Error when adding migration to EntityFramework project - entity-framework

Question

Working on a .NETCore MVC back end that's using EntityFramework.
The Solution as a Service project, EntityFramework project, and an API (MVC) project. The API project is the envy point/startup.
It's an existing project, and there was an initial migration file which I used to setup the database from the command line using: dotnet ef database update.
Now, I've added a new model to the EntityFramework project, and added the appropriate public DbSet to the Context class. Now I'm trying to generate the update migration file for it so I can update the database.
Using dotnet ef migrations add migration_002 gave an error of:
Unable to create an object of type 'StartNetContext'. Add an
implementation of 'IDesignTimeDbContextFactory' to
the project, or see https://go.microsoft.com/fwlink/?linkid=851728 for
additional patterns supported at design time.
Searching for that turned up posts and blogs related to upgrades from EF 1 -> 2 ... not appropriate in this case. Or pointing to issues when creating the initial migration when there's also seeding happening... again, also not appropriate in this case. When I add the -v option, I get a bit more info:
Microsoft.EntityFrameworkCore.Design.OperationException: Unable to
create an object of type 'StartNetContext'. Add an implementation of
'IDesignTimeDbContextFactory' to the project, or see
https://go.microsoft.com/fwlink/?linkid=851728 for additional patterns
supported at design time. ---> System.MissingMethodException: No
parameterless constructor defined for this object.
Ugh. What? Why do I need that? Searching that points to re-arranging how the startup is configured... specifically making sure that the. AddDbContext is being called - which it is...
I feel like I'm going in circles... it should work... someone created the initial migration, so it had to have worked, right?
NOTE: I'm on a Mac, so there's no using the Package Manager console, so selecting target projects, etc isn't a solution either. This all needs to be done right from the CLI.

Answer 1

Add the following class in your main project.
public class DesignTimeDbContextFactory : IDesignTimeDbContextFactory<YourDbContext>
{
public YourDbContext CreateDbContext(string[] args)
{
IConfigurationRoot configuration = new ConfigurationBuilder()
.SetBasePath(Directory.GetCurrentDirectory())
.AddJsonFile("appsettings.json")
.Build();
DbContextOptionsBuilder<YourDbContext> builder = new DbContextOptionsBuilder<YourDbContext>();
string connectionString = configuration.GetConnectionString("YourDbContextConnectionName");
builder.UseSqlServer(connectionString);
return new YourDbContext(builder.Options);
}
}
Hope your error will go away!

Answer 2

So it turns out in my case it was a simple flag on the CLI... Despite having the web api project marked as the startup project in the solution, I needed to specify it on the command line as well.
dotnet ef migrations add migration_002 -s ../MyWebApp.Api
I got that from this post.
Adding the -s flag worked, even without the code and adding the class (which I did try first, but ran into problems with the return statement).
It's the simple things in life.