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How to use Fluent API for creating a simple one-to-many relationship

Let me first give some background: I'm creating an application, which should handle a DB. That DB might evolve (extra tables/columns/constraints might be added, but nothing gets removed, in fact the DB gets more and more elaborated).
I started with a "Database First" approach and as a result, I have created an Entity Framework diagram, with according classes in *.cs files. Two of those files are (only some interesting fields):
Area.cs:
public partial class Area
{
public Area() { }
public string Name { get; set; }
public int Id { get; set; }
}
Location.cs:
public partial class Location
{
public Location() { }
public string Name { get; set; }
public int Id { get; set; }
...
public Nullable<int> AreaId { get; set; }
}
This is generated from a version of the DB, which does not cover constraints, and now I would like to add a ForeignKeyConstraint to the corresponding Entity Framework model:
Location.AreaId is a foreign key towards Area.Id
There are many Location objects for one Area object
It's the idea to prevent deletion of Area objects, being referred to by Location objects).
I believe this should be done as follows:
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<Area>().HasKey(t => t.Id); // Creation of primary key
modelBuilder.Entity<Location>().HasKey(t => t.Id); // Creation of primary key
modelBuilder.Entity<Location>().HasRequired(n => n.AreaId)
.WithMany(...)
.HasForeignKey(n => n.AreaId);
...
This, obviously, does not work. I'm missing following information:
My "Area.cs" file does not contain a reference to the Location object (as this version of the DB does not contain constraints, this has not been added by the "database first" wizard), should I add this or can I solve my issue without?
What do I need to fill in instead of the ellipsis .WithMany(...)?
Extra question: I'm aware of the ForeignKey directive. Should I replace public Nullable<int> AreaId { get; set; } in "Location.cs" by [ForeignKey("AreaId")], followed by public virtual Area Area { get; set; }?
Edit
Important remark: as "Location.cs" and "Area.cs" are auto-generated, I like to minimise changes in those files.
Next edit
Meanwhile I've updated my "Location.cs" file as follows:
...
// public Nullable<int> AreaId { get; set; }
[ForeignKey("AreaId")]
public Area Area { get; set;}
....
My OnModelCreating() has been changed into:
modelBuilder.Entity<Location>().HasRequired(n => n.Area)
.WithMany(...)
.HasForeignKey(n => n.Area);
That leaves only the ellipsis problem to be solved.
Another edit
Since it takes such a long time for an answer (even for a comment), I've decided to add following line of source code to my "Area.cs" file:
public virtual ICollection<Location> Locations { get; set; }
I've then filled in the ellipsis as follows:
modelBuilder.Entity<Location>().HasRequired(l => l.Area)
.WithMany(a => a.Locations)
.HasForeignKey(l => l.Area);
Now just one question: how can I mention that the link between the Area and the Location should be handled by Location.AreaId and Area.Id (I know that Location.AreaId is the foreign key, but how can I know that it refers to Area.Id)?
Thanks in advance

The simple answer to your last question. EF is recognizing that Area.Id is a primary key so connects Location.AreaId to Area.Id
Also, here is a simple guide on how to do it.

How to migrate an already set up one-to-many relationship in Entity Framework?

I have a C# WPF application using an SQLite database with Entity Framework. I have a Contact class which can have multiple messages, so there is a one-to-many relationship, set up the following way (simplified version):
public class Message {
public int PK { get; set; }
public int SenderKey { get; set; }
public Contact Sender { get; set; }
}
public class Contact {
public int PK { get; set; }
public ICollection<Message> Messages { get; set; }
}
Then I set the relationship using the Fluent API, the following way:
protected override void OnModelCreating(DbModelBuilder modelBuilder) {
...
modelBuilder.Entity<Message>()
.HasOptional(e => e.Sender)
.WithMany(e => e.Messages)
.HasForeignKey(e => e.SenderKey);
}
I'm going to have lots of data (and also have lots of troubles with this circular dependency, especially on editing the objects in detached mode) so it's not a good idea to have that Messages collection in the memory for every contact. To avoid this, I'd like to get rid of that list, so it would be great to implement the 'Convention 1' from the docs (to just have the Contact object in the Message class and that's all).
The problem with this solution is that my app is already published, so I can't just simply change the structure, I need a migration. My question is that how can I migrate this kind of relation set up by the Fluent API?
I tried to remove the relationship from the OnModelCreating, but I got the following exception when I started the app:
System.Data.SQLite.SQLiteException: SQL logic error no such column: Extent1.Sender_PK (what is that Extent1 table?)

