如何在Entity Framework中使用动态DbSet？

Question

coluld you please be so kind to tell me how do I choose DbSet depending on string variable? What I have is the following:

public class DataContext : DbContext
{
    public DataContext() : base("myDb") { }

    public DbSet<Entry> RurEntries { get; set; }
    public DbSet<Entry> UsdEntries { get; set; }
    public DbSet<Entry> EurEntries { get; set; }
}

There are 3 tables for each currency: Rur,Usd,Eur. All have same structure. There is string variable named CurrentCurrency which is changed from UI and may be one of 3 currencies. In my previous code without Entity Framework I had code that read db with pure sql, someting like:

string sqlQuery = "Select * from " + CurrentCurrency 

Now I decided to rewrite code with Entity Framework and faced that problem. Any answer will be appreciated. Thanks in advance.

Answer 1

You can simply switch on your CurrentCurrency string to get set that you need:

 var db = new DataContext();
        var CurrentCurrency = "RUR";
        DbSet<Entry> set = null;
        switch (CurrentCurrency) {
            case "RUR":
                set = db.RurEntries;
            break;
            case "EUR":
                set = db.EurEntries;
            break;
            case "USD":
                set = db.UsdEntries;
            break;
            default:
                throw new Exception();
        }
        var res = set.ToList();

Answer 2

You can only use an entity class T in a DbSet<T> once per DbContext. Your code won't run. See also Entity Framework 6 Creating Two table from the same entity object .

Given your comment:

All 3 tables have unique_id field, which I receive from another software. I used unique flag on that column and it might be a problem if i put all entries to the same table

You just need a composite primary key, comprised of Currency and ExternalId, as explained in Composite Key with EF 4.1 Code First :

public class Entry
{
    [Key]
    [Column(Order = 0)]
    public string Currency { get; set; }

    [Key]
    [Column(Order = 1)]
    public string ExternalId { get; set; }
}

Then you can read the "EUR" rows like this:

var eurRows = dbContext.Entries.Where(e => e.Currency == "EUR");

Answer 3

You can execute raw SQL queries in Entity Framework. Like this:

var curs = context.Database.SqlQuery<Entry>("Select * from " + CurrentCurrency").ToList();

You can also make that more neat and create a stored procedure, where you send a parameter and execute a SELECT statement based on that parameter. Then using Entity framework, you call that procedure an map the results to a List<Entry> .

However, and as I stated in the comments I do personally prefer to have only one table with a CurrenyCode column. Instead of having 3 tables with the exact same structure but with different data. Then you can query like this:

var curs = var curs = context.Currencies.Where(x=> x.CurrencyCode = CurrentCurrency)
                                        .ToList();