Entity Framework 6 code-first migration - starting with the CreateDatabaseIfNotExists initializer - sql-server

Question

I have started an EF6 project to store measurement results from analytical instruments. Each instrument has a built-in PC with and it's own results database.
Initially, the database initializer CreateDatabaseIfNotExists was used. On database creation, it creates an entry in the __MigrationHistory table with a non-unique MigrationId entry (timestamp differs from instrument to instrument, e.g. 201706011336597_InitialCreate), the ContextKey if the fully qualified type of my derived DbContext.
After a while, it was decided to add more result data to the database... Furtunately, only three new tables are required. There are no changes in the existing tables.
For that, I wanted to use the MigrateDatabaseToLatestVersion initializer. But I have to support the following two scenarios:
Existing database with the non-unique MigrationId, that has to be migrated to the extended version with the three new tables.
No database, create the database with the the MigrateDatabaseToLatestVersion initializer.
How can I do this?
I have created an initial migration using the add-migration PM console command from the initial DbContext. That works well with scenario 2 (no database exists). From that starting point I can update my DbContext and create a new migration with the three new tables.
But how to support scenario 1? The Up() method of the initial migration contains the table creation code, that is not nessessary, because the tables already exist. Is an empty migration (add-migration -IgnoreChanges) helpful, maybe with a later timestamp than the initial migration?
Note: I have no access from the PM console to the target database(s), only on my developer machine to a test database.
Thanks and best regards
Karsten
Update:
I have modified the created initial migration with the static flag TablesAlreadyCreated.
public partial class InitialMigraCreate : DbMigration
{
/// <summary>
/// Set this field to true, if the tables are already created by the
/// CreateDatabaseIfNotExists database initializer. Then, the Up()
/// and Down() methods do nothing, but the
/// migration is added to the __MigrationHistory table.
/// </summary>
public static bool TablesAlreadyCreated = false;
public override void Up()
{
if (TablesAlreadyCreated)
return;
// several CreateTable calls here
}
/// <inheritdoc/>
public override void Down()
{
if (TablesAlreadyCreated)
return;
// several Drop... calls here
}
}
I have also implemented a new database initializer class as follows:
public class MigrateDatabaseToLatestVersionEx<TContext, TMigrationsConfiguration> : IDatabaseInitializer<TContext>
where TContext : DbContext
where TMigrationsConfiguration : DbMigrationsConfiguration<TContext>, new()
{
...
/// <inheritdoc />
public virtual void InitializeDatabase(TContext context)
{
if (context == null)
throw new ArgumentNullException("context");
// check whether a first migration exists from the CreateDatabaseIfNotExists database initializer
var firstConfig = new ConfigurationAutoCreatedDatabase();
firstConfig.TargetDatabase = _config.TargetDatabase;
var firstMigrator = new DbMigrator(firstConfig);
var firstDbMigrations = firstMigrator.GetDatabaseMigrations();
// create the default migrator with the current configuration
var migrator = new DbMigrator(_config);
if (1 == firstDbMigrations.Count(migra => migra.EndsWith("_InitialCreate", StringComparison.InvariantCultureIgnoreCase)))
{ // This database was created using the CreateDatabaseIfNotExists database initializer.
// That's an indication whether it's an old database
// Do the custom migration here!
InitialMigraCreate.TablesAlreadyCreated = true;
migrator.Update();
}
else
{ // do the default migration the database was created with this MigrateDatabaseToLatestVersionEx initializer
InitialMigraCreate.TablesAlreadyCreated = false;
migrator.Update();
}
}
}
It checks, whether the initial migration entry is from the CreateDatabaseIfNotExists initializer and disables the table creation/drop calls in the Up()/Down() methods in that case. ConfigurationAutoCreatedDatabase is a manually created derived DbMigrationsConfiguration class:
internal sealed class ConfigurationAutoCreatedDatabase : DbMigrationsConfiguration<MyNamespace.MyDbContext>
{
/// <summary>
/// Creates a <c>ConfigurationAutoCreated</c> object (default constructor).
/// </summary>
public ConfigurationAutoCreatedDatabase()
{
this.AutomaticMigrationsEnabled = false;
this.AutomaticMigrationDataLossAllowed = false;
this.ContextKey = "MyNamespace.MyDbContext";
}
}
So, it works for both scenarios. I hope that helps other guys with a similar problem. It would be interesting, if there is an out-of-the-box EF workflow for that task.

Answer 1

This is a very common scenario that EF handles nicely. The non-migration initializers (CreateDatabaseIfNotExists, etc) should be used in very early development (when you don't care about the data except for stuff that is seeded).
Once you are switching to migrations you should generate a baseline migration that takes a snapshot of your current model as you indicate (add-migration MyStartPoint -IgnoreChanges). This adds a migration with no Up() code and stores the current state of your code first model so that when you change the model only those changes are reflected. You could accomplish the same thing by commenting out the items that exist from the Up() code.
Now when you run against an existing database, it will check __MigrationHistory to see which migrations have been applied. If the database does not exist, it will be created. See here and here for more info.
Not sure what you are talking about with the MigrationId. EF handles that automatically unless you change your namespace (there is a workaround for that as well).