.NET Core EF如何从模型生成表？添加DbSet无效问题咨询

Hey there! Let's break down your questions about EF Core Code First and how migrations work, since you're just getting started with .NET Core:

1. How does Update-Database know which models to sync to the database? #

EF Core identifies models to include in your database through a few key mechanisms:

• Explicit DbSet<T> declarations: Any model you add as a DbSet<T> property in your DbContext (like your MvcMovieContext) is automatically recognized.
• Model configuration in OnModelCreating: Even if you don't add a DbSet<T>, if you configure the model using modelBuilder.Entity<YourModel>() inside the OnModelCreating method of your DbContext, EF Core will include it.
• Automatic discovery: If your model classes are in the same assembly as your DbContext (or you've configured EF to scan other assemblies), EF Core will detect classes with a primary key (usually a property named Id or annotated with [Key]) as entities to include.

If adding a DbSet<T> didn't work for you, here are common fixes:

• Double-check that your model has a primary key (EF Core requires this to track entities).
• Make sure you've added the correct using statement for your model's namespace in your DbContext file.
• You need to generate a new migration (via Add-Migration <MigrationName>) after modifying your model or DbContext—Update-Database only applies existing migration files, not your current code directly.

2. Are Migrations the key here? What's the exact mechanism? #

Yes, the Migrations folder is central to how EF Core syncs your model to the database. Here's the step-by-step flow:

1. Snapshot creation: When you run Add-Migration, EF Core creates a snapshot of your current model state in YourContextModelSnapshot.cs (inside the Migrations folder). This snapshot acts as a reference point for future migrations.
2. Migration generation: EF Core compares the current model (from your code) to the last snapshot. It generates a new migration file with Up() and Down() methods that contain the SQL needed to apply or revert the changes (like creating a table, adding a column, etc.).
3. Applying migrations: Update-Database executes all pending migration files (those not yet applied to the database) in order. It also keeps track of which migrations have been applied in the __EFMigrationsHistory table in your database.

So Update-Database doesn't read your current model directly—it relies on the migration files that were generated when you ran Add-Migration.

3. Is scaffolding required? Is there a list to specify which models go into the database? #

Scaffolding is completely optional—it's just a tool to quickly generate controllers and views for CRUD operations. You don't need it at all if you don't need a UI for your database tables.

To specify which models are included in the database, you have these options (no "magic list" exists, but these are the explicit ways):

• Add a DbSet<T> property to your DbContext for each model you want to include (this is the most common and readable approach).
• Configure the model in OnModelCreating with modelBuilder.Entity<YourModel>()—this works even if you don't have a DbSet<T> for it.
• Use assembly scanning: You can configure your DbContext to scan specific assemblies for model classes (useful if your models are in a separate project) by specifying the assembly in OnModelCreating or via context options.

内容的提问来源于stack exchange，提问作者aykl104001