Entity mapping issue - asp.net

Question

When post gets triggered, it does not save user first and the last name. I have a user class, which inherits from identityuser class and i have declared two properties first and last name as in the code snippet. But problem is when I add the post to the repo it does not save user name rather it shows null value, which throws an exception.
I don't understand how to map, so that user name gets saved in the database.
public abstract class AuditableEntity
{
[Required]
public string Title { get; set; }
[Required]
public string Description { get; set; }
public DateTime PostCreatedDate { get; set; }
[Required]
public User User { get; set; }
}
public class TechPost : AuditableEntity
{
[Key]
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public Guid Id { get; set; }
}
public class User : IdentityUser
{
[Required]
public string FirstName { get; set; }
[Required]
public string LastName { get; set; }
}
public class TechPostCreation
{
public string FirstName { get; set; }
public string LastName { get; set; }
public string Title { get; set; }
public DateTime PostCreatedDate { get; set; }
public string Description { get; set; }
}
[HttpPost()]
public IActionResult CreatePost([FromBody] TechPostCreation techCreatePost)
{
if (techCreatePost == null)
{
return BadRequest();
}
var techPostEntity = _mapper.Map<TechPost>(techCreatePost);
_repo.AddTechBlogPost(techPostEntity);
if (!_repo.Save())
{
return StatusCode(500, "A problem happend and could handle your request");
}
var techPostReturn = _mapper.Map<TechPostViewModel>(techPostEntity);
return CreatedAtRoute("GetTechPost", new { id = techPostReturn.Id },
techPostReturn);
}
public RepoMapping()
{
CreateMap<TechPost, TechPostViewModel>()
.ForMember(dest => dest.Name, opt => opt.MapFrom(src =>
$"{src.User.FirstName} {src.User.LastName}"));
CreateMap<TravelPost, TravelPostViewModel>()
.ForMember(dest => dest.Name, opt => opt.MapFrom(src =>
$"{src.User.FirstName} {src.User.LastName}"));
CreateMap<TechPostCreation, TechPost>();
}

Answer 1

This line:
var techPostEntity = _mapper.Map<TechPost>(techCreatePost);
Uses this map:
CreateMap<TechPostCreation, TechPost>();
Since your User class is a child of TechPost, you need a MapFrom clause. You may instead be able to rename the fields to UserFirstName/UserLastName so the convention kicks in.
Change your map:
CreateMap<TechPostCreation, TechPost>()
.ForMember(dest => dest.User.FirstName, opt => opt.MapFrom(src => src.FirstName))
.ForMember(dest => dest.User.LastName, opt => opt.MapFrom(src => src.LastName));
Also, I am not sure what your repo is doing, but you may need to retrieve or create the User with a UserManager at some point in your process.