten if the code is regenerated.
// </auto-generated>
//------------------------------------------------------------------------------

namespace WebAppNet.Models
{
    using System;
    using System.Collections.Generic;
    
    public partial class KeyMain2
    {
        [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
        public KeyMain2()
        {
            this.KeySubset2 = new HashSet<KeySubset2>();
        }
    
        public System.Guid Key2ID { get; set; }
        public Nullable<System.Guid> KeyID { get; set; }
        public string Key2Name { get; set; }
    
        public virtual KeyMain KeyMain { get; set; }
        [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
        public virtual ICollection<KeySubset2> KeySubset2 { get; set; }
    }
}

//------------------------------------------------------------------------------
// <auto-generated>
//     This code was generated from a template.
//
//     Manual changes to this file may cause unexpected behavior in your application.
//     Manual changes to this file will be overwritten if the code is regenerated.
// </auto-generated>
//------------------------------------------------------------------------------

namespace WebAppNet.Models
{
    using System;
    using System.Collections.Generic;
    
    public partial class KeySubset
    {
        [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
        public KeySubset()
        {
            this.KeySubset2 = new HashSet<KeySubset2>();
        }
    
        public System.Guid KeySubsetID { get; set; }
        public Nullable<System.Guid> KeyID { get; set; }
        public string KeySubsetName { get; set; }
    
        public virtual KeyMain KeyMain { get; set; }
        [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
        public virtual ICollection<KeySubset2> KeySubset2 { get; set; }
    }
}

//------------------------------------------------------------------------------
// <auto-generated>
//     This code was generated from a template.
//
//     Manual changes to this file may cause unexpected behavior in your application.
//     Manual changes to this file will be overwritten if the code is regenerated.
// </auto-generated>
//------------------------------------------------------------------------------

namespace WebAppNet.Models
{
    using System;
    using System.Collections.Generic;
    
    public partial class KeySubset2
    {
        public System.Guid KeySubset2ID { get; set; }
        public System.Guid KeySubsetID { get; set; }
        public string KeySubset2name { get; set; }
        public Nullable<System.Guid> KeyMain2ID { get; set; }
    
        public virtual KeyMain2 KeyMain2 { get; set; }
        public virtual KeySubset KeySubset { get; set; }
    }
}

 增改查都没什么好说的，我们来看一下设置了为唯一级连删除的地方：在keymain2与keysubse2之间

 这两张表中有各有一条数据，删除子表没什么好说的。

我们来看一下删除主表，当删除主表时：

点击删除，删除成功。同时可以看到子表数据也一起删除了：

 

我们在来删除一下没有设置级连删除的主表：

删除主表报错。

我们来分析一下两种做法优缺点：

1.设置了级连删除

   优点：删除主表时很方便的同时删除了子表，不需要写过多的代码，逻辑也是自动处理的。

   缺点：可能会不小心删除很多关连的数据，造成数据的误删。

2.未设置了级连删除（数据库默认）

   优点：删除逻辑清楚，删除主表时，子表有数据不能直接删除主表【因为会报错】。需要自己手动写逻辑，容易理清数据之间的关系。

   缺点：所有删除逻辑都需要自己手写。

我们通过项目中的需求和实际情况，来考虑如何使用这些功能。