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Entity Framework Core: Many To Many relationship with some null values for some records

Let's say I have the following 3 classes - a Company that employs ConstructionWorkers and TruckDrivers. Let's say those people can be employed at many companies (as they work part time) - so it's a many to many relationship.
Some companies will only employ TruckDrivers, others will only employ ConstructionWorkers and others yet will employ both. This means in my CompanyEmployeeMapping table, the record could look like this:
CompanyEmployeeMapping Table:
+-------------+------------------+------------------------+
| CompanyId | TruckDriverId | ConstructionWorkerId |
+-------------+------------------+------------------------+
| 1 | 10 | NULL |
+-------------+------------------+------------------------+
(ie - Company only employs Truck Drivers)
This is how I set up my Class Structure + Fluent API:
public class Company
{
public int Id { get; set; }
public string Name { get; set; }
public ICollection<CompanyEmployeeMapping> Employees { get; set; }
}
public class ConstructionWorker
{
public int Id { get; set; }
public string Name { get; set; }
public ICollection<CompanyEmployeeMapping> Companies { get; set; }
}
public class TruckDriver
{
public int Id { get; set; }
public string Name { get; set; }
public ICollection<CompanyEmployeeMapping> Companies { get; set; }
}
public class CompanyEmployeeMapping
{
public int CompanyId { get; set; }
public int TruckDriverId { get; set; }
public int ConstructionWorkerId { get; set; }
[JsonIgnore]
public Company LinkedCompany { get; set; }
[JsonIgnore]
public TruckDriver LinkedTruckDriver { get; set; }
[JsonIgnore]
public ConstructionWorker LinkedConstructionWorker { get; set; }
}
Fluent API setup:
builder.Entity<CompanyEmployeeMapping>()
.HasKey(x => new { x.CompanyId, x.TruckDriverId, x.ConstructionWorkerId });
builder.Entity<CompanyEmployeeMapping>()
.HasOne(c => c.LinkedCompany)
.WithMany(m => m.LinkedEmployees)
.HasForeignKey(x => x.CompanyId)
.HasPrincipalKey(x => x.Id);
builder.Entity<CompanyEmployeeMapping>()
.HasOne(c => c.LinkedTruckDriver)
.WithMany(m => m.LinkedCompanies)
.HasForeignKey(x => x.TruckDriverId)
.HasPrincipalKey(x => x.Id);
builder.Entity<CompanyEmployeeMapping>()
.HasOne(c => c.LinkedConstructionWorker)
.WithMany(m => m.LinkedCompanies)
.HasForeignKey(x => x.ConstructionWorkerId)
.HasPrincipalKey(x => x.Id);
This seems to work correctly for me - I get an object with companies, containing a list of TruckDrivers and a list of ConstructionWorkers. Each of those also have their corresponding companies.
QUESTION
When I try to add a new company, which only employs TruckDrivers I get an exception:
The value of CompanyEmployeeMapping.ConstructionWorkerId is unknown when attempting to save changes. This is because the property is also part of a foreign key for which the principal entity in the relationship is not known.
What am I doing wrong here?

It looks like each instance of the mapping entity will only map to either TruckDriver or ConstructionWorker.
Therefore, your TruckDriver and ConstructionWorker ids need to be nullable.
This will then create a new challenge that your composite key cannot contain a nullable property.
I think the way forward is to make those ints nullable and then create a new single property EmployeeMapping Id for the mapping entity key.

How to Map Twitter follower/following type of relation in EF Core 5

How do you configure something similar to Twitter Following and Follower type of relationship using EF Core 5 with the Fluent API? I tried various different ways of configuring it and the only few ways I was able to get it to work is if I ignored the navigation properties on the User entity. I am currently migrating my code from EF Core 2.1 to 5. The following configuration worked earlier. (Not sure if it is misconfigured)
public class User
{
public long Id { get; set; }
public string Name { get; set; }
public ICollection<UserFollower> Followers { get; set; }
public ICollection<UserFollower> Following { get; set; }
}
public class UserFollower
{
public long UserId { get; set; }
public User User { get; set; }
public long FollowedById { get; set; }
public User FollowedBy { get; set; }
}
public class UserFollowerConfiguration : IEntityTypeConfiguration<UserFollower>
{
public void Configure(EntityTypeBuilder<UserFollower> builder)
{
builder.HasKey(p => new { p.UserId, p.FollowedById });
builder.HasOne(p => p.User)
.WithMany(i => i.Followers)
.HasForeignKey(i => i.UserId);
builder.HasOne(p => p.FollowedBy)
.WithMany(i => i.Following)
.HasForeignKey(i => i.FollowedById);
}
}
This configuration throws an error when saving to the database.
SqlException: Violation of PRIMARY KEY constraint 'PK_UserFollower'.
Cannot insert duplicate key in object 'dbo.UserFollower'. The duplicate key value is (111, 111).
Even when trying to directly add to the DbContext and calling SaveChanges() on it.
Context.Add(new UserFollower() {UserId = 222, FollowedById = 111});
What is the recommended way of mapping such a relationship with EF Core 5? Note that I do need to access the UserFollowers table without going through the Navigation properties of the User.
Edit #1
The following is the OnModelCreating() for the DbContext
protected override void OnModelCreating(ModelBuilder builder)
{
base.OnModelCreating(builder);
builder.ApplyConfigurations(typeof(DbContext).Assembly);
/*few configurations unrelated to UserFollower entity*/
}
User entity has the following configuration,
builder.HasKey(i => i.Id);
builder.Property(i => i.Id).ValueGeneratedOnAdd();

