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Relational Database SQL Query in Asp.NET Core

public async Task<List<Note>>ShowAssigned()
{
return await _context.Notes
.Where(x => x.List.OwnerId != x.OwnerId)
.ToListAsync()
}
I get no syntax εrrors, but it seems you can't access attributes from related Data in this way.
Basically the goal is: A user creates a List, then some Notes for this List. Then he should be able to assign one of that Notes to another User. When that other User logs on, he should be able to see that new Note that was assigned to him.
Can anyone help me out with this?
public class List
{
public Guid ListId { get; set; }
public string OwnerId { get; set; }
public List<Note> Notes { get; set; }
}
public class Note
{
public Guid ID { get; set; }
public string OwnerId { get; set; }
[ForeignKey("ListId")]
public Guid ListId { get; set; }
public List List { get; set; }
}
And the context class:
public DbSet<Note> Notes { get; set; }
public DbSet<List> Lists { get; set; }
When i try to access Data the same way in a view like that:
#model List<Project.Models.Note>
#foreach (var item in Model)
{
if (item.List.OwnerId == item.OwnerId)
i get this error when running the web app (no syntax errors):
NullReferenceException: Object reference not set to an instance of an object

First write your model classes as follows:
public class List
{
public Guid ListId { get; set; }
public string OwnerId { get; set; }
public virtual List<Note> Notes { get; set; }
}
public class Note
{
public Guid ID { get; set; }
public string OwnerId { get; set; }
[ForeignKey("List")] // Not ListId, its List
public Guid ListId { get; set; }
public virtual List List { get; set; }
}
If your project is on ASP.NET Core < 2.1
Then write your query as follows:
await _context.Notes.Include(n => n.List).ToListAsync()
If your project is on ASP.NET Core >= 2.1
Then in the ConfigureServices() method in Startup class:
services.AddDbContext<ApplicationDbContext>(options =>
options.UseLazyLoadingProxies().UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
Don't forget to install appropriate version of Microsoft.EntityFrameworkCore.Proxies nuget package because UseLazyLoadingProxies() resides in this package.
Then write your query as follows:
await _context.Notes.ToListAsync()

So I found the answer to my problem, in some parts with the help of TanvirArjel (but i basically did it differently)
public async Task<List<Note>> GetAssignedItemsAsync(ApplicationUser user)
{
var lists = await _context.Lists.Include(l => l.Notes).Where(x => x.OwnerId != user.Id).ToListAsync();
var notesListe = new List<Note>();
foreach (List l in lists)
{
foreach (Note n in l.Notes)
{
if (n.OwnerId == user.Id)
{
notesListe.Add(n);
}
}
}
return notesListe;
}

c# asp.net make a get request on relational table with entity framework database

I am trying to get data from a table with a get request with a controller. When I make the request with a normal table (TestTable) it is ok, but if I make the request with a relational table I get the fail message:
"The 'ObjectContent`1' type failed to serialize the response body for
content type 'application/xml; charset=utf-8'."
My controller (Mdata):
namespace ScThAsp.Controllers
{
public class MDataController : ApiController
{
public IEnumerable<Måledata> Get()
{
using (var e = new SCTHDBEntities())
{
return e.Måledata.ToList();
}
}
public TestTable Get(int id)
{
using (SCTHDBEntities entities = new SCTHDBEntities())
{
return entities.TestTable.FirstOrDefault(e => e.Id == 1);
}
}
}
}
My Table for måledata is:
public partial class Måledata
{
public int MDid { get; set; }
public Nullable<int> BBid { get; set; }
public Nullable<decimal> Måling1 { get; set; }
public Nullable<decimal> Måling2 { get; set; }
public Nullable<decimal> Måling3 { get; set; }
public Nullable<decimal> Måling4 { get; set; }
public Nullable<System.DateTime> RegTid { get; set; }
public virtual BuildBoard BuildBoard { get; set; }
}
My database looks like:
Database
See link..
I think I mayby should make a inner join with the other table connected to Måledata table - I am not sure how to do that in a EF environment.
I have really tried a lot now - hope for an answer. Thanks

Your class Måledata contains more data that you presented (it is marked as partial) and probably contains stuff related to EF. This magic stuff is not serializable. To avoid problem rewrite results to a plain object with properties you need. This object must be serializable (if contains plain properties and classes it will).

