How do I implement DbContext inheritance for multiple databases in EF7 / .NET Core

I would like to bring this post from the OP’s GitHub issue to everyone’s attention:

I was able to resolve this without a hack by providing a protected constructor that uses DbContextOptions without any type. Making the second constructor protected ensures that it will not get used by DI.

public class MainDbContext : DbContext {
    public MainDbContext(DbContextOptions<MainDbContext> options)
        : base(options) {
    }

    protected MainDbContext(DbContextOptions options)
        : base(options) {
    }
}

public class SubDbContext : MainDbContext {
    public SubDbContext (DbContextOptions<SubDbContext> options)
        : base(options) {
    }
}