Model1)
public class Student {
int StudentID {get;set;}
string FirstName {get;set;}
string LastName {get;set;}
ICollection courses = {get; set;}
}
Model2)
public class Course{
int CourseID {get;set;}
string CourseName {get;set;}
int CreditHours {get;set;}
}
Model3)
public class StudentCourse{
int StudentCourseID {get;set;}
int StudentID {get;set;}
int CourseID {get;set;}
}
Now I add a bunch of data to the database...
and the relationship between student to course (one to many) is lost.
That is now I only have three data tables based on the models above in the database
with no relationship between them whatsoever.
If I create another application, write 3 models matching exactly as the models above and define the one to many relationship between
student and course using fluent API, will that work? That is will I be able to relate the data that already exists?
For example,
After doing the step above and querying a list of student and doing student.courses.ToList(); will the courses with matching studentID be returned as a list with that query?
and the relationship between student to course (one to many) is lost.
The relationship isn't working because you're only setting a link on the Student side of it. You also need to add an endpoint for the relationship on Courses:
public class Student {
public virtual ICollection<Course> Courses { get; set; }
}
public class Course {
public virtual Student Student { get; set; }
}
This should set up the 1-many relationship.
many students take the same courses though, so I'd set it up as a many-many relationship:
public class Student {
public virtual ICollection<Course> Courses { get; set; }
}
public class Course {
public virtual ICollection<Student> Students { get; set; }
}
I believe that this should eliminate the need for the StudentCourse table, but I stand under correction on this point.
Now, for your link between them (StudentCourse), you'll want to properly structure your relationships on this one, too:
public class StudentCourse
{
public int Id { get; set; }
public virtual Student Student { get; set; }
public virtual Course Course { get; set; }
}
Related
I'm trying to create an asp.net core web api.
So i wrote these classes for a Student with a list of Exams:
public class Student
{
[Key]
public int Id { get; init; }
public string? Name { get; set; }
public List<Exam>? Exams { get; init; }
}
public class Exam
{
[Key]
public int Id { get; init; }
public string? Type { get; set; }
public int Grade { get; set; }
}
On my database i have both classes as tables, though the exams table has an extra column holding the Id of the student the exam belongs to.
My database context class looks like this
public class LehrerDBContext : DbContext
{
public LehrerDBContext(DbContextOptions<LehrerDBContext> options) : base(options) {
}
public DbSet<Student> Students { get; set; }
public DbSet<Exam> Exams { get; set; }
}
if i do _dbContext.Students.ToList() now, it returns me a List with all students but the students' exam lists are null.
The database tables look like this:
Feel free to ask if something is unclear as this is my first stack overflow question.
I tried adding a getter function to the student's exam list, that gets all exams by the students Id, though i don't know how to get a database context here.
You could check this document to learn how to load related data(Eager loading,Explicit loading,Lazy loading)
If you want to try with Lazy loading,you have to install this package:
regist your dbcontext as below:
services.AddDbContext<yourdbcontext>(options =>
{
options.UseLazyLoadingProxies();
options.UseSqlServer(Configuration.GetConnectionString("yourdbcontext"));
}) ;
Set the navigation properties as virtual
public virtual List<Exam> Exams { get; set; }
Result:
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"
}
}
I have the following models:
public class ItemRental
{
[Key]
public Int32 ItemRentalId { get; set; }
public Int32 ItemId { get; set; }
public Int32 ChargeItemId { get; set; }
[ForeignKey("ItemId")]
public Item Item { get; set; }
[ForeignKey("ChargeItemId")]
public Item ChargeItem { get; set; }
}
public class Item
{
[Key]
public Int32 ItemId { get; set; }
public Int32? ItemRentalId { get; set; }
[ForeignKey("ItemRentalId")]
public ItemRental ItemRental { get; set; }
}
ItemRental has a 1..N relation with Item AND has a 1..N relation with ChargeItem.
Problem is that I needed a relation from Item back to the ChargeItem, so added the property ItemRentalId on the Item. This is nullable because not every Item has to have an ItemRental.
Is it possible to create this relation with just annotations?
I tried the fluent api:
modelBuilder.Entity<Item>()
.HasOptional(m => m.ItemRental)
.WithRequired(c => c.ChargeItem)
.Map(p => p.MapKey("ItemRentalId"));
But after doing a migration is it not using the ChargeItemId as a relation.
