c# - How to have Many to Many Association in Entity Framework Code First

Question

I am just getting started with EF and I watched some great tutorial videos. I am stuck with the following.

I have a class for a collection of files, I would like these to be tied to events and/or people

public class file{
    public int id {get;set;}
    public string path {get;set;}
}

public event {
    public int id {get;set;}
    public string eventname {get;set}
    public virtual ICollection<file> files {get;set;}
    public event(){ files = new list<file>();}
}

public person {
    public int id {get;set;}
    public string name {get;set}
    public virtual ICollection<file> files {get;set;}
    public person(){ files = new list<file>();}
}

Now when I generate the database my file table has a PersonID AND EventID.

I want to be able to let users attach files to people and/or events.

Answer 1

What you are getting is the default behavior of EF Code First in terms of mapping a 1 to Many association to the database. For example you have a ICollection<File> on Person class, as a result, EF will create a FK on Files table (PersonId) and map it to Id PK on Persons table.

Now, my guess is that you like to have a many to many relationship between File and Person, so that each file can relates to many Persons and each Person can have many files (same story for Event object as well). One way to achieve this is to put navigation properties on File class pointing to Event and Person classes. So, your model should be changed to this:

public class File {
    public int Id { get; set; }
    public string Path { get; set; }
    public virtual ICollection<Event> Events { get; set; }
    public virtual ICollection<Person> Persons { get; set; }
}

public class Event {
    public int Id { get; set; }
    public string EventName { get; set; }
    public virtual ICollection<File> Files {get;set;}
}

public class Person {
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual ICollection<File> Files { get; set; }
}

public class MyContext : DbContext {
    public DbSet<Person> Persons { get; set; }
    public DbSet<Event> Events { get; set; }
    public DbSet<File> Files { get; set; }
}

As a result, EF will create link tables (Events_Files and Files_Persons) to map these many to many associations to the database.

Update:
When using POCOs with EF, if you mark your navigation properties as virtual you will opt-in to some of the additional EF supports like Lazy Loading and Relationship Fixup. So in general have a virtual keyword in navigation properties considered to be a good practice.