Building upon Piotr Stapp's answer you need to create a DTO (Data Transfer Object) for your Måledata which contains properties as your model, Måledata other than the EF properties. Use some sort of Mapper, maybe AutoMapper to map the required properties in your final response.
public class MaledataDTO
{
public int MDid { get; set; }
public int? BBid { get; set; }
public decimal? Måling1 { get; set; }
public decimal? Måling2 { get; set; }
public decimal? Måling3 { get; set; }
public decimal? Måling4 { get; set; }
public DateTime? RegTid { get; set; }
//... other properties
}
public IEnumerable<MaledataDTO> Get()
{
using (var e = new SCTHDBEntities())
{
var result = e.Måledata.ToList();
return Mapper.Map<List<MaledataDTO>>(result);
}
}

I found 2 solutions.
1) Solution was with Automapper (thanks Abdul). Installing automapper and a Using Automapper. Added a class called MåledataDTO : ` public class MåledataDTO
{
public int MDid { get; set; }
public int? BBid { get; set; }
public decimal? Måling1 { get; set; }
public decimal? Måling2 { get; set; }
public decimal? Måling3 { get; set; }
public decimal? Måling4 { get; set; }
public DateTime? RegTid { get; set; }
//... other properties
}
`
In my controller I used the following code
public IEnumerable<MåledataDTO> Get()
{
using (var e = new SCTHDBEntities())
{
Mapper.Initialize(config =>
{
config.CreateMap<Måledata, MåledataDTO>();
});
var result = e.Måledata.ToList();
return Mapper.Map<List<MåledataDTO>>(result);
2: In the second solution: In the picture you see the relations bewtween the tables - made in VS - but that creates a problem in the tables Get SET classes. The relation creates a Virtual object in the class - like mentioned before
public virtual BuildBoard BuildBoard { get; set; }
If you delete the relations and make the public partial class Måledata like
in this video
https://www.youtube.com/watch?v=4Ir4EIqxYXQ
the controller should then have one of two solutions:
using (SCTHDBEntities e = new SCTHDBEntities()) {
//this works
//var knud = new List<Måledata>();
//knud = (e.BuildBoard.Join(e.Måledata, c => c.BBid, o => o.BBid,
// (c, o) => o)).ToList();
//return knud;
//this works too
return (from p in e.BuildBoard
where p.BBid == 1
from r in e.Måledata
where r.BBid == p.BBid
select p).ToList();
That was that
Gorm

EntityFramework: How to load List of objects from database?

I have two entities:
public class Booking
{
[Key]
public int Id { get; set; }
public int RoomId { get; set; }
[ForeignKey("RoomId")]
public Room Room { get; set; }
public DateTime StartDate { get; set; }
public DateTime EndDate { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public string DocumentNumber { get; set; }
public string ContactPhone { get; set; }
}
public class Room
{
[Key]
public int RoomId { get; set; }
public int Number { get; set; }
public int Size { get; set; }
public bool HasBalcony { get; set; }
public int Beds_1 { get; set; }
public int Beds_2 { get; set; }
public double DayPrice { get; set; }
public List<Booking> Bookings { get; set; }
...
public int BookingsCount()
{
return Bookings.Count;
}
public bool IsFree(DateTime dateTime)
{
MessageBox.Show(BookingsCount().ToString());
return true;
}
}
and DbContext:
public class HotelContext : DbContext
{
public DbSet<Employee> Employees { get; set; }
public DbSet<Room> Rooms { get; set; }
public DbSet<Booking> Bookings { get; set; }
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<Booking>()
.HasRequired(b => b.Room)
.WithMany(r => r.Bookings)
.HasForeignKey(b => b.RoomId);
}
}
When MessageBox.Show is called I'm getting exception: An unhandled exception of type 'System.NullReferenceException' occurred in Hotel.exe
When I'm trying to access Room::Bookings, the list is always null. There is one row in Bookings table and multiple rows in Rooms table.
How can I load all of Bookings into Room object?

Depends where you are in the learning curve, however some things stand out
Firstly
You either want to create a relationship via FluentApi or Annotations, not both
Ie. you have this on your Room entity
[ForeignKey("RoomId")]
And this in fluent
modelBuilder.Entity<Booking>()
.HasRequired(b => b.Room)
.WithMany(r => r.Bookings)
.HasForeignKey(b => b.RoomId);
You need to pick one or the other, otherwise you may end-up with multiple Ids in your Booking i.e RoomId and Room_Id
Secondly
If you want to be able to Lazy Load bookings you need to make Bookings collection Virtual
public virtual List<Booking> Bookings { get; set; }
Lastly
To access your data (presuming your connection string is correct)
using(var db = new HoteContext())
{
var rooms = db.Rooms.Include(x => x.Bookings).ToList();
}
Note : Although EF Lazy loads relationships, you might want to make sure you have included the Room->Booking relationship

