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Many to many EF core - Unable to determine the relationship

I'm trying to solve a many to many relationship in EF core, but I keep getting an exception (Unable to determine the raltionship represented by navigation property 'Category.contacts' of type List)
I'm learning EF core and I've read a lot of posts about this issue already but I cannot solve it on my own. I believe it will be asked on the exam.
I've made a class to solve the many to many problem, but how do I configure this correctly using the fluent api?
This is my code:
public class Contact{
public int PersonId {get; set;}
public List<Category> Categories {get; set;}
}
public class Category{
public int CategoryId {get; set;}
public List<Category> Categories {get; set;}
}
public class ContactCategory{
public int PersonId {get; set;}
public int CategoryId {get; set;}
public Contact Contact {get; set;}
public Category Category {get; set;}
}
//Inside the DbContext class:
protected override void OnModelCreating(ModelBuilder modelBuilder){
modelBuilder.Entity<ContactCategory>().HasKey(x => new {x.PersonId, x.CategoryId});
}
The exception itself:
Unhandled Exception: System.TypeInitializationException: The type initializer for 'Contacts.UI.CA.Program' threw an exception. ---> System.InvalidOperationException: Unable to determine the relationship represented by navigation property 'Category.Contacts' of type 'List'. Either manually configure the relationship, or ignore this property using the '[NotMapped]' attribute or by using 'EntityTypeBuilder.Ignore' in 'OnModelCreating'.

The many-to-many relationship is a tricky one; we need to understand how these kinds of
relationships are built. Usually, we will build two different entities that require a many-to-many relationship, create an entity that will be purely used to join the first two entities, and then map one-to-many between this entity (created to join two separate one-to-many relationships) and the two entities (created first) separately:
public class Contact
{
public int Id {get; set;}
public ICollection<ContactCategory> ContactCategories {get; set;}
}
public class Category
{
public int Id { get; set; }
public ICollection<ContactCategory> ContactCategories { get; set; }
}
public class ContactCategory
{
public int Id { get; set; }
public int ContactId {get; set;}
public Contact Contact {get; set;}
public int CategoryId {get; set;}
public Category Category {get; set;}
}
The Contact and Category entities require many-to-many relationships for which the ContactCategory entity is created, and its only job is to join the Contact and Category entities.
We should configure the relationship in the Fluent API in two different one-to-many
relationships between the Contact and ContactCategory and Category and ContactCategory entities. The HasForeignKey method doesn't need to be generic, since one-to-many relationships don't need to mark the dependent type explicitly:
The many-to-many relationship between Contact and Category entities require the following
configuration:
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<ContactCategory>()
.HasOne(x => x.Category)
.WithMany(x => x.ContactCategories)
.HasForeignKey(x => x.CategoryId);
modelBuilder.Entity<ContactCategory>()
.HasOne(x => x.Contact)
.WithMany(x => x.ContactCategories)
.HasForeignKey(x => x.ContactId);
}
Good luck!

How to get records from table that depends on another tables in entity framework core?

I want to get some records from Database that depends on three tables.
Three tables are:
1.Company(Id,Name)
2. Car(Id,CompanyId,Name)
3. Showroom(Id,CarId,Name)
Now a one company contains many cars and many cars may exist in many showrooms.
I want to get records from showroom table where company '2' cars exist along with cars. Is it possible to do it in entity framework core?

I think your entities will be like :
Company
public class Company
{
public int Id {get; set;}
public string Name {get; set;}
public ICollection<Car> Cars {get; set;}
}
Car:
public class Car
{
public int Id{get; set;}
public string Name {get; set;}
public int CompanyId{get; set;}
public Company Company {get; set;}
}
ShowRoom:
public class ShowRoom
{
public int Id{get; set;}
public string Name {get; set;}
public int CarId{get; set;}
public Car Car{get; set;}
}
In your method:
var context = new SomeContext();
var showRooms= context.ShowRooms
.Include(x=> x.Car)
.ThenInclude(x=> x.Company)
.Where(x=> x.Car.Company.Id== 2)
.ToList();

