One to One relationship in Entity Framework Code First

Question

I have a parent table as CustomerRequest that has a relationship with Loan table as 1 to 1-0, that means customer request can have 1 or 0 Loan in Loan Table. I have problem in getting a correct relationship in Entity Framework code first.

This is My CustomerRequest Table Script with correct relationship:

CREATE TABLE [Loan].[CustomerRequest]
(
 [Id]               INT IDENTITY(1,1)
,[CorrelationId]    VARCHAR(500)
,[CustomerNumber]   BIGINT
....

 CONSTRAINT PK_CustomerRequest PRIMARY KEY([Id])
)

this is loan table:

 CREATE TABLE [Loan].[Loan]
 (
   [Id]                             INT    IDENTITY(1,1)
   ,[CustomerRequestId]             INT
   .....
 CONSTRAINT PK_Loan PRIMARY KEY([Id])
   ,CONSTRAINT FK_Loan_CustomerRequestId FOREIGN KEY([CustomerRequestId])  REFERENCES [Loan].[CustomerRequest]([Id])
 )

This is my Customer Request Model:

  public class CustomerRequest
{
    public int Id { get; set; }
    public string CorrelationId { get; set; }
    .....        
    public virtual Loan Loan { get; set; }
}

and Loan Model:

  public class Loan
   {
    public int Id { get; set; }
    public int CustomerRequestId { get; set; }
    ....
    public virtual CustomerRequest CustomerRequest { get; set; }
      }

I have this relationship in CustomerRequestMap:

    HasKey(t => t.Id).HasOptional(t => t.Loan).WithRequired(t => t.CustomerRequest);

I get this error when I try to insert to Loan table:

 {"Cannot insert explicit value for identity column in table 'Loan' when IDENTITY_INSERT is set to OFF."}

Answer 1

You have to modify your entities and configurations a bit like this.

public class CustomerRequest
{
    public int Id { get; set; }
    public string CorrelationId { get; set; }
    public virtual Loan Loan { get; set; }
}

public class CustomerRequestMap : EntityTypeConfiguration<CustomerRequest>
{
    public CustomerRequestMap()
    {
        Property(x => x.Id)
            .IsRequired()
            .HasDatabaseGeneratedOption(System.ComponentModel.DataAnnotations.Schema.DatabaseGeneratedOption.Identity);
    }
}

public class Loan
{
    //public int Id { get; set; }
    [Key, ForeignKey("CustomerRequest")]
    public int CustomerRequestId { get; set; }
    public virtual CustomerRequest CustomerRequest { get; set; }
}

public class LoanMap : EntityTypeConfiguration<Loan>
{
    public LoanMap()
    {
        HasRequired(m => m.CustomerRequest)
        .WithOptional(m => m.Loan)
        ;
    }
}

With this configuration you can have 1 - 0 or 1 relation but you have to modify your schema script.

You can't have Id as primary key and CustomerRequestId as foreign key, EF does not allow this in 1-0 or 1 relation. And it's a wrong design from schema point of view too.

Imagine the situation when you have CustomerRequest with Id=1 . Then you insert two Loan entry with CustomerRequestId=1 but with different Id primary key. How will you query the two Loan entry from EF model in 1-0 or 1 relation?

If the schema is fix and you can't modify it then you must go with 1-many relation.