指向实体框架中相同表的多个外键是无法区分的

Question

I'm unable to distinguish between foreign keys when two or more point to the same table through the database context.

For example, if I have a db with two tables, Users and Invoices.

Users has two fields: userId and userName
Invoices has three fields: invoiceNumber, cashierUserId, supervisorUserId

cashierUserId and supervisorUserId are foreign keys pointing to userId in the Users table.

Now in entity framework via the db context when I try to get the cashier or supervisor's user name I can't tell which user name I'm going to get.

dbContext.Invoices.Single(i => i.invoiceNumber == 1).Users.userName;

Which user name do I get? The cashier's or the supervisor's? How do I tell it which one I want?

Answer 1

First off, your code has an issue. The "Users" collection wouldn't have a property called "userName". It should probably read:

dbContext.Invoices.Single(i => i.invoiceNumber == 1).Users.First().userName;

Obviously, .First() is not going to help with the "distinguishing" problem. For that, I see two solutions:

1. Modify your context so each navigation property has a special name ("Cashier", "Supervisor"). Then you would just write:

    dbContext.Invoices.Single(i => i.invoiceNumber == 1).Cashier.userName; 

2. Give your user a "type" field. This could be a string stored in the user table, or likely a foreign key into a "UserTypes" table (in a normalized database). Then your query becomes:

    dbContext.Invoices.Single(i => i.invoiceNumber == 1).Users.First(u => u.UserType == "Cashier").userName; 

Using a different table would change the predicate a bit, but you get the idea. To help guide your design, remember that EF isn't very different than standard SQL. How would you craft a standard SQL query to distinguish the two? You would base it on foreign key (the first option), or another field on the users table (the second option).