Try configuring it like this.
builder.Entity<User>().HasMany(s => s.Followers)
.WithOne(f => f.FollowedBy);
builder.Entity<User>().HasMany(s => s.Following)
.WithOne(f => f.);
Also, The PK is missing for the UserFollower table, I don't know if an Id is being generated somehow somewhere. If not, maybe this is why it's trying to wrongly use FollowedById as key, but define an Id for the UserFollower table and see.
public class UserFollower
{
public long Id {get;set;}
public long UserId { get; set; }
public User User { get; set; }
public long FollowedById { get; set; }
public User FollowedBy { get; set; }
}
Even if this works, I would recommend you change the structure of your model, it looks ambigous for the twitter requirements you described. If I query Userfollowers
var userFollowers = _context.UserFollowers.ToList();
For each result in the list, there is no way for me to tell if the user is following or being followed. You could change your models to these ones;
public class User
{
public long Id { get; set; }
public string Name { get; set; }
public ICollection<UserFollower> Followers { get; set; }
public ICollection<UserFollowing> Following { get; set; }
}
public class UserFollower
{
public long UserId { get; set; }
public User User { get; set; }
public long UserFollowingMeId { get; set; }
public User UserFollowingMe { get; set; }
}
public class UserFollowing
{
public long UserId { get; set; }
public User User { get; set; }
public long UserIAmFollowingId { get; set; }
public User UserIAmFollowing { get; set; }
}
This way, everybody knows when they check the UserFollowings table, the UserId is the Id of the person that is following and vice versa for the UserFollowers table. If I had an Id of 8 in the system, I can query my followers and people I follow like this;
var myFollowers = _context.UserFollowers.Where(UserId = 8);
var peopleIFollow = _context.UserFollowing.Where(UserId = 8);

Is it possible to use an owned type for many-to-many relations?

In Entity Framework Core version 2.2 or 3.0, is it possible to use owned/complex types in such a way that this kind of configuration is possible:
public class Product {
public int Id { get; set; }
public string Name { get; set; }
public ProductProperties Properties { get; set; }
}
public class ProductProperties {
public List<ProductSize> Sizes { get; set; }
}
public class Size {
public int Id { get; set; }
public string Name { get; set; }
}
public class ProductSize {
public int ProductId { get; set; }
public Product Product { get; set; }
public int SizeId { get; set; }
public Size Size { get; set; }
}
modelBuilder.Entity<ProductSize>()
.HasOne(x => x.Product)
.WithMany(x => x.Properties.Sizes)
.HasForeignKey(x => x.ProductId);
modelBuilder.Entity<ProductSize>()
.HasOne(x => x.Size)
.WithMany()
.HasForeignKey(x => x.SizeId);
The error message which is seen for this kind of approach usually ends up in:
'x => x.Properties.Sizes' is not a valid property expression. The expression should represent a simple property access: 't => t.MyProperty'
An earlier found answer is almost exactly matching my question, but this was posted in 2013. By the time it was almost certainly not possible.
HasForeignKey relationship through a Complex Type property
The sources on Microsoft are only giving examples for creating an entity with the complex type itself, not for creating relationships between them.

The cause of the issue
In your sample code it's quite clear there is no specific Many to Many relation. To make my argument a bit more convincing what follows is a model of your entities and their relations:
The new class structure
For a Many to Many relation to work in EF the product and size tables need to have an implicit relation with each other through a singular junction table. In my proposed solution I've chosen the ProductProperty table. There I've added the fields from the productsize junction table:
public class Product
{
public int Id { get; set; }
public string Name { get; set; }
public ICollection<ProductProperty> Properties { get; set; }
}
public class ProductProperty
{
public int ProductId { get; set; }
public Product Product { get; set; }
public int SizeId { get; set; }
public Size Size { get; set; }
}
public class Size
{
public int Id { get; set; }
public string Name { get; set; }
public ICollection<ProductProperty> Properties { get; set; }
}
The functions
modelBuilder.Entity<ProductProperty>()
.HasKey(pp => new { pp.ProductId, pp.SizeId });
modelBuilder.Entity<ProductProperty>()
.HasOne(pp => pp.Product)
.WithMany(p => p.Properties)
.HasForeignKey(pp => pp.ProductId);
modelBuilder.Entity<ProductProperty>()
.HasOne(pp => pp.Size)
.WithMany(p => p.Properties)
.HasForeignKey(pp => pp.SizeId);
Additional advice (EDIT)
Make the "Size" class a generic property class. This way the Many-to-Many relation won't get broken and querying will also be very easy:
public class Property
{
public int Id { get; set; }
public PropertyType propType { get; set; }
public string propValue { get; set; }
}
public enum PropertyType
{
Size,
Fontsize,
...
}
As a final argument this change will make it easier to change existing properties or add new ones
Sources
https://www.learnentityframeworkcore.com/configuration/many-to-many-relationship-configuration