Finally I found the solution. I didn't have to implement a specific migration, just modify the following things:
Remove the relationship setup from the OnModelCreating
The previous step caused that "sql logic error" posted in the question. This was because the foreign key column name wasn't specified and the Entity Framework searched for a default column, which is in fact Sender_PK. So, in order to solve this, I added an annotation in the Message class, which tells the Entity Framework what is the foreign key column name for that Contact object:
public class Message {
public int PK { get; set; }
public int SenderKey { get; set; }
[ForeignKey("SenderKey")]
public Contact Sender { get; set; }
}
Remove the message list reference (public ICollection<Message> Messages { get; set; }) from the Contact class.
So, after this three step modification I had the one-to-many relationship between the two tables and I could get rid of that list. Nothing else needed, it works perfectly with the old databases.

Create Self-Referencing Hierarchical Table in Entity Framework

Background
I have a class that looks more or less like this:
public class MyClass
{
[Id]
public long Id { get; set; }
public string MyProperty { get; set; }
public bool MyBoolean { get; set; }
public string AnotherProperty { get; set; }
public MyClass ChildOne { get; set; }
public MyClass ChildTwo { get; set; }
}
I will need to use a stored procedure to load a set of records, but that's ok as long as the structure itself is correct.
For any instance of MyClass, one or both of the children can be null. Any instance of MyClass can be used in a parent class - but the child itself doesn't need to know about this relationship, and a child can be used by any number of parents.
Problem
With this structure, I get the following error when creating a new migration:
Unable to determine the principal end of an association between the
types 'MyClass' and 'MyClass'. The principal end of this association
must be explicitly configured using either the relationship fluent API
or data annotations.
This error makes sense - when given a structure of an object with a foreign key to itself, I am not surprised that EF has a hard time determining the principal end. I'm not sure how to fix this, though.
I've tried some different Fluent mappings:
modelBuilder.Entity<MyClass>().HasOptional(x => x.ChildOne).WithOptionalPrincipal(x => x.ChildOne);
modelBuilder.Entity<MyClass>().HasOptional(x => x.ChildOne).WithOptionalDependent(x => x.ChildOne);
modelBuilder.Entity<MyClass>().HasOptional(x => x.ChildOne);
(Note: I didn't try these concurrently - I did one at a time & duplicated it for ChildTwo.)
I was able to get a migration to work by adding a ChildThree property to MyClass, but that doesn't make sense and isn't a useful property; it just creates another foreign key on the table but this isn't needed in my model.
So, in summary:
How do I get this structure to work the way I want? I think the secret is in some Fluent mapping voodoo but I'm very unfamiliar with that library and I don't know how to get that to work.
Why does adding a third (unneeded, unwanted) property fix everything and allow the migration to scaffold?

Your fluent mapping is totally wrong.
You should do something like this:
modelBuilder.Entity<MyClass>().HasOptional(p => p.ChildOne).WithOptionalDependent();
modelBuilder.Entity<MyClass>().HasOptional(p => p.ChildTwo).WithOptionalDependent();

Unidirectional One to One relationship entity framework, cascade delete doesn't work

I want to implement a unidirectional one to one relationship; however on cascade delete doesn't work.
I have the following classes:
public class Student
{
public int Id { get; set; }
public string Name { get; set; }
public Address Address { get; set; }
}
public class Address
{
public int Id { get; set; }
public string Street { get; set; }
//I don't want the StudentId foreign key or the property of Student class here
}
In my Context class, I'm mapping the relationship like this:
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<Student>()
.HasRequired(s => s.Address)
.WithOptional()
.Map(m => m.MapKey("Address_Id"))
.WillCascadeOnDelete();
}
For some reason, it's not deleting the address when, student object is deleted.
Moreover, I also want to add the foreign key property (i.e. AddressId) in the Student class like this:
[ForeignKey("Address")]
[Column("Address_Id")]
public string AddressId { get; set; }
However, I get this error when I try to add a new migration:
Address_Id: Name: Each property name in a type must be unique. Property name 'Address_Id' is already defined.
I do believe I'm mixing things up (with MapKey and the attributes of AddressId). However, I don't know how to fix this.
I went through this SO question and this article; however, no luck so far.
Link to DotNetFiddle. It won't work cause there is no database.

