Creating a new entity with reference to an entity from another context - entity-framework

Question

I have 2 projetcs PRJ1 and PRJ2 which uses their own databases DB1 and DB2. Each of these databases uses EF Code First Migration.
The PRJ1 is for managing stock of products (already exists since 4 years).
The PRJ2 is for orders (brand new project still in dev)
Now let's talk about the second project only. In my project PRJ2 I need to access data from the other database DB1. So I need to place orders for products.
Here is what I got so far for PRJ2
Note that I defined 2 different contexts.
// Context for accessing entities in DB1
public class DB1Context : DbContext
{
static DB1Context()
{
Database.SetInitializer<DB1Context>(null);
}
public DbSet<Product> Products { get; set; }
}
// Context for accessing entities in DB2
public class DB2Context : DbContext
{
static DB2Context()
{
Database.SetInitializer(new MigrateDatabaseToLatestVersion<DB2Context, DAL.Migrations.Configuration>());
}
public DbSet<Anything> Anythings { get; set; }
public DbSet<Order> Orders { get; set; }
}
What works: I can query data from DB1Context (Products) or DB2Context (Anythings).
What didn't works yet: Creating my Orders entity.
// My Orders entity
public class Orders
{
public int Id { get; set; }
public int ProductId { get; set; }
public int Quantity { get; set; }
public virtual Product Product { get; set; }
}
This entity located in DB2Context is referencing the Product entity which is part of DB1Context.
Now the problem is that as soon as I add this Entity inside my context DbSet<Order> I see that there is a migration waiting for Product. This migration is for creating Product in my DB2Context. That's not what I want. This entity already exists in DB1Context. It seems I cannot create this Order entity which is referencing Product from the other context.
Can you confirm this ? Did I missed something ? Otherwise what is the best alternative ?

Answer 1

I think you can't do that using Entity Framework.
That problem looks like a No-SQL database's problem. When you have multiple database like that you have to control all the CRUD for all databases. The ADO can't do it to you because you don't have data integrity.
A possible solution is you put your CRUD in your business logic layer.. or something like that.
Let suppose you have a OrderBll to control:
public class OrderBll
{
private DB1Context _DB1Context = new DB1Context();
private DB2Context _DB2Context = new DB2Context();
public List<Orders> GetOrders()
{
var orders = _DB2Context.Orders.Where(???).ToList();
var productIds = orders.Select(x => x.ProductId).Distinct().ToArray();
var products = _DB1Context.Products.Where(x => productIds.Contains(x.Id)).ToList(); // Optimize the load of all products in orders
// Set the product object in the order list
foreach( var order in orders )
{
order.Product = products.FirstOrDefault(x=>x.Id == order.ProductId);
}
return orders;
}
}
Remember you have to map the Product property in Orders like Ignore.
So, you have to set the Foreign Key by yourself and do all the constraints checks.
But, if someone else have a better solution to do that, I'll be glad to know.

Answer 2

Ideally, this is what you should aim for.
Put orders and products in the same database. You would then be able to create relations between orders and products. You would end up with a single EF context, and this would give you a solid solution.
If for whatever reasons you cannot put orders in the same database as products, then solutions are still available with their own limitations.
You may replicate the products table from DB1 to DB2, replication running every minute, for example. You can write your own replication component or use replication functionality of your database. If products can be deleted in DB2, replication could delete the products and orders in DB1, or just flagged deleted products. It is up to you to decide. If PRJ2 can update the products table, then replication has to be both ways. This gets more complicated. The EF context with this solution would contain relations from orders to products, and products to orders.
Another solution would be to keep in DB1 a "proxy" products table that contains the Id's of the products that are referenced in orders. Everywhere in your business logic, you decide whether you need to access the actual products table from DB1 or not. For example, when creating a new order, you would access products from DB1, and insert their Id's in the proxy table if not there. When displaying the products of an order, you would first retrieve the product Id's of the order from DB1, and then their full description from DB1. When updating the inventory of a product, as part of creating an order, you would access DB1, probably with a transaction that spans DB! and DB2.