Entity Framework “A relationship multiplicity constraint violation occurred”

问题

I received this error from Entity Framework this morning:

A relationship multiplicity constraint violation occurred: An EntityReference can have no more than one related object, but the query returned more than one related object. This is a non-recoverable error.

The error refers to a very simple One-To-Many relationship between User and Address

public class User
{
    public Guid Id { get; set; }
    public virtual IList<Address> Addresses { get; set; }
} 

public class Address
{
    public Guid Id { get; set; }
    public User User { get; set; }
} 

I'm using Fluent API to configure the relationship between entities, but in this case the relationship seems to simple that at first I didn't specify any particular rule and I let EF "deduce" the relationship:

public class AddressConfiguration : EntityTypeConfiguration<Address>
{
    public AddressConfiguration()
    {
        HasKey(a => a.Id);
        Property(a => a.Id).HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity).IsRequired();
    }
}

public class UserConfiguration : EntityTypeConfiguration<User>
{
    public UserConfiguration()
    {
        HasKey(u => u.Id);
        Property(u => u.Id).HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity).IsRequired();
    }
}

But after receiving the error I tried to specify the following rule in the AddressConfiguration:

public class AddressConfiguration : EntityTypeConfiguration<Address>
{
    public AddressConfiguration()
    {
        HasKey(a => a.Id);
        Property(a => a.Id).HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity).IsRequired();
        HasOptional(x => x.User).WithMany(); // I tried adding this
    }
}

After doing that, I tried generating a new automatic migration, and this is what I obtained:

public partial class AddressFixMigration : DbMigration
{
    public override void Up()
    {
        DropForeignKey("dbo.Addresses", "User_Id", "dbo.Users");
        AddColumn("dbo.Addresses", "User_Id1", c => c.Guid());
        CreateIndex("dbo.Addresses", "User_Id1");
        AddForeignKey("dbo.Addresses", "User_Id1", "dbo.Users", "Id");
    }

    public override void Down()
    {
        DropForeignKey("dbo.Addresses", "User_Id1", "dbo.Users");
        DropIndex("dbo.Addresses", new[] { "User_Id1" });
        DropColumn("dbo.Addresses", "User_Id1");
        AddForeignKey("dbo.Addresses", "User_Id", "dbo.Users", "Id");
    }
}

I found this very odd. The Db seemed ok even before, with the Addresses Table having a Foreign Key "User_Id", but after specifying the one-to-many relationship in the Configuration file EF wants to create a different Foreign Key. Why??

I also tried to specify HasOptional(x => x.User).WithMany().Map(x => x.MapKey("User_Id"));, but in that case when I try to create the automatic migration I receive the following error:

User_Id: Name: Each property name in a type must be unique. Property name 'User_Id' is already defined.

It seems like something is clearly wrong with my DB, but I can't see what and why.

回答1:

There are other techniques, but I prefer to put the FK in explicitly:

public class Address
{
    public Guid Id { get; set; }
    public Int? User_Id { get; set; }
    public User User { get; set; }
} 

Then use this fluent code:

HasOptional(x => x.User).WithMany(x => x.Addresses).HasForeignKey(x => x.User_Id);

回答2:

I realized that I needed to change my Configuration file as it follows:

public class AddressConfiguration : EntityTypeConfiguration<Address>
{
    public AddressConfiguration()
    {
        HasKey(a => a.Id);
        Property(a => a.Id).HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity).IsRequired();
        HasOptional(x => x.User).WithMany(x => x.Addresses).Map(x => x.MapKey("User_Id"));
    }
}

.WithMany() (without parameter) is for when there is no navigation property on the other side of the relationship, while in this case I needed to spacify .WithMany(x => x.Addresses) because User actually contains a list of Addresses.

来源：https://stackoverflow.com/questions/43000646/entity-framework-a-relationship-multiplicity-constraint-violation-occurred