I have an existing database which I would like to use Entity Framework Code First against in the most simple way possible. It is only a small database.
I have created simple POCO classes which mirror the database tables:
e.g.
public class Author
{
[Key]
public int AuthorID { get; set; }
public string AuthorName { get; set; }
public virtual ICollection<Books> Books { get; set; }
}
public class Books
{
[Key]
public int BookID { get; set; }
public string BookName { get; set; }
public int AuthorID { get; set; }
}
And a DbContext as follows:
public class Entities : DbContext
{
public Entities(string connString)
: base(connString)
{
}
public DbSet<Author> Authors { get; set; }
public DbSet<Books> Books { get; set; }
When I run my application, and select the first Author from my database, the AuthorID and AuthorName properties are populated correctly. However, the collection of Books is not populated. Instead there is an exception of type 'System.Data.EntityCommandExecutionException', and an inner exception of: 'Invalid column name 'Author_AuthorID'.
How can I establish correctly the link between Author and Books? (i.e. one to many, one Author can have many Books). I have created the Code First very simply - no migrations or auto-generation in any way, and would like to keep it as simple as this.
Many thanks for any help,
Martin
Fluent API Approach :
modelBuilder.Entity<Author>().HasMany(a => a.Books).WithRequired().HasForeignKey(b => b.AuthorID);
update :
with this fluent API you don't have to add property Author in Class Books
if you want to set Books to not required, you must set property AuthorID to int?
modelBuilder.Entity<Author>().HasMany(a => a.Books).HasForeignKey(b => b.AuthorID);
Books class :
public class Books
{
[Key]
public int BookID { get; set; }
public string BookName { get; set; }
public int? AuthorID { get; set; }
}
Add Author property (without it single AuthorID property not matter at context of relation with Authors table.):
public class Books
{
[Key]
public int BookID { get; set; }
public string BookName { get; set; }
public int AuthorID { get; set; }
//add this(attribute is not needed if you use EF6 or higher):
[ForeignKey("AuthorID")]
public virtual Author Author { get; set; }
}
Well, this is embarassing, I found an answer minutes after my post. I will post the answer, rather than deleting my question in case it helps anyone else:
public class Books
{
[Key]
public int BookID { get; set; }
public string BookName { get; set; }
[ForeignKey("Author")]
public int AuthorID { get; set; }
public Author Author { get; set; }
}
Related
I am using EF 6.1.1.
I am unable to figure out how to create One-to-One relationship between two classes/tables with both entities have their owns PKs. I originally posted question link but could not get much help on it OR i am not able to get it. So, here i am putting my question in simple way.
Appreciate if someone can share thoughts on it.
My Requirement:
I would like create One-To-One relationship between Principle and Dependant with 'Id' from Principle class acts as Foreign Key in dependant class.
Principle Class
public class Student
{
public string FullName {get; set;}
}
Dependant Class
public class StudentReport
{
public string RollNumber { get; set; }
public string StudentType { get; set; }
}
Add PKs – EF requires this:
public class Student
{
public int Id { get; set; }
public string FullName { get; set; }
}
public class StudentReport
{
public int Id { get; set; }
public string RollNumber { get; set; }
public string StudentType { get; set; }
}
Note that EF 5 and later supports naming conventions: Id indicates a primary key. Alternately, it also supports the name of the class followed by "Id", so the above keys could have been StudentId for Student and StudentReportId for StudentReport, if you wished.
Add the foreign relation as a navigation property to at least one of the tables – in this case, you stated that StudentReport is the dependent, so let's add it to that one:
public class Student
{
public int Id { get; set; }
public string FullName { get; set; }
}
public class StudentReport
{
public int Id { get; set; }
public string RollNumber { get; set; }
public string StudentType { get; set; }
public Student Student { get; set; }
}
Again – by naming convention – EF determines that a single Student property on StudentReport indicates that this is a navigational property associated with a foreign key. (By defining only the Student property, but no foreign key property, you are indicating that you don't care what EF names the associated FK ... basically, you're indicating you'll always access the related Student via the property.)
If you did care about the name of the FK property, you could add it:
public class Student
{
public int Id { get; set; }
public string FullName { get; set; }
}
public class StudentReport
{
public int Id { get; set; }
public string RollNumber { get; set; }
public string StudentType { get; set; }
public int StudentId { get; set; }
public Student Student { get; set; }
}
Again – by naming convention – EF determines that StudentId is the FK associated with the Student property because it has the class name, "Student", followed by "Id".
