Performing projections on multiple databases with only one dbContext

Question

We are using EF.Core for a current project. The project has three databases which is a real nuisance that can\'t really be avoided. The databases have the same structure. So we

Answer 1

So the questioner (P.Joe) answers his own question here (https://stackoverflow.com/a/43326729/861352), but I can't actually get that answer to work. The reason it doesn't work is

[Table("dbname.scheme.Stock", Schema = "dbo")]

will get rendered in the sql as

[dbo].[dbname.scheme.Stock]

when what you actually want in the SQL is

[dbname].[dbo].[Stock]

Even if you try and mess around with it wont work. I tried various combinations

[Table("Stock", Schema = "dbname.dbo")] // SQL => [dbname.dbo].[Stock]
[Table("Stock", Schema = "[dbname].[dbo]")] // SQL => [[dbname]].[dbo]]].[Stock]
[Table("Stock", Schema = "dbname].[dbo")] // SQL => [dbname]].[dbo].[Stock]
[Table("Stock", Schema = "dbname.[dbo")] // SQL => [dbname.[dbo].[Stock]

...etc. The basic problem is that you need to separate the dbname with brackets that get escaped when it's parsed.

This actually appears to be a known issue, with a solution in the pipeline (although it hasn't been prioritised yet):

https://github.com/aspnet/EntityFrameworkCore/issues/4019

I did however find an interim solution to this problem, and it's based on two sources:

https://stackoverflow.com/a/26922902/861352 (EF6 solution) https://weblogs.asp.net/ricardoperes/interception-in-entity-framework-core

And here it is:

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How To Do (Same Server) Cross DB Joins With One EF Core DbContext

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You'll need to install the Microsoft.Extensions.DiagnosticAdapter Nuget Package

using System;
using System.Data.Common;
using Microsoft.EntityFrameworkCore.Diagnostics;
using Microsoft.Extensions.DiagnosticAdapter;

namespace Example
{
    public class CommandInterceptor
    {
        [DiagnosticName("Microsoft.EntityFrameworkCore.Database.Command.CommandExecuting")]
        public void OnCommandExecuting(DbCommand command, DbCommandMethod executeMethod, Guid commandId, Guid connectionId, bool async, DateTimeOffset startTime)
        {
            var secondaryDatabaseName = "MyOtherDatabase";
            var schemaName = "dbo";
            var tableName = "Stock";

            command.CommandText = command.CommandText.Replace($" [{tableName}]", $" [{schemaName}].[{tableName}]")
                                                     .Replace($" [{schemaName}].[{tableName}]", $" [{secondaryDatabaseName}].[{schemaName}].[{tableName}]");
        }
    }
}

Replace 'MyOtherDatabase', 'dbo' and 'Stock' with your Database name, table schema and table name, maybe from a config etc.

Then attach that interceptor to your context.

using System.Diagnostics;
using Microsoft.EntityFrameworkCore.Infrastructure;

var context = new MultipleDatabasesExampleDbContext(optionsBuilder.Options);

// Add interceptor to switch between databases
var listener = context.GetService();
(listener as DiagnosticListener).SubscribeWithAdapter(new CommandInterceptor());

In my case I put the above in MultipleDatabasesExampleDbContextFactory method.

Now you can just use the context as if you were referencing one database.

context.Customers // Default database defined in connection string
context.Stocks    // MyOtherDatabase (a different database on the same server)