For some reason, it's not deleting the address when, student object is deleted.
That's the normal behavior for the relationship you have defined. It's not a matter of data annotations or fluent configuration. If you have different expectations, you'd better revisit your model.
Every relationship has a one side called principal and another side called dependent. The principal side (a.k.a. master, primary) is the one being referenced. The dependent side (a.k.a. detail, secondary) is the one that is referencing the principal. The foreign key is put on the dependent side and must always point to an existing principal or null when the relationship is optional. The cascade delete works by deleing all the dependent records when the principal record is deleted.
As explained in the How Code First Determines the Principal and Dependent Ends in an Association? section of the article mentioned by you, EF always uses the required side as principal and allows you to chose the one only when both are required.
With all that being said, let see what you have.
Address is required, Student is optional. Also you want to put FK in Student, i.e. Student references Address.
All that means that in your relationship, Address is the principal and Student is the dependent. Which means the Address may exists w/o Student referencing it. If cascade delete is turned on (as you did), deleting the Address will delete the Student, not the opposite.
I think all that should explain why it's is working the way it is now, and that no attributes or configuration can help to achieve what are you asking for. If you want it differently, the same article (and related from the same series) explains how to configure the relationship to either use Shared Primary Key Association or Foreign Key Association at the Address side. Whether it is unidirectional or bidirectional absolutely has nothing in common with the problem - see Should We Make the Associations Bidirectional? section in the article.

You foreign key should be like this :
public class Student
{
public int Id { get; set; }
public string Name { get; set; }
[ForeignKey("AddressId")]
public Address Address { get; set; }
[Column("Address_Id")]
public int AddressId { get; set; }
}
In your fluent mapping you just need:
modelBuilder.Entity<Student>()
.HasRequired(s => s.Address)
.WillCascadeOnDelete(true);
Or you can force cascade delete with an annotation:
[Required]
[ForeignKey("AddressId")]
public Address Address { get; set; }
Now update your database and your mapping should be correct and the delete should cascade.

One-to-many with Join Table in Entity Framework 6

I realize this has been answered more than once, here for example, but nothing seems to be working for me and I'm not sure why.
My database has Sites and Users. A User might own a Site, or he might work at a Site. He might own Sites A and C, but work on Site B. However, each site has only one worker and one owner. Therefore, I have created a join table structure with the following three tables: User, Site, and User_Site, and User_Site contains a column called role that can be either "worker" or "owner".
To simplify things, I have created two views, view_Worker and view_Owner. view_Owner, for example, is
SELECT User_Site.site_id, User.*
FROM User_Site
JOIN User ON User_Site.user_id = User.user_id
WHERE User_Site.role = "owner"
Therefore, each row in view_Owner contains all of the information on User and the site_id for which the User is joined as an "owner".
Now, I'm trying to build an API around this database using Entity Framework 6. I've gotten a number of errors trying a number of different things, so I'm not sure which errors to post here. I'll post my most recent error:
dbContext.cs
public DbSet<User> Users { get; set; }
public DbSet<Site> Sites { get; set; }
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<Site>()
.HasOptional<User>(s => s.Owner)
.WithMany()
.Map(m =>
{
m.MapKey("site_id").ToTable("view_Owner");
});
}
user.cs
[Table("User")]
public class User
{
[Key, Column("user_id")]
public int ID { get; set; }
}
site.cs
[Table("Site")]
public class Site
{
[Key, Column("site_id")]
public int ID { get; set; }
public virtual User Owner { get; set; }
}
The error message I get with this configuration is
The specified table 'view_Owner' was not found in the model. Ensure
that the table name has been correctly specified.
So, the next step is to try and add this table to the model. I change [Table("User")] to [Table("view_Owner")] and I get the following error message:
(66,6) : error 3021: Problem in mapping fragments starting at line
66:Each of the following columns in table User is mapped to multiple
conceptual side properties: User.site_id is mapped to
(66,6) : error 3025: Problem in mapping fragments starting at line
66:Must specify mapping for all key properties (User.user_id) of table
User.
Clarification
Ideally, I'd like a way to simply tell EntityFramework to load the Owner property on Site from the view_Owner table by joining view_Owner.site_id to the Site.site_id primary key. But, since the Owner property is still of type User, it doesn't need a new definition.