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I have the following situation: I have a class Company, it is as simple as possible

public class Company

{

    public int ID { get; set; } 



    public string Name { get; set; }

}

I also need to extend the IdentityUser class from ASP. NET Identity Core:

public class User : IdentityUser

{

    public Company Company { get; set; }



    public int CompanyID { get; set; }

}

Not bad so far. But I also need a class Device :

public class Device

{

    public Company Company { get; set; }



    public int CompanyID { get; set; }

}

Accordingly, we add the Devices property of the List<Device> type to the Company class and Users property of the List<User> type.

The problem is that I want to separate the data (Device table) from the data of ASP .NET Identity Core in different databases. Table Company I want to put in the database with Auth data. And table Device in other one. How can I tell the Entity Framework that the Company property of the Device class is from another database?

If the Company property is from another database, you may need to add a second database context for it, if you haven't already. Something like this:

public class CompanyModelContext : DbContext

{

    public CompanyModelContext(DbContextOptions options) : base(options){ }

    public DbSet<Company> Company { get; set; }

}

You can try to set your EF model like this:

Companies table:

[Table("Companies")]

public class Company

{

    [Key]

    public int ID { get; set; } 



    public string Name { get; set; }



    public ICollection<Device> Devices { get; set; }

}

Devices table:

[Table("Devices")]

public class Device

{

    [Key]

    public int Id { get; set; }



    public Company Company { get; set; }



    public int CompanyID { get; set; }

}

ApplicationDbContext class:

public DbSet<Company> Companies { get; set; }



public DbSet<Device> Devices { get; set; }



protected override void OnModelCreating(ModelBuilder builder)

{

    base.OnModelCreating(builder);



    builder.Entity<Company>(buildAction =>

    {

        buildAction.ToTable("Companies");



        buildAction.HasMany(x => x.Devices)

                   .WithOne(x => x.Company)

                   .OnDelete(DeleteBehavior.Cascade);

    };

}

I've added Devices as a list in Company class, then overriding the FK in the method OnModelCreating.

Once you delete an item in table Company, all items with the same Id in table Devices (comparing to CompanyID) will be delted automatically.

Since we use the 2 properties [Table] and [Key] to define table name and the primary key. So in the OnModelCreating, you can ignore to define the name, PK, FK of table Device.

Company has many User

In this case:

User class:

public class User : IdentityUser

{

    public int CompanyId { get; set; }



    public Company Company { get; set; }

}

Company class:

[Table("Companies")]

public class Company

{

    [Key]

    public int ID { get; set; } 



    public string Name { get; set; }



    public ICollection<Device> Devices { get; set; }



    public ICollection<User> Users { get; set; }

}

ApplicationDbContext class:

protected override void OnModelCreating(ModelBuilder builder)

{

    base.OnModelCreating(builder);



    builder.Entity<Company>(buildAction =>

    {

        buildAction.ToTable("Companies");



        buildAction.HasMany(x => x.Devices)

                   .WithOne(x => x.Company)

                   .OnDelete(DeleteBehavior.Cascade);



        buildAction.HasMany(x => x.Users)

                   .WithOne(x => x.Company)

                   .OnDelete(DeleteBehavior.Cascade);

    };

}