c# - How to insert a record into a table with a foreign key using Entity Framework in ASP.NET MVC -


i'm new entity framework code-first. learning in asp.net mvc, using code-first database creation.

i have 2 classes:

public class student {     public int studentid { get; set; }     public string name { get; set; }     public int standard { get; set; }                 public int subjectid { get; set; }          [foreignkey("subjectid")]     public icollection<subject> subjects { get; set; } }  public class subject {     [key]     public int subjectid{ get; set; }     public string subjectname { get; set; } }

i'm trying insert student record student table, has foreign key subjectid referencing subject table.

i'm trying out in 2 possible ways:

first approach 

using(var cxt = new schoolcontext()) {     subject sub = new subject() { subjectid = 202, subjectname ="geology" };     student stu = new student() { name = "riya", subjectid = 202 };     cxt.subjects.add(sub);     cxt.students.add(stu);                 cxt.savechanges(); }

here created new subject instance, has subjectid=202. when create student object , assign value 202 subjectid, there insert statement conflict. though there subject subjectid = 202, why there insert conflict? , when debug, see navigation property subjects null here. don't understand point here.

second approach:

using( var cxt=new schoolcontext()) {     student stu = new student() { name = "riya" };     subject sub = new subject() { subjectid = 202, subjectname = "geology" };     stu.subjects.add(sub);     cxt.students.add(stu);                     cxt.savechanges(); }

but null reference exception

object reference not set instance of object

why stu.subjects null here?

so questions here are:

  1. what subjectid in student class mean? i.e. value pertain to? can explicitly set it, if yes, refer primary key of subject table? if no, specified ef code conventions purpose?

  2. similarly: navigation property role? why null , when not null?

my basic understanding of navigation property is used make ef determine relationship between 2 entities.

can please clarify bit examples appreciated.

you're creating new student, , new subject, in both approaches. understand, you're trying do, create new student, , assign existing subject (with subjectid = 202) - right??

the subjectid in student class makes absolutely no sense in setup - since have 1:n relationship between student , subject. need use icollection<subject> handle 0:n subjects student enrolled in.

for - use code:

using(var ctx = new schoolcontext()) {     // create *new* student     student stu = new student() { name = "riya" };      // existing subject id=202     subject sub = ctx.subjects.firstordefault(s => s.subjectid == 202);      // add existing subject new student's "subjects" collection     stu.subjects.add(sub);      // add new student context, , save all.     ctx.students.add(stu);                 ctx.savechanges(); }

that - new student inserted database table, , 1:n relationship between student , subjects established.


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