All of this, so far, has been using conventions as defined in Entity Framework Code First Conventions, but Data Annotations are also an option, if you wish:
public class Student
{
[Key]
public int Id { get; set; }
public string FullName { get; set; }
}
public class StudentReport
{
[Key]
public int Id { get; set; }
public string RollNumber { get; set; }
public string StudentType { get; set; }
[ForeignKey("Student")]
public int StudentId { get; set; }
public Student Student { get; set; }
}
Doing this is actually a good idea, because it makes clearer your intent to other programmers that might not be aware of EF Conventions – but can easily infer them from simply looking at EF Data Annotations – and is still less cumbersome than Fluent API.
UPDATE
I just realized, I left this as a one-to-many, with enforcement of the one-to-one relationship being left to do in the code using this model. To enforce the one-to-one in the model, you could add a navigation property to the Student class going the other way:
public class Student
{
public int Id { get; set; }
public string FullName { get; set; }
public StudentReport StudentReport { get; set; }
}
public class StudentReport
{
public int Id { get; set; }
public string RollNumber { get; set; }
public string StudentType { get; set; }
public Student Student { get; set; }
}
However, that's going to break, because EF doesn't know which entity to insert first on an add. To indicate which is dependent, you have to specific that the dependent class' PK is the FK to the principal class (this enforces one-to-one because – in order for a Student/StudentReport pair to be associated – their Id properties must be the exact same value):
public class Student
{
public int Id { get; set; }
public string FullName { get; set; }
public StudentReport StudentReport { get; set; }
}
public class StudentReport
{
[ForeignKey("Student")]
public int Id { get; set; }
public string RollNumber { get; set; }
public string StudentType { get; set; }
public Student Student { get; set; }
}
or, using the full set of Data Annotations from earlier:
public class Student
{
[Key]
public int Id { get; set; }
public string FullName { get; set; }
public StudentReport StudentReport { get; set; }
}
public class StudentReport
{
[Key, ForeignKey("Student")]
public int Id { get; set; }
public string RollNumber { get; set; }
public string StudentType { get; set; }
public Student Student { get; set; }
}
I've the following situation I try to solve : I've 2 tables, a Course table with some fields and a CourseDescription table which is optional (so Course may have a CourseDescription but CourseDescription must have a Course). I'm trying to set this up. So far, here's what I have :
public class Course
{
[Key, Column("Key_Course")]
public int ID { get; set; }
public string Name { get; set; }
public virtual CourseDescription CourseDescription { get; set; }
}
public class CourseDescription
{
[Key, ForeignKey("Course")]
public int ID { get; set; }
public string Description { get; set; }
public string PreRequis { get; set; }
public int CoursesID { get; set; }
[ForeignKey("CoursesID")]
public Course Course { get; set; }
}
This "works" meaning that EF doesn't complains about my model but the relation is not properly done because EF associate the PK of CourseDescription with the PK of Course. In my database, this is not the case (ex : CourseDescription.ID=1 is associated with CourseDescription.CoursesID=3, not 1).
Is there a way to fix that with data annotation ? I know I can use the fluent API but I don't want to override the model building just for that (unless there's no other way).
Thanks
Well, I think you have two choices:
Configure an one to many relationship
If you want to map the FK of the relationship between Course and CourseDescription, and you don't want to declare that FK property as Key of the CourseDescription entity, then, you don't have other choice that configure an one-to-many relationship. In that case your model would be like this:
public class Course
{
[Key, Column("Key_Course")]
public int ID { get; set; }
public string Name { get; set; }
public virtual ICollection<CourseDescription> CourseDescriptions { get; set;}
}
public class CourseDescription
{
[Key]
public int ID { get; set; }
public string Description { get; set; }
public string PreRequis { get; set; }
[ForeignKey("Course")]
public int CourseID { get; set; }
public Course Course { get; set; }
}
Configure an one-to-one relationship but not map the FK of the
relationship
The only way that EF lets you map the FK in an one-to-one relationship is when the FK is declared as a PK too, so if you want to have diferent Ids in both entities and you want to stablish an one-to-one relationship, then you could do something like this:
public class Course
{
[Key, Column("Key_Course")]
public int ID { get; set; }
public string Name { get; set; }
public CourseDescription CourseDescription { get; set;}
}
public class CourseDescription
{
[Key]
public int ID { get; set; }
public string Description { get; set; }
public string PreRequis { get; set; }
[Required]
public Course Course { get; set; }
}
And work with the navigations properties.
It looks like you should not use ForeignKey attribute for ID property of CourseDescription class as you don't want to have an association between primary keys. Try to remove it.
