如何使M：N（多对多）关系，其中M和N是相同的实体？

Question

I am having difficulties with Entity Framework. I am trying to set up datamodel with entity refering to its opposite via a connection table.

I have created an entity Style and connection table StyleXStyle.

[Table("Styles")]
public class Style : FullAuditedEntity
{
    public virtual string Name { get; set; }

    public virtual string ShortName { get; set; }

    public List<StyleXStyle> Opposites { get; set; } = new List<StyleXStyle>();
}

[Table("StylesXStyles")]
public class StyleXStyle: FullAuditedEntity
{
    public virtual int StyleId { get; set; }
    public Style Style { get; set; }

    public virtual int OppositeId { get; set; }
    public Style Opposite { get; set; }
}

When I try to add database migration, I got this error:

Unable to determine the relationship represented by navigation property 'Style.Opposites' of type 'List<StyleXStyle>'. Either manually configure the relationship, or ignore this property using the '[NotMapped]' attribute or by using 'EntityTypeBuilder.Ignore' in 'OnModelCreating'.

Answer 1

Since you need to self-refer to a collection of Style entities you should have 2 collection properties for this many-to-many mapping. One to navigate to your opposites and one to "alikes". For my examples I am including a second list property called Alikes .

With that you can include a custom mapping to tell EF how these are related:

Using attribute-based config:

public class Style : FullAuditedEntity
{   
    [InverseProperty("Style")] 
    public List<StyleXStyle> Alikes { get; set; } = new List<StyleXStyle>();
    [InverseProperty("Opposite")]
    public List<StyleXStyle> Opposites { get; set; } = new List<StyleXStyle>();
}

public class StyleXStyle : FullAuditedEntity
{
    [ForeignKey("Opposite")]
    public virtual int OppositeId { get; set; }
    public Style Opposite { get; set; }

    [ForeignKey("Style")]
    public virtual int StyleId { get; set; }
    public Style Style { get; set; }
}

Using Fluent API:

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    // configures one-to-many relationship
    modelBuilder.Entity<StyleXStyle>()
        .HasRequired<Style>(s => s.Style)
        .WithMany(g => g.Alikes)
        .HasForeignKey<int>(s => s.StyleId);          

    // configures one-to-many relationship
    modelBuilder.Entity<StyleXStyle>()
        .HasRequired<Style>(s => s.Opposite)
        .WithMany(g => g.Opposites)
        .HasForeignKey<int>(s => s.OppositeId);          
}

The above performs the mapping on the joining entity/table so you can use that as your many-to-many relation. This is necessary if you need to track any information on the relation itself (for example, CreatedDate, CreatedBy, RelationDate, etc.), however, EF provides its own way to map this by convention IF you don't require additional information on the relation entity.

In the latter case, you can map your entities directly to each other and omit the relation table altogether. Here is a quick example:

[Table("Styles")]
public class Style : FullAuditedEntity
{
    public virtual string Name { get; set; }

    public virtual string ShortName { get; set; }

    public ICollection<Style> Opposites { get; set; }
    public ICollection<Style> Alikes { get; set; }
}

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{

    modelBuilder.Entity<Style>()
                .HasMany<Style>(s => s.Alikes)
                .WithMany(c => c.Opposites)
                .Map(cs =>
                        {
                            cs.MapLeftKey("OppositeId");
                            cs.MapRightKey("StyleId");
                            cs.ToTable("StyleXStyle");
                        });

}

The code above was put together for your example, I did not fully test it but have implemented this the same way before.

HTH