How to use EF Core when two properties in the Principle entity point to the same attribute in the dependent entity?

Question

I have 2 Model classes:

---

Department.cs

cs
public class Department
{
    public string DepartmentId { get; set; }

    public string Name { get; set; }

    public string Description { get; set; }

    [ForeignKey("FacultyId")]
    public IList<Faculty> Faculties{ get; set; }

    [ForeignKey("FacultyId")]
    public Faculty HOD { get; set; }

}

---

Faculty.cs

cs
public class Faculty
{
    public string FacultyId { get; set; }

    public string FacultyName { get; set; }

    public FacultyStatus FacultyStatus { get; set; }

    public Designation Designation { get; set; }

    [DataType(DataType.EmailAddress)]
    public string Email { get; set; }

    [DataType(DataType.PhoneNumber)]
    public string PhoneNumber { get; set; }

    public string Address { get; set; }

    public int Experience { get; set; }

    public string Qualifications { get; set; }

    public string Description { get; set; }

    public Department Department{ get; set; }

}

---

I am having trouble determining how to set the HOD for the department. What I thought of was, making the HOD column a foreign key, but I don't know how good or useful that would be.
I referred this link to learn, but I am unable to apply the same

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Error:

Unable to determine the relationship represented by navigation 'Department.Faculties' of type 'ICollection<Faculty>'. Either manually configure the relationship, or ignore this property using the '[NotMapped]' attribute or by using 'EntityTypeBuilder.Ignore' in 'OnModelCreating'.

Answer 1

Hi @Uchchay Dugal ,

From your code, you want to configure two entities (Department and Faculty) with more than one relationship: One-to-One and One-to-Many. If that is the case, you could consider using the InverseProperty attribute.

For example:

public class Department  
{  
    public string DepartmentId { get; set; }  
    public string Name { get; set; }  
    public string Description { get; set; }  

    [InverseProperty("FacultyDepartment")]  
    public IList<Faculty> Faculties { get; set; } //navigation property, One-to-Many relationship  

    [InverseProperty("HODDepartment")]  
    public Faculty HOD { get; set; }        //navigation property, One-to-One relationship  
}  
public class Faculty  
{  
    [ForeignKey("HODDepartment")]  
    public string FacultyId { get; set; }   // this property is the primary key, and the one-to-one relationship foreign key. You can also add a new Primary key.  

    public string FacultyName { get; set; }  
       
       
    [DataType(DataType.EmailAddress)]  
    public string Email { get; set; }  
    [DataType(DataType.PhoneNumber)]  
    public string PhoneNumber { get; set; }  
    public string Address { get; set; }  

    public int Experience { get; set; }  

    public string Qualifications { get; set; }  

    public string Description { get; set; }  

    public Department HODDepartment { get; set; }  // one-to-one relationship, navigation property.  

    [ForeignKey("FacultyDepartment")]  
    public string FacultyDepartmentId { get; set; }  //one-to-many relationship foreign key.  
    public Department FacultyDepartment { get; set; } //one to-many relationship , navigation property.  
}  

Then, after migration, the result as below:

Update:

HOD (this should be the Id of the Faculty who is the HOD)

To add the HodId in the DepartMent table, based on the previous sample code, you can add a foreign key in the Departments class:

public class Department  
{  
    public string DepartmentId { get; set; }  
    public string Name { get; set; }  
    public string Description { get; set; }  
    [InverseProperty("FacultyDepartment")]  
    public IList<Faculty> Faculties { get; set; }  
  
    [ForeignKey("HOD")]  
    public string HODId { get; set; }  
    public Faculty HOD { get; set; }  
}  

The Faculty class as below:

public class Faculty  
{  
    [ForeignKey("HODDepartment")]  
    public string FacultyId { get; set; }  

    public string FacultyName { get; set; }  
       
       
    [DataType(DataType.EmailAddress)]  
    public string Email { get; set; }  
    [DataType(DataType.PhoneNumber)]  
    public string PhoneNumber { get; set; }  
    public string Address { get; set; }  

    public int Experience { get; set; }  

    public string Qualifications { get; set; }  

    public string Description { get; set; }  
    public Department HODDepartment { get; set; }  

    [ForeignKey("FacultyDepartment")]  
    public string FacultyDepartmentId { get; set; }  
    public Department FacultyDepartment { get; set; }  
}  

Then, after migration, the table as below:

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Best regards,
Dillion