在 LINQ Entity Framework asp.net MVC 5 中切换数据库上下文

Question

In laravel they have eloquent that can change between databases.

For example:

$db = (statement) ? "mysql1" : "mysql2" ;

$users = DB::connection($db)   
              ->select(...);

How do you do you do this in LINQ?

Model data =  (from table in context_db <--- how to i dynamically change this?? 
               .Model
               where table.ref_no == ref_no
               select new Model
                      {
                             ...
                      });

my web.config contextdb

 <add name="DBContext1" connectionString=" ... / >
 <add name="DBContext2" connectionString=" ... / >   

Answer 1

You should register your another DbContext in services, in ConfigureServices method in Startup.cs and pass it to your constructor of your class like:

Startup.cs

services.AddDbContext<YourDbContextA>(options =>
                options.UseSqlServer(
                Configuration.GetConnectionString("YourConnectionString")));

YourClass.cs (or YourClass.cshtml.cs)

public class YourClass
{
        private readonly YourDbContext _context;

        public CreateModel(YourDbContext context)
        {
            _context = context;
        }
 }

You can register multiple DbContext objects in services and can pass multiple DbContext objects to your constructor .

Later you can switch between your dbContextes:

 private DbContext getDbContext(bool statement)
 {
     if(statement)
       return _dbcontext1;
     else if(statement2)
       return _dbcontext2;
     else
       return _someOtherDbContext;
 }



private void foo()
{
 Model data =  (from table in getDbContext(statement)
               where table.ref_no == ref_no
               select new Model
                      {
                             ...
                      });
}

Answer 2

You can do it by editing your ContextDB constructor.

first you have to set a connection string on your web.config

 <add name="DBContext1" connectionString=" ... / >
 <add name="DBContext2" connectionString=" ... / >   

second on your contextdb constructor add a parameter for your connection.

public partial class YourContextDB: DbContext
{
        public YourContextDB(string connection = "name=DBContext1")
            : base(connection )
        {
        }
        ... //basically ur models below
}

3rd you can now do this.

string connection = (statement) ? "name=DBContext2" : "name=DBContext1";
YourContextDB dbcontext = new YourContextDB(connection); //default is "name=DBContext1"

Model data =  (from table in dbcontext <--- dynamic dbcontext
               .Model
               where table.ref_no == ref_no
               select new Model
               {
                ...
               });

NOTE: you cannot use code-first Update-Database with this solution.