The problem is when I run this migration it doesn't honor the ItemRentalId as a FK navigation property.
So if I understand correctly, your problem is with mapping the one to zero-or-one relationship.
What you are experiencing is a by-design feature of Entity Framework. Handling one-to-one relationships (and their optional counterparts) is tricky for a lot of reasons. When you do it like this, you cannot specify the foreign key in your model — instead the primary key of your entity will also be the foreign key to the principal end, no extra FK can be specified.
See below for more details on this.
Mapping one-to-zero or one
So let's say that you have the following model:
public class Person
{
public int PersonId { get; set; }
public string Name { get; set; }
}
public class Car
{
public int CarId { get; set; }
public string LicensePlate { get; set; }
}
public class MyDemoContext : DbContext
{
public DbSet<Person> People { get; set; }
public DbSet<Car> Cars { get; set; }
}
And now you want to set it up so that you can express the following specification: one person can have one or zero car, and every car belongs to one person exactly (relationships are bidirectional, so if CarA belongs to PersonA, then PersonA 'owns' CarA).
So let's modify the model a bit: add the navigation properties and the foreign key properties:
public class Person
{
public int PersonId { get; set; }
public string Name { get; set; }
public int CarId { get; set; }
public virtual Car Car { get; set; }
}
public class Car
{
public int CarId { get; set; }
public string LicensePlate { get; set; }
public int PersonId { get; set; }
public virtual Person Person { get; set; }
}
And the configuration:
public class CarEntityTypeConfiguration : EntityTypeConfiguration<Car>
{
public CarEntityTypeConfiguration()
{
this.HasRequired(c => c.Person).WithOptional(p => p.Car);
}
}
By this time this should be self-explanatory. The car has a required person (HasRequired()), with the person having an optional car (WithOptional()). Again, it doesn't matter which side you configure this relationship from, just be careful when you use the right combination of Has/With and Required/Optional. From the Person side, it would look like this:
public class PersonEntityTypeConfiguration : EntityTypeConfiguration<Person>
{
public PersonEntityTypeConfiguration()
{
this.HasOptional(p => p.Car).WithOptional(c => c.Person);
}
}
Now let's check out the db schema:
Look closely: you can see that there is no FK in People to refer to Car. Also, the FK in Car is not the PersonId, but the CarId. Here's the actual script for the FK:
ALTER TABLE [dbo].[Cars] WITH CHECK ADD CONSTRAINT [FK_dbo.Cars_dbo.People_CarId] FOREIGN KEY([CarId])
REFERENCES [dbo].[People] ([PersonId])
So this means that the CarId and PersonId foregn key properties we have in the model are basically ignored. They are in the database, but they are not foreign keys, as it might be expected. That's because one-to-one mappings does not support adding the FK into your EF model. And that's because one-to-one mappings are quite problematic in a relational database.
The idea is that every person can have exactly one car, and that car can only belong to that person. Or there might be person records, which do not have cars associated with them.
So how could this be represented with foreign keys? Obviously, there could be a PersonId in Car, and a CarId in People. To enforce that every person can have only one car, PersonId would have to be unique in Car. But if PersonId is unique in People, then how can you add two or more records where PersonId is NULL(more than one car that don't have owners)? Answer: you can't (well actually, you can create a filtered unique index in SQL Server 2008 and newer, but let's forget about this technicality for a moment; not to mention other RDBMS). Not to mention the case where you specify both ends of the relationship...
The only real way to enforce this rule if the People and the Car tables have the 'same' primary key (same values in the connected records). And to do this, CarId in Car must be both a PK and an FK to the PK of People. And this makes the whole schema a mess. When I use this I rather name the PK/FK in Car PersonId, and configure it accordingly:
public class Person
{
public int PersonId { get; set; }
public string Name { get; set; }
public virtual Car Car { get; set; }
}
public class Car
{
public string LicensePlate { get; set; }
public int PersonId { get; set; }
public virtual Person Person { get; set; }
}
public class CarEntityTypeConfiguration : EntityTypeConfiguration<Car>
{
public CarEntityTypeConfiguration()
{
this.HasRequired(c => c.Person).WithOptional(p => p.Car);
this.HasKey(c => c.PersonId);
}
}
Not ideal, but maybe a bit better. Still, you have to be alert when using this solution, because it goes against the usual naming conventions, which might lead you astray. Here's the schema generated from this model:
So this relationship is not enforced by the database schema, but by Entity Framework itself. That's why you have to be very careful when you use this, not to let anybody temper directly with the database.