Invalid column name error from SQL generated by EF

Below you can see the SQL should join by using [ClassId1] instead of [Class1_ClassId] since the latter doesn't exist.
I'm pretty sure I can use Fluent API to correct this but not sure what methods.
Generated SQL
SELECT ...
FROM [dbo].[School] AS [Extent1]
LEFT OUTER JOIN [dbo].[Student] AS [Extent2] ON [Extent1].[SchoolId] = [Extent2].[SchoolId]
LEFT OUTER JOIN [dbo].[Class] AS [Extent3] ON [Extent2].[Class1_ClassId] = [Extent3].[ClassId]
LEFT OUTER JOIN [dbo].[Class] AS [Extent4] ON [Extent2].[Class2_ClassId] = [Extent4].[ClassId]
WHERE ...
Tables
School
- SchoolId
- Name
- StudentId
Student
- StudentId
- Name
- Class1Id
- Class2Id
Class
- ClassId
- Name
Models
public class School
{
[Required]
public long SchoolId { get; set; }
[Required]
public string Name { get; set; }
[Required]
public long StudentId { get; set; }
public virtual Student Student { get; set; }
}
public class Student
{
[Required]
public long StudentId { get; set; }
[Required]
public string Name { get; set; }
[Required]
public long ClassId1 { get; set; }
public long? ClassId2 { get; set; }
public virtual Class Class1 { get; set; }
public virtual Class Class2 { get; set; }
}
public class Class
{
[Required]
public long ClassId { get; set; }
[Required]
public string Name { get; set; }
}

[Required]
public long ClassId1 { get; set; }
public long? ClassId2 { get; set; }
public virtual Class Class1 { get; set; }
public virtual Class Class2 { get; set; }
You haven't setup any relationship between these properties. Since you haven't defined foreign key columns for Class1 or Class2, it'll create them for you: Class1_ClassId and Class2_ClassId. Creating a migration should create those columns for you; but you'd end up with duplicates (Class1Id and Class1_ClassId for example).
I believe EntityFramework will resolve relationships between properties if the name ends with Id. Which means your setup should be:
[Required]
public long Class1Id { get; set; }
public long? Class2Id { get; set; }
public virtual Class Class1 { get; set; }
public virtual Class Class2 { get; set; }
However, I find it's better to be explicit - purely for readability and to ensure EF doesn't try to get too smart. I'd write it like this:
[Required]
public long ClassId1 { get; set; }
public long? ClassId2 { get; set; }
[ForeignKey("ClassId1")]
public virtual Class Class1 { get; set; }
[ForeignKey("ClassId2")]
public virtual Class Class2 { get; set; }
This should properly setup your foreign key relationships in the database.

I think Entity Framework constructed this SQL from your linq, because the relations between the classes in your model are unclear.
According to your model a School has only one Student, a Student doesn't know which School he attends, and is obliged to have one Class, and possibly a second one. A class does not know in which School it is, nor which Students are in the Class.
Are you sure about your model?
I'd gather that a School would have zero or more Students. A School also has zero or more Classes. Each class is a class in a School.
In database terms this is a typical one-to-many relationship. See Entity Framework Configure One-to-Many Relationship
Furthermore a Student attends zero or more Classes, a Class has one or more Students.
In database terms this is a typical many-to-many relationship. See: Entity Framework configure many-to-many relationship
These articles also describe schools, students and couses. Summarized the class definitions ought to be:
class School
{
public int Id {get; set;}
// a School has many Students:
public virtual ICollection<Student> Students {get; set;}
// a School has many Classes:
public virtual ICollection<Class> Classes {get; set;}
...
}
public class Student
{
public int Id {get; set;}
// A student belongs to one School via Foreign Key
public int SchoolId {get; set;}
public virtual School School {get; set;}
// A student attends many classes
public virtual ICollection<Class> Classes {get; set;}
...
}
class Class
{
public int Id {get; set;}
// a class belongs to one School via foreign key:
public int SchoolId {get; set;}
public virtual School School {get; set;}
// a class has many Students
public virtual ICollection<Student> Students {get; set;}
...
}
After this the DbContext will be as follows:
class MyDbContext : DbContext
{
public DbSet<School> Schools {get; set;}
public DbSet<Student> Students {get; set;}
public DbSet<Class> Classes {get; set;}
}
If you model it like this, entity framework will automatically recognize the one-to-many relationships between School and Students, and create proper foreign keys for it. It will also recognize the many-to-many relationship between Students and Classes. It will even create a table for the many-to-many, which you won't need in your LINQ queries.
Entity Framework uses default conventions If you follow them, you won't need to tell the model about Table names and column names, about primary keys and foreign keys etc.
Back to your question
You want to tell your model that it should use a certain column name for a property instead of the column name it constructed from your class relations.
This can be done using Data Annotations within your class, or using Fluent API within your DbContext. I prefer using Fluent Api, because it allows you to use the same classes in different database structures without having to change the classes. If you want different table names, or different names for primary keys, different precision for decimals, etc, all you have to do is create a new DbContext. You don't have to change your classes, users of your classes won't notice the changes.
Fluent API is described here.
In you case: specify a table name instead of the default table name.
In my example, A Class would be put in table Classs, while of course you'd want it in table Classes:
class MyDbContext : DbContext
{
public DbSet<School> Schools {get; set;}
public DbSet<Student> Students {get; set;}
public DbSet<Class> Classes {get; set;}
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
// Entity Class should be put in table Classes
modelBuilder.Entity<Class>().ToTable("Classes");
// property Student.ClassId in column "ClassId1"
modelBuilder.Entity<Student>() // from class Student
.Property(student => student.ClassId) // take property ClassId
.HasColumnName("ClassId1"); // give it the column name "ClassId1"
}
}

How to update related entities using GraphDiff?

