Entity Framework code first relationships and navigation property?

Question

i have the following classes

public class Subject{ public int SubjectId { get; set; } public String SubjectName { get; set; } public String SubjectCategory { get; set; } } public class QuestionDescriptor { public int QuestionDescriptorId { get; set; } public String QuestionText { get; set; } public String Answer { get; set; } public int SubjectId { get; set; } public virtual Subject Subject { get; set; } }

i have configured it using the following code ,i want that a Subject can have many QuestionDescriptors

 modelBuilder.Entity<QuestionDescriptor>() .HasRequired(qd => qd.Subject) .WithMany() .HasForeignKey(qd => qd.SubjectId) .WillCascadeOnDelete(true);

Now i have the following question

have i done it correctly ?
do i need a navigation property in the Subject class?

what happems if i do this

public class Subject { public int SubjectId { get; set; } public String SubjectName { get; set; } public String SubjectCategory { get; set; } public int QuestionDescriptorId {get;set;} public virtual QuestionDescriptor {get;set;} }

if i do the above what changes do i need in the configuration and why?
if i want all the questions belonging to a particular subject then i can get them by querying the QuestionDescriptor ,why then do i need a bi-directional property ?

Slauma · Accepted Answer · 2011-10-01 11:29:37Z

1) have i done it correctly ?

Yes.

2) do i need a navigation property in the Subject class?

No. You don't need it. It can be helpful for certain queries but it is not required.

3) what happems if i do this ...

That's another relationship. It would represent a one-to-one relationship. But because you want a one-to-many relationship you must have a navigation collection on your entity:

public class Subject { public int SubjectId { get; set; } public String SubjectName { get; set; } public String SubjectCategory { get; set; } public virtual ICollection<QuestionDescriptor> Descriptors {get;set;} }

4) if i do the above what changes do i need in the configuration and why?

For the change above you can leave you mapping configuration as it is - with the only exception that you now must specify the collection as the other side of the relationship. Instead of .WithMany() you use

.WithMany(s => s.Descriptors)

5) if i want all the questions belonging to a particular subject then i can get them by querying the QuestionDescriptor ,why then do i need a bi-directional property ?

You don't need it.

In the 3 question if i have just a navigation property ,doesn't that change my table structure in the database ?
@ashutoshraina: In your example in the question with public virtual QuestionDescriptor, yes, it would change the table structure because it's a one-to-one relationship. In my example with public virtual ICollection<QuestionDescriptor> Descriptors it doesn't change the database because the relationship is still the same (one-to-many).
yes, you are right.i tried it out and there was no change in the database structure and it made my querying straightforward.Many thanks for the help.

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