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{
}
protected override void OnModelCreating(ModelBuilder builder)
{
base.OnModelCreating(builder);
// ...
}
}
Add a DbSet property to the ApplicationDbContext , right below the
constructor:
public ApplicationDbContext(
DbContextOptions<ApplicationDbContext> options)
: base(options)
{
}
public DbSet<TodoItem> Items { get; set; }
// ...
51
Update the context
A DbSet represents a table or collection in the database. By creating a
DbSet<TodoItem> property called Items , you're telling Entity
Framework Core that you want to store TodoItem entities in a table
called Items .
You've updated the context class, but now there's one small problem: the
context and database are now out of sync, because there isn't actually an
Items table in the database. (Just updating the code of the context class
doesn't change the database itself.)
In order to update the database to reflect the change you just made to
the context, you need to create a migration.
If you already have an existing database, search the web for
"scaffold-dbcontext existing database" and read Microsoft's
documentation on using the Scaffold-DbContext tool to reverse-
engineer your database structure into the proper DbContext and
model classes automatically.
52
Create a migration
Create a migration
Migrations keep track of changes to the database structure over time.
They make it possible to undo (roll back) a set of changes, or create a
second database with the same structure as the first. With migrations,
you have a full history of modifications like adding or removing columns
(and entire tables).
In the previous chapter, you added an Items set to the context. Since
the context now includes a set (or table) that doesn't exist in the
database, you need to create a migration to update the database:
dotnet ef migrations add AddItems
This creates a new migration called AddItems by examining any changes
you've made to the context.
If you get an error like No executable found matching command
"dotnet-ef" , make sure you're in the right directory. These