Entity Framework - Composite Primary Key formed by 3 Foreign Keys

Question

i'm creating a relation between 4 tables: Provider, CostumerSite, DrmType and Drm.

Relation:

in the Drm table there is a composite key formed by the primary keys of the other 3 tables.. this is the Code:

public partial class Provider
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int ProviderId { get; set; }

    [StringLength(128)]
    [Required]
    public string Title { get; set; }

    // one-to-many
    public virtual ICollection<Content> Contents { get; set; }

    public virtual Drm Drm { get; set; }

}

 public partial class CustomerSite
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int CustomerSiteId { get; set; }

    [Required]
    [StringLength(128)]
    public string Name { get; set; }

    [Required]
    [StringLength(128)]
    public string Username { get; set; }

    [Required]
    [StringLength(128)]
    public string Password { get; set; }

    // many-to-many
    public virtual ICollection<Content> Contents { get; set; }

    // one-to-one
    public virtual Drm Drm { get; set; }
}

 public partial class DrmType
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int DrmTypeId { get; set; }

    [Required]
    [StringLength(128)]
    public string Name { get; set; }

    // one-to-one
    public virtual Drm Drm { get; set; }
}

public partial class Drm
{
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int DrmId { get; set; }

    // one-to-one relation
    [Key, Column(Order = 1), ForeignKey("CustomerSite")]
    public int CustomerSiteId { get; set; }

    // one-to-one relation
    [Key,  Column(Order = 2), ForeignKey("Provider")]
    public int ProviderId { get; set; }

    // one-to-one relation
    [Key,  Column(Order = 3), ForeignKey("DrmType")]
    public int DrmTypeId { get; set; }

    public virtual Provider Provider { get; set; }
    public virtual CustomerSite CustomerSite { get; set; }
    public virtual DrmType DrmType { get; set; }
}

Is this correct? After this, i'm using the onModel function:

protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {

        modelBuilder.Entity<Drm>()
        .HasKey(d => new { d.ProviderId, d.CustomerSiteId, d.DrmType });

    }

But in this way, when i launch the "Enable-Migrations -EnableAutomaticMigrations -Force" command it shows an error:

One or more validation errors were detected during model generation:

Drm_CustomerSite_Source: : Multiplicity is not valid in Role 'Drm_CustomerSite_Source' in relationship 'Drm_CustomerSite'. Because the Dependent Role properties are not the key properties, the upper bound of the multiplicity of the Dependent Role must be ' '. Drm_Provider_Source: : Multiplicity is not valid in Role 'Drm_Provider_Source' in relationship 'Drm_Provider'. Because the Dependent Role properties are not the key properties, the upper bound of the multiplicity of the Dependent Role must be ' '.

Can you explain how to solve this? Thanks

Answer 1

Based on your model, Drm should not have its own "id". So, it should look like this:

public partial class Drm
{
    //remove this property
    //[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    //public int DrmId { get; set; }

    // one-to-one relation
    [Key, Column(Order = 1), ForeignKey("CustomerSite")]
    public int CustomerSiteId { get; set; }

    // one-to-one relation
    [Key,  Column(Order = 2), ForeignKey("Provider")]
    public int ProviderId { get; set; }

    // one-to-one relation
    [Key,  Column(Order = 3), ForeignKey("DrmType")]
    public int DrmTypeId { get; set; }

    public virtual Provider Provider { get; set; }
    public virtual CustomerSite CustomerSite { get; set; }
    public virtual DrmType DrmType { get; set; }
}

Since you provided the [Key] attribute at the class declaration, you do not need to declare HasKey in the model binding. So, remove this line:

modelBuilder.Entity<Drm>()
    .HasKey(d => new { d.ProviderId, d.CustomerSiteId, d.DrmType });

EDIT

Provider , CustomerSite and DrmType must have a collection of Drm .

public partial class Provider
{
    //...
    //public virtual Drm Drm { get; set; }
    public virtual ICollection<Drm> Drms { get; set; }
}

public partial class CustomerSite
{
    //...
    //public virtual Drm Drm { get; set; }
    public virtual ICollection<Drm> Drms { get; set; }
}

public partial class DrmType
{
    //public virtual Drm Drm { get; set; }        
    public virtual ICollection<Drm> Drms { get; set; }
}

Hope this helps!

Answer 2

The easiest way to solve this is by giving the DRM table its own ID and making the 3 fields Foreign keys. I do not know to what extend you are normalising but it is something often done to prevent unnecessary complex code. keep it clean, keep it simple