I'd like to define relationship where Student can have only one favorite Course. I expect it would look like this in DB:
STUDENT
ID
Name
FavoriteCourseID
COURSE
ID
Name
How to achieve this with entity framework? I'd prefer to specify it just by attributes. I tried:
public class Student
{
public int ID { get; set; }
public string Name { get; set; }
public Course FavoriteCourse { get; set; }
public int? FavoriteCourseID { get; set; }
}
public class Class
{
public int ID { get; set; }
public string Name { get; set; }
}
which gave me this DB model:
STUDENT
ID
Name
FavoriteCourseID
COURSE
ID
Name
StudentID // how to remove this?
Note, that it may happen that several students have the same favorite class and therefore this is unacceptable solution.
Another question: what type of relationship this is? (1:1 / 1:N ?)
To specify 1 to 1 relationship, it is assumed, that primary key for the related entity matches the primary key of first entity. Also you should specify a virtual property to related entity:
public class Student
{
[Key]
public int ID { get; set; }
public string Name { get; set; }
public Course FavoriteCourse { get; set; }
public int? FavoriteCourseID { get; set; }
}
public class Class
{
[Key]
[ForeignKey("Student")]
public int ID { get; set; }
public string Name { get; set; }
public virtual Student Student { get; set; }
}
And it will be one-to-zero-or-one relationship. Check this tutorial.
If you will mark FavouriteCourse property with RequiredAttribute, it seems, that it will result in strong one to one relationship.
It will result in adequate database structure:
STUDENT
ID
Name
FavoriteCourseID
COURSE
ID
Name
However, if many students could have one favourite course, this structure will be a problem, as you want one-to-many instead of one-to-one. And you will have a duplicate records in database, because one course can refer only to one student. You have to think about your db design.
You can try this:
public class Student
{
public int ID { get; set; }
public string Name { get; set; }
[ForeignKey("FavoriteCourseId")]
public Course FavoriteCourse { get; set; }
public int? FavoriteCourseId { get; set; }
}
Normally, you define one of the following relations:
Optional:Optional
Required:Optional
Optional:Many
Required:Many
Many:Many
Having Required:Required is not a usual relation, inserting the first entry with such a relation needs special treatment.
I Suppose you want Required:Many as in "Each student has one favorite course but many students may chose the same favorite course".
I have the Student, Course and a relationship between them as StudentCourse. The fields in these classes are as follows:
public class Student
{
public int StudentId { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public int CourseId { get; set; }
[ForeignKey("CourseId")]
public Course Course { get; set; }
}
public class Course
{
public int CourseId { get; set; }
public string CourseName { get; set; }
}
public class StudentCourse
{
public int ID { get; set; }
public virtual Student Student { get; set; }
public virtual Course Course {get;set;}
}
When I delete the students in student table , then I want to remove the corresponding rows from the relationship StudentClass. How can I do it?
I believe that you actually want a many-to-many relationship between Student and Course: A student can participate in many courses and a course can have many students.
In this case you can simplify your model:
public class Student
{
public int StudentId { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public ICollection<Course> Courses { get; set; }
}
public class Course
{
public int CourseId { get; set; }
public string CourseName { get; set; }
public ICollection<Student> Students { get; set; }
}
The "join entity" StudentCourse is not needed. EF will create three tables from this model: A Students table, a Courses table and StudentCourses (or CourseStudents) table that will have a composite primary key (StudentId, CourseId) (or named similar). Both parts are foreign keys to their respective tables.
For the two FK relationships in the database cascading delete will be turned on by default. So, if a Student gets deleted the link records in the join table will be deleted automatically. The same when a Course gets deleted.
You can also define the detailed names for join table and join table columns explicity and you can also work with only a single collection, for example only the Courses collection in Student but without the Students collection in Course. You must use Fluent API for this:
modelBuilder.Entity<Student>()
.HasMany(s => s.Courses)
.WithMany() // no parameter if there is no collection in Course
.Map(m =>
{
m.MapLeftKey("StudentId");
m.MapRightKey("CourseId");
m.ToTable("StudentCourses");
});