EF core and creating a many to many table. Creates extra field. Why ? - ef-code-first

Question

Why is there a UserProgramRefProgramCharacteristics.RefProgramCharacteristicsId field??? There should only be 2 fields not 3. Right? Below are the 3 classes and the OnModelCreating that is needed to create a many to many table
public class RefProgramCharacteristic
{
public int Id { get; set; }
public string ProgramCharacteristic { get; set; }
public List<UserProgramRefProgramCharacteristic> UserProgramRefProgramCharacteristics { get; set; }
// public ICollection<UserProgram> userPrograms { get; } = new List<UserProgram>();
// public virtual ICollection<UserProgram> UserPrograms { get; set; }
}
public class UserProgram
{
public int Id { get; set; }
//UserProgramSaved
public bool MyList { get; set; }
public float MyPriorityRating { get; set; }
public int Similarity { get; set; }
public bool Compare { get; set; }
//UserProgramSimilarity
public int OverallSimilarityScore { get; set; }
public int DeltaProfileElement1_WorkExp { get; set; }
public int DeltaProfileElement2_VolExp { get; set; }
public int DeltaProfileElement3_ResExp { get; set; }
public int DeltaProfileElement4_Pubs { get; set; }
public int DeltaProfileElement5_Step1 { get; set; }
public int DeltaProfileElement6_Step2ck { get; set; }
public int DeltaProfileElement7_Aoa { get; set; }
public int DeltaProfileElement8_Nspecialties { get; set; }
public int DeltaProfileElement9_PercentApps { get; set; }
//UserComparisonSaved
// public RefProgramCharacteristic RefProgramCharacteristic { get; set; }
public string RefProgramCharacteristicList { get; set; }
public string ApplicationUserId { get; set; }
public ApplicationUser ApplicationUser { get; set; }
public int MedicalProgramId { get; set; }
public RefProgramDetailData MedicalProgram { get; set; }
public List<UserProgramRefProgramCharacteristic> UserProgramRefProgramCharacteristics { get; set; }
// public ICollection<RefProgramCharacteristic> RefProgramCharacteristics { get; } = new List<RefProgramCharacteristic>();
// public virtual ICollection<RefProgramCharacteristic> RefProgramCharacteristics { get; set; }
}
public class UserProgramRefProgramCharacteristic
{
// public int Id { get; set; }
public int UserProgramId { get; set; }
public UserProgram UserProgram { get; set; }
public int RefProgramCharacteristicsId { get; set; }
public RefProgramCharacteristic RefProgramCharacteristic { get; set; }
}
protected override void OnModelCreating(ModelBuilder builder)
{
builder.Entity<UserProgramRefProgramCharacteristic>()
.HasKey(t => new { t.UserProgramId, t.RefProgramCharacteristicsId });
base.OnModelCreating(builder);
}

Answer 1

Why is there a UserProgramRefProgramCharacteristics.RefProgramCharacteristicsId field?
Because you are telling EF Core to create such field here:
public int RefProgramCharacteristicsId { get; set; }
// ^
While the navigation property is called RefProgramCharacteristic (no s). And by EF Core conventions:
If the dependent entity contains a property named <primary key property name>, <navigation property name><primary key property name>, or <principal entity name><primary key property name> then it will be configured as the foreign key.
RefProgramCharacteristicsId does not match any of these rules, so EF Core creates a shadow FK property with default name RefProgramCharacteristicId.
Either rename the property to RefProgramCharacteristicId (best), or map it explicitly using ForeignKey data annotation:
[ForeignKey(nameof(RefProgramCharacteristicsId))]
public RefProgramCharacteristic RefProgramCharacteristic { get; set; }
or
[ForeignKey(nameof(RefProgramCharacteristic))]
public int RefProgramCharacteristicsId { get; set; }
or using HasForeignKey fluent API:
builder.Entity<UserProgramRefProgramCharacteristic>()
.HasOne(e => e.RefProgramCharacteristic)
.WithMany(e => e.UserProgramRefProgramCharacteristics)
.HasForeignKey(e => e.RefProgramCharacteristicsId);