asp.net - LINQ Query with relational Data - asp.net

Question

I have two tables Category and Document. See relationships in picture
See picture
I wrote the following query to select data from both tables based on relationship
public List<DocumentViewModel> All()
{
var docs = _context.Document.ToList();
List<DocumentViewModel> docList = docs.Select(x => new DocumentViewModel
{ DocumentId = x.DocumentId,
DocumentPath = x.DocumentPath,
CategoryId = x.CategoryId,
CategoryName = x.Category.CategoryName }).ToList();
return docList;
}
when this function is called , I get the following error
System.NullReferenceException: 'Object reference not set to an instance of an object.'
Here are my modals
document
public class Document
{
[Key]
public int DocumentId { get; set; }
[Required]
public string DocumentPath { get; set; }
public Nullable<int> CategoryId { get; set; }
public virtual Category Category { get; set; }
}
Category
public class Category
{
[Key]
public int CategoryId { get; set; }
[Required]
public string CategoryName { get; set; }
public virtual ICollection<Document> Documents { get; set; }
}
DocumentViewModel
public class DocumentViewModel
{
public int DocumentId { get; set; }
public string DocumentPath { get; set; }
public int? CategoryId { get; set; }
public string CategoryName { get; set; }
}
Any Idea where am doing mistake?

Answer 1

In this case there is no reason to get a List in memory and then do the projection, you can do this directly from EF instead. Even if there is no relationship defined EF will return null for CategoryName if you project the the results. If you go to memory first then an NRE is expected if there is no Category relationship.
public List<DocumentViewModel> All()
{
return _context.Document.Select(x => new DocumentViewModel
{ DocumentId = x.DocumentId,
DocumentPath = x.DocumentPath,
CategoryId = x.CategoryId,
CategoryName = x.Category.CategoryName}).ToList();
}
Original reason why it is failing.
There is at least one entity that does not have a corresponding relationship with Category.
You do not have lazy loading enabled (which is a good thing) and if that is the case you should use Include to return the relationship.