I have the following model:
public class Customer
{
public int Id {get; set;}
public string Name {get; set;}
public int AddressId {get; set;}
public virtual Address Address {get; set;}
public virtual ICollection<CustomerCategory> Categories {get; set;}
}
public class CustomerCategory
{
public int Id {get; set;}
public int CustomerId {get; set;}
public int CategoryId {get; set;}
public virtual Category Category {get; set;}
}
public class Address
{
public int Id {get; set;}
public string Street{get; set;}
public virtual PostCode PostCode {get; set;}
}
From the above, and using GraphDiff, I want to update the customer aggregate as follows:
dbContext.UpdateGraph<Customer>(entity,
map => map.AssociatedEntity(x => x.Address)
.OwnedCollection(x => x.Categories, with => with.AssociatedEntity(x => x.Category)));
But the above is not updating anything!!
What is the correct way to use GraphDiff in this case?

GraphDiff basically distinguishes two kinds of relations: owned and associated.
Owned can be interpreted as "being a part of" meaning that anything that is owned will be inserted/updated/deleted with its owner.
The other kind of relation handled by GraphDiff is associated which means that only relations to, but not the associated entities themselves are changed by GraphDiff when updating a graph.
When you use the AssociatedEntity method, the State of the child entity is not part of the aggregate, in other words, the changes that you did over the child entity will not be saved, just it will update the parent navegation property.
Use the OwnedEntity method if you want to save tha changes over the child entity, so, I suggest you try this:
dbContext.UpdateGraph<Customer>(entity, map => map.OwnedEntity(x => x.Address)
.OwnedCollection(x => x.Categories, with => with.OwnedEntity(x => x.Category)));
dbContext.SaveChanges();

Entity Framework mapping between multiple tables

I am trying to get the correct mapping between 4 tables.
MainTables
Class(Id, ClassName)
Course(Id, CourseName)
Student(Id, StudentName)
Relationship tables
ClassCourse(Id, ClassId, CourseId)
ClassCourseStudent(ClassCourseId, StudentId)
Class to Course has Many to Many mapping. So we use a relationship table ClassCourse to store the relationship
Student has one to Many mapping with ClassCourse.
So my question is how can I do the mapping for Student and ClassCourse
My code is
public class Class
(
public int Id {get;set;}
public string ClassName {get;set;}
public virtual ICollection<Course> Courses {get;set;}
)
public class Course
(
public int Id {get;set;}
public string CourseName {get;set;}
public virtual ICollection<Student> Students {get;set;}
)
public class Student
(
public int Id {get;set;}
public string StudentName {get;set;}
)
modelBuilder.Entity<Class>().ToTable("Class");
modelBuilder.Entity<Course>().ToTable("Course");
modelBuilder.Entity<Student>().ToTable("Student");
modelBuilder.Entity<Class>().HasMany(c => c.Courses).WithMany().Map(m => m.ToTable("ClassCourse")
m.MapLeftKey("ClassId")
m.MapRightKey("CourseId")
)
modelBuilder.Entity<Course>().HasMany(c => c.Students).WithMany().Map(m =>
m.ToTable("ClassCourseStudent")
m.MapLeftKey("ClassCourseId")
m.MapRightKey("StudentId")
The last mapping is the one I am looking for.
Thanks in advance.

I think you have to revisit your design. Right now you're trying to assign a composite key as foreign key, which can't be done.
What I would do is create a separate model that simply stores the course-class combination and provides a single key to reference. This will result in an extra table, but allows you to do what you want.
class Student {
public int StudentId {get; set;}
}
class Class {
public int ClassId {get; set;}
}
class Course {
public int CourseId {get; set;}
}
class ClassCourse {
public int ClassCourseId {get; set;}
public int ClassId {get; set;}
public int CourseId {get; set;}
}
Now every class should have a list of ClassCourse objects instead of Course, and every Course should have a list of ClassCourse objects. Now they're not directly linked together but are still connected trough an intermediate object and you can connect your Student objects to the primary key of ClassCourse.