Consider the following code.
using System;
using System.Collections.Generic;
using System.Data.Entity;
using System.Linq;
namespace ConsoleApp
{
class Program
{
static void Main(string[] args)
{
using (MyDbContext dbContext = new MyDbContext())
{
dbContext.Departments.Add(new Department()
{
Name = "Some Department1",
Employees=new List<Employee>()
{
new Employee() { Name = "John Doe" }
}
});
dbContext.SaveChanges();
var department = dbContext.Departments.FirstOrDefault(d => d.Name == "Some Department1");
if (department.Employees != null)
{
foreach (var item in department.Employees)
{
Console.WriteLine(item.Name);
}
}
}
}
}
public class Department
{
public int Id { get; set; }
public string Name { get; set; }
public List<Employee> Employees { get; set; }
}
public class Employee
{
public int Id { get; set; }
public string Name { get; set; }
}
public class MyDbContext : DbContext
{
public DbSet<Department> Departments { get; set; }
public DbSet<Employee> Employees { get; set; }
}
}
If you have the code in above way, the control will not go into if condition, because department.Employees is null. Now, change the Department entity as follows.
public class Department
{
public int Id { get; set; }
public string Name { get; set; }
public virtual List<Employee> Employees { get; set; }
}
And now you should be able to see control go into if condition and outputs the employees name.
That is called Lazy Loading.
If you want to eagerly load, you don't have to put virtual to the property. You can Include the properties as follows.
var department = dbContext.Departments.Include(d => d.Employees).FirstOrDefault(d => d.Name == "Some Department1");
Now you can see the employees names are getting outputted.

You will absolutely run into performance trouble with your design here.
The temptation with EF is to completely map your object model to the DB and have EF do all the magic for you behind the scenes. But you need to think about it in terms of only getting specifically what you need from the db at any point in time. Otherwise you will get all kinds of cartesian product issues. I highly suggest you get yourself a copy of Hibernating Rhino's EF Profiler or similar so you can analyze your code statically and at runtime for EF performance issues (and see what SQL it is generating). For this what you want is a purpose built call to the DB to get the count. Otherwise what will happen is you will pull the entire table of Bookings and then have C# give you the count. That only makes sense if you want to do something with the whole list. Two options would be:
1) Create a VIEW against the Bookings table and map that to EF. The view would look something like SELECT ROOMS.ROOMID, COUNT(*) - you map this view to your model and voila now you have a list of counts by room (id) and you can use them individually or sum it up to get your total count for all rooms. If you have 1,000 bookings in 10 rooms, you are getting back only 10 rows from the DB. Whereas with your design, you are pulling back all 1,000 bookings with all their fields and then filtering down in C#. Bad juju.
2) The architecturally and conceptually simpler approach is going to be to do a direct query as such (obviously this returns only a single int from the db):
public int BookingsCount()
{
int count = 0;
try
{
using (var context = new HotelContext())
{
var sql ="SELECT COUNT(*) FROM Bookings WHERE ROOMID=" + this.RoomId;
count = context.Database.SqlQuery<int>(sql).First();
}
} catch (Exception ex)
{
// Log your error, count will be 0 by default
}
return count;
}

A simple solution would be making the Bookings property virtual.
public class Room
{
[Key]
public int RoomId { get; set; }
public int Number { get; set; }
public int Size { get; set; }
public bool HasBalcony { get; set; }
public int Beds_1 { get; set; }
public int Beds_2 { get; set; }
public double DayPrice { get; set; }
public virtual List<Booking> Bookings { get; set; }
}
More information on Entity Framework Loading Related Entities,
https://msdn.microsoft.com/en-us/library/jj574232(v=vs.113).aspx

EntityFrameworkCore using wrong column in query

I am trying to write my first web api with .net core. I am using VS2017 and core 1.1. I've got everything working except for one of my objects (I've tried it with that last line commented and uncommented...it makes no difference):
public class Tag
{
public int ID { get; set; }
public string Name { get; set; }
public bool ShowInFilter { get; set; }
public ICollection<SubscriberTag> SubscriberTags { get; set; }
}
My repository code looks like this:
private SubscriptionContext db;
public TagRepository(SubscriptionContext context) { db = context; }
public Tag Find(int key) => db.Tags.SingleOrDefault(a => a.ID == key);
That is being called from my TagController:
private iTagRepository TagItems { get; set; }
public TagController(iTagRepository tagItems) {TagItems = tagItems; }
[HttpGet("{id}", Name = "GetTag")]
public IActionResult Get(int id) { return new ObjectResult( TagItems.Find(id) ); }
The problem is when I run it, the query that is executed is:
exec sp_executesql
N'SELECT TOP(2) [a].[ID], [a].[Name],
[a].[ShowInFilter], [a].[SubscriberID]
FROM [Tags] AS [a]
WHERE [a].[ID] = #__key_0',N'#__key_0 int',#__key_0=1
which throw and error because Tags doesn't contain a column called SubscriberID.
I've searched all my code and SubscriberID only shows up two places (in other classes which are not being used here). I have no partial classes in my entire project (saw that was an issue on a related question.)
Why is EF adding this column to its query and how do I fix it?
As requested here is the class that contains subscriberID:
public class SubscriberTag
{
public long ID { get; set; }
public long subscriberID { get; set; }
public int tagID { get; set; }
public Subscriber Subscriber { get; set; }
public Tag Tag { get; set; }
}
Subscriber class (lots of irrelevant properties removed):
public class Subscriber
{
public Subscriber()
{
//a few value initalizers/defaults
}
public long ID { get; set; }
[StringLength(200)]
public string FirstName { get; set; }
//.......
public ICollection<Subscribers.Models.Subscription> Subscriptions { get; set; }
public ICollection<Subscribers.Models.Tag> Tags { get; set; }
}

