Interview Question

Does Entity Framework Core support creating migrations for multiple databases?

Answer

Yes, EF Core supports creating and managing migrations for multiple databases. There are two main approaches:

1. Using Multiple DbContext Classes

// First DbContext for Orders database
public class OrdersContext : DbContext
{
    public OrdersContext(DbContextOptions<OrdersContext> options)
        : base(options) { }
        
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlServer("Server=.;Database=Orders;Trusted_Connection=True;");
    }
}

// Second DbContext for Products database
public class ProductsContext : DbContext
{
    public ProductsContext(DbContextOptions<ProductsContext> options)
        : base(options) { }
        
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlServer("Server=.;Database=Products;Trusted_Connection=True;");
    }
}

// Creating migrations for each context
// dotnet ef migrations add InitialCreate --context OrdersContext
// dotnet ef migrations add InitialCreate --context ProductsContext

2. Using Migration History Table

public class MultiDbContext : DbContext
{
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        // Configure based on environment/tenant
        var dbConnection = GetDatabaseConnection(); // Your logic here
        optionsBuilder.UseSqlServer(dbConnection);
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        // Specify different migration history table for each database
        modelBuilder.HasDefaultSchema("schema_name");
        modelBuilder.HasHistoryTable("__EFMigrationsHistory", "schema_name");
    }
}

Key Points 💡

• Each DbContext can target a different database
• Use --context flag to specify which context to use for migrations
• Migrations are stored separately for each context
• Can use different schemas within same database
• Migration history tables can be customized

Common Follow-up Questions

1. How do you handle transactions across multiple databases?
2. How do you ensure migration consistency across databases?
3. What are the best practices for managing multiple database connections?