Microsoft.EntityFrameworkCore.DbUpdateException - asp.net-core

Question

I have a comment table where it have a two foreign key(i.e Userid and postid). I was trying to insert data into comment table using these two foreign key but was unable to insert. This is my post table
public class Post
{
public int Id { get; set; }
public string PostText { get; set; }
public string Title { get; set; }
public bool Status { get; set; }
public DateTime PostDate { get; set; }
public virtual List<Comment> Comments { get; set; }
public ApplicationUser ApplicationUser { get; set; }
}
And this is my comment table
public class Comment
{
public int Id { get; set; }
public string CommentText { get; set; }
public DateTime CommentTime { get; set; }
public bool Status { get; set; }
public ApplicationUser CommentBy { get; set; }
public Post Posts { get; set; }
}
Comment service
public void Save(Comment comment)
{
_context.Set<Comment>().Add(comment);
_context.SaveChanges();
}
And this is my controller
[HttpPost]
public ObjectResult SaveComment([FromBody] Comment comment)
{
if (ModelState.IsValid)
{
try
{
_commentService.Save(comment);
return Ok("comment saved");
} catch (Exception e)
{
return BadRequest(e);
}
} else
{
return BadRequest("Model is not valid");
}
}
And the error is
{Microsoft.EntityFrameworkCore.DbUpdateException: An error occurred while updating the entries. See the inner exception for details. ---> System.Data.SqlClient.SqlException: Violation of PRIMARY KEY constraint 'PK_AspNetUsers'. Cannot insert duplicate key in object 'dbo.AspNetUsers'. The duplicate key value is (966fc417-8757-4bac-89b2-9975d4f2cd41).
Cannot insert explicit value for identity column in table 'Posts' when IDENTITY_INSERT is set to OFF.
The statement has been terminated.
this is my api request

Answer 1

The reason is that you will also insert a new comment.CommentBy when you're inserting a brand new comment:
// the `comment` here is constructed by model binding, which is a brand new entity
[HttpPost]
public ObjectResult SaveComment([FromBody] Comment comment)
{
// ...
_commentService.Save(comment); // save the brand new comment
// ...
}
"A brand new comment" means this Comment entity and its related propreties are all untracked. When saving a brand new entity, EF Core will also create related entities for you automatically.
To avoid this behavior, you could mark the state of comment.CommentBy as Unchanged,
_context.Entry(comment.CommentBy).State= EntityState.Unchanged;
so that the EF Core will not create a new CommentBy (i.e. an ApplicationUser) for you. But be careful: you must make sure the ApplicationUser already exists.
The same goes Post.
Another approach is much safer. As suggested by #Khai Nguyen in the comment, you should get ApplicationUser and Post instance from database, so that the EF Core knows there's already a ApplicationUser and a Post within database and won't insert new ApplicationUser or Post for you.