Edit: It looks like I misunderstood the question previous time.
You can have your CourseDescription this way.
public class CourseDescription
{
[Key, ForeignKey("Course")]
public int ID { get; set; }
public string Description { get; set; }
public string PreRequis { get; set; }
public Course Course { get; set; }
}
In this case you don't need to have CoursesID field. Entities will be connected by primary keys.
Right now I'm learning ADO.NET Entity Framework and there's one thing that I can't explain to myself. Here is a source code from a tutorial I've been using recently:
public class Blog
{
public int BlogId { get; set; }
public string Name { get; set; }
public string Url { get; set; }
public User UserId { get; set; }
public virtual List<Post> Posts { get; set; }
}
public class Post
{
public int PostId { get; set; }
public string Title { get; set; }
public string Content { get; set; }
public int BlogId { get; set; }
public virtual Blog Blog { get; set; }
}
public class User
{
public int UserId { get; set; }
public string Username { get; set; }
public string DisplayName { get; set; }
}
First I thought that the using of List<> is the way to implement Foreign Key-like behaviour but now knowing that's not the case why we need and for what purpose we use List<> in our entites?
To show that Blog have a lot of Posts, when you will build your project in DB will be the relation 1xBlog--->NxPost where N=unlimited. This will show that each Blog can have unlimited amount of Posts
I have an EF code-first model with a table having several one-to-many relationships with other tables:
public class Note
{
[Key]
public Guid Id { get; set; }
public string NoteText { get; set; }
public string Author { get; set; }
public DateTime? CreationDate { get; set; }
}
public class Foo
{
public Foo()
{
Notes = new HashSet<Note>();
}
[Key]
public Guid Id { get; set; }
public virtual ICollection<Note> Notes { get; set; }
// other properties ommited...
}
public class Bar
{
public Bar()
{
Notes = new HashSet<Note>();
}
[Key]
public Guid Id { get; set; }
public virtual ICollection<Note> Notes { get; set; }
// other properties ommited...
}
As you can see, both Foo and Bar have their own list of Notes, but a Note belongs to either a Foo or a Bar.
When scaffolding the migration, EF creates a foreign key for Foo and Bar in the Notes table, which I think is not correct. I would like, instead, that a link table is created between Foo and Notes and another one between Bar and Notes.
Is there a way to automatically do this? Or do I have to manually create these classes in my code-first implementation?
This has already been answered in this other post!
But to save you a little googling, you are getting a one-to-many association, which is correct. you want a many-to-many relationship in your code, so what you will need to do is :
in your Notes class:
public class Note
{
[Key]
public Guid Id { get; set; }
public string NoteText { get; set; }
public string Author { get; set; }
public DateTime? CreationDate { get; set; }
public DateTime? CreationDate { get; set; }
public virtual ICollection<Foo> Foos { get; set; }
public virtual ICollection<Bar> Bars { get; set; }
}
Hope this helps
Person and EHR(electronic health record) are one to one related.
Person has EHRId nullable and EHR has PersonId not nullable.
At the same time EHR and Person must be many to many related.
Because a person can have many medics (represented by person entity) and a medic can have many EHRs.
I would like to have extra attributes on the join table.
I dont know how to define this in EF.
Please help.
Here are my classes.
public class Person
{
public int ID { get; set; }
public string firstName { get; set; }
public string lastName { get; set; }
public ICollection<UserSpecialist> patients { get; set; }
public int ehrID { get; set; }
public virtual EHR ehr { get; set; }
}
public class EHR
{
public int ID { get; set; }
public bool asthmatic{ get; set; }
public ICollection<UserSpecialist> specialists { get; set; }
public int PersonID { get; set; }
public virtual Person Person { get; set; }
}
public class UserSpecialist
{
public int ID { get; set; }
public DateTime creationDate { get; set; }
public int PersonID { get; set; }
public int EHRID { get; set; }
public virtual Person Person { get; set; }
public virtual EHR EHR { get; set; }
}
When EF tries to create the database throws this error
Unable to determine the principal end
of an association between the types
'Project.Person' and 'Project.EHR'.
The principal end of this association
must be explicitly configured using
either the relationship fluent API or
data annotations.
Please help
Person and EHR are not one-to-one related and they cannot be in EF. What you have defined is bidirectional one-to-many. You have also declared both relations as required because FK's are not nullable.
Real one-to-one can be defined in EF only if EHR's PK (Id) is also FK to Person. Once you define this the part with many-to-many becomes really strange because Person will be related with EHRs of other persons. You domain description is most probably not correct.