You can check the owned entity types released in 2019 Check documentation here
An example from the link is the following:
public class Distributor
{
public int Id { get; set; }
public ICollection<StreetAddress> ShippingCenters { get; set; }
}
The owns many function should help you like this:
modelBuilder.Entity<Distributor>().OwnsMany(p => p.ShippingCenters, a =>
{
a.WithOwner().HasForeignKey("OwnerId");
a.Property<int>("Id");
a.HasKey("Id");
});
Let me know if I misunderstood your question.

Entity Framework with Condition Relationship

Is it possible to have a relationship that is based on a condition in Entity Framework? My model looks something like this...
public class Document
{
public int Id { get; set; }
public string Name { get; set; }
public OwnerType OwnerType { get; set; }
public int OwnerId { get; set; }
public virtual Organization OrganizationOwner { get; set; }
public virtual User UserOwner { get; set; }
}
public enum OwnerType
{
Organization = 1,
User = 2
}
public class Organization
{
public int Id { get; set; }
public string Name { get; set; }
//[other properties specific to Organization]
public virtual List<Documents> Documents { get; set; }
}
public class User
{
public int Id { get; set; }
public string Name { get; set; }
//[other properties specific to User]
public virtual List<Documents> Documents { get; set; }
}
So, what I'd like to is set up a relationship so that the OrganizationOwner property of a Document instance automatically gets populated when the OwnerType == OwnerType.Organization, and the UserOwner property is populated when OwnerType == OwnerType.User.
Is this possible to set up this kind of relationship in EntityFramework - Code First? Something like this in the mapping...
EntityTypeConfiguration<Document>.HasOptional(d => d.OrganizationOwner)
.WithMany(o => o.Documents)
.HasForeignKey(d => d.OwnerId)
.Where(d => d.OwnerType == OwnerType.Organization);
EntityTypeConfiguration<Document>.HasOptional(d => d.UserOwner)
.WithMany(u => u.Documents)
.HasForeignKey(d => d.OwnerId)
.Where(d => d.OwnerType == OwnerType.User);
I would like to be able to leverage joins on the OrganizationOwner and UserOwner when setting up my Linq queries on the context so that I don't have to do a separate selects on those entities for each Document. Is this type of relationship supported or is there a better way to do this? Thanks.

How do you map many-to-many relationships?

How does one map many to many relationships?
One-to-one relationships are easy....
Assuming...
public class ProductDTO
{
public int Id { get; set; }
public string Name { get; set; }
public int SupplierId { get; set; }
}
public class Product
{
public int Id { get; set; }
public string Name { get; set; }
public double Cost { get; set; }
public Supplier Supplier { get; set; }
}
public class Supplier
{
public int Id { get; set; }
public string Name { get; set; }
public int Rating { get; set; }
public ICollection<Product> Products { get; set; }
}
Mapper.CreateMap<Product, ProductDTO>()
.ForMember(d => d.SupplierId, m => m.MapFrom(s => s.Supplier.Id));
Works assuming every product has only one supplier and a supplier can have many products. How do I map it if a product can also have many suppliers?
I changed supplier line in Products to
public ICollection<Supplier> Supplier { get; set; }
and in ProductDTO doing the same
public ICollection<int> SupplierId { get; set; }
How do I alter CreateMap since the Ids are now collections? Autocomplete no longer shows Id and all I get are functions.
I'm new to C# so I many be missing something obvious. Am I supposed to iterate in a for loop and map the ids one by one?

You may try to use:
Mapper.CreateMap<Product, ProductDTO>()
.ForMember(d => d.SupplierIds, m => m.MapFrom(p => p.Suppliers.Select(s => s.Id)));
Another option:
Mapper.CreateMap<Supplier, int>().ConvertUsing(s => s.Id);
Mapper.CreateMap<Product, ProductDTO>()
.ForMember(d => d.SupplierIds, m => m.MapFrom(p => p.Suppliers));
One more thing if you are this using DTO to pass data from Web/WCF service you may consider using
public ICollection<SupplierDTO> Supplier { get; set; }
instead if passing supplier ids only. In most cases it's better (and more effective) to pass more data in one call to the service than doing few calls.