避免“引入FOREIGN KEY约束可能会导致循环或多个级联路径”

Question

I reproduced a simple example of a problem I have with Entity Framework.

I want to have three tables:

Users, Projects, WorkOrders

Table Users has information about users for all other tables (in example only two). WorkOrders has information about which User has to work on this work order and to which Project it belongs.

Here are the classes:

public class User
{
    [Key]
    public int Id { get; set; }
    public string Name { get; set; }

    public virtual ICollection<WorkOrder> WorkOrders { get; set; }
    public virtual ICollection<Project> Projects { get; set; }
}

public class Project
{
    [Key]
    public int Id { get; set; }
    public string Name { get; set; }
    public int ManagerId { get; set; }
    public DateTime Start { get; set; }

    public virtual User Manager { get; set; }
    public virtual ICollection<WorkOrder> WorkOrders { get; set; }
}

public class WorkOrder
{
    [Key]
    public int Id { get; set; }
    public int Number { get; set; }
    public string Type { get; set; }
    public int AssigneeId { get; set; }
    public int ProjectId { get; set; }

    public virtual Project Project { get; set; }
    public virtual User Assignee { get; set; }
}

When I try to run the program, it throws an exception:

'Introducing FOREIGN KEY constraint 'FK_dbo.WorkOrders_dbo.Projects_ProjectId' on table 'WorkOrders' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints.

Then I went another way. I tried DB-first approach with EF. I first created the tables and connections in SQL Server Management Studio:

Then the generated models by EF look almost the same as mine, with code-first approach.

public partial class User
{
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
    public User()
    {
        this.Project = new HashSet<Project>();
        this.WorkOrder = new HashSet<WorkOrder>();
    }

    public int Id { get; set; }
    public string Name { get; set; }

    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<Project> Project { get; set; }
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<WorkOrder> WorkOrder { get; set; }
}

public partial class Project
{
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
    public Project()
    {
        this.WorkOrder = new HashSet<WorkOrder>();
    }

    public int Id { get; set; }
    public string Name { get; set; }
    public int ManagerId { get; set; }
    public System.DateTime Start { get; set; }

    public virtual User User { get; set; }
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<WorkOrder> WorkOrder { get; set; }
}

public partial class WorkOrder
{
    public int Id { get; set; }
    public int Number { get; set; }
    public string Type { get; set; }
    public int AssigneeId { get; set; }
    public int ProjectId { get; set; }

    public virtual Project Project { get; set; }
    public virtual User User { get; set; }
}

So the code is almost identical, except the SuppressMesages and constructors in WorkOrder and User classes. The second approach works.

I would like to know, what makes the difference? Also the context class is identical as my own. Where or how are defined this FK constraints or cascade delete settings?

Answer 1

It is unfortunate default behavior of EF code first to crate FK with on delete cascade. So while defining relations you need to simply change this setting:

//in context
protected override void OnModelCreating(DbModelBuilder modelBuilder) { 
    modelBuilder.Entity<Project>()
        .HasRequired<User>(s => s.User)
        .WithMany()
        .WillCascadeOnDelete(false);

Also there might be a convention for it in modelBuilder.Conventions if you want to just change behavior for all FK.