Because of the Property Tags on Subscriber:
public ICollection<Subscribers.Models.Tag> Tags { get; set; }
Entity Framework expects the Tag object to have a foreign-key to Subscriber so it builds a query with it. It looks like to configure the many-to-many relationship needs to change the property to:
public ICollection<Subscribers.Models.SubscriberTag> SubscriberTag{ get; set; }
Configuring a Many-to-Many Relationship.
Thanks for the insight Ivan

Converting infinitely nested objects in .NET Core

EDIT: I originally worded this question very poorly, stating the problem was with JSON serialization. The problem actually happens when I'm converting from my base classes to my returned models using my custom mappings. I apologize for the confusion. :(
I'm using .NET Core 1.1.0, EF Core 1.1.0. I'm querying an interest and want to get its category from my DB. EF is querying the DB properly, no problems there. The issue is that the returned category has a collection with one interest, which has one parent category, which has a collection with one interest, etc. When I attempt to convert this from the base class to my return model, I'm getting a stack overflow because it's attempting to convert the infinite loop of objects. The only way I can get around this is to set that collection to null before I serialize the category.
Interest/category is an example, but this is happening with ALL of the entities I query. Some of them get very messy with the loops to set the relevant properties to null, such as posts/comments.
What is the best way to address this? Right now I'm using custom mappings that I wrote to convert between base classes and the returned models, but I'm open to using any other tools that may be helpful. (I know my custom mappings are the reason for the stack overflow, but surely there must be a more graceful way of handling this than setting everything to null before projecting from base class to model.)
Classes:
public class InterestCategory
{
public long Id { get; set; }
public string Name { get; set; }
public ICollection<Interest> Interests { get; set; }
}
public class Interest
{
public long Id { get; set; }
public string Name { get; set; }
public long InterestCategoryId { get; set; }
public InterestCategory InterestCategory { get; set; }
}
Models:
public class InterestCategoryModel
{
public long Id { get; set; }
public string Name { get; set; }
public List<InterestModel> Interests { get; set; }
}
public class InterestModel
{
public long Id { get; set; }
public string Name { get; set; }
public InterestCategoryModel InterestCategory { get; set; }
public long? InterestCategoryId { get; set; }
}
Mapping functions:
public static InterestCategoryModel ToModel(this InterestCategory category)
{
var m = new InterestCategoryModel
{
Name = category.Name,
Description = category.Description
};
if (category.Interests != null)
m.Interests = category.Interests.Select(i => i.ToModel()).ToList();
return m;
}
public static InterestModel ToModel(this Interest interest)
{
var m = new InterestModel
{
Name = interest.Name,
Description = interest.Description
};
if (interest.InterestCategory != null)
m.InterestCategory = interest.InterestCategory.ToModel();
return m;
}
This is returned by the query. (Sorry, needed to censor some things.)

This is not .NET Core related! JSON.NET is doing the serialization.
To disable it globally, just add this during configuration in Startup
services.AddMvc()
.AddJsonOptions(options =>
{
options.SerializerSettings.ReferenceLoopHandling = ReferenceLoopHandling.Ignore;
}));
edit:
Is it an option to remove the circular references form the model and have 2 distinct pair of models, depending on whether you want to show categories or interests?
public class InterestCategoryModel
{
public long Id { get; set; }
public string Name { get; set; }
public List<InterestModel> Interests { get; set; }
public class InterestModel
{
public long Id { get; set; }
public string Name { get; set; }
}
}
public class InterestModel
{
public long Id { get; set; }
public string Name { get; set; }
public InterestCategoryModel InterestCategory { get; set; }
public class InterestCategoryModel
{
public long Id { get; set; }
public string Name { get; set; }
}
}
Note that each of the models has a nested class for it's child objects, but they have their back references removed, so there would be no infinite reference during deserialization?