DataInitializer.cs

using BooksWebApi.Entities;
using Microsoft.AspNetCore.Identity;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace BooksWebApi.Data
{
  public static class DataInitializer
  {
    static Category _category = new Category { Description = "Science", Books = new List<Book>() };
    static List<Book> _sample = new List<Book>
    {
        new Book()
        {
          Name = "A science book",
          AuthorName = "A good author",
          Edition = 1,
          PublicationDate = new DateTime(2010, 5 , 1),
          ISDN = "1111111111111111",
          Category = _category
        }
    };

    internal static void Seed(UserManager<User> userMgr, RoleManager<IdentityRole> roleMgr, BooksCatalogDbContext ctx)
    {
      ctx.Database.EnsureCreated();
      if (userMgr.FindByNameAsync("myapiuser").Result == null)
      {
        if (!roleMgr.RoleExistsAsync("Administrator").Result)
        {
           IdentityRole role = new IdentityRole();
           role.Name = "Administrator";
           IdentityResult result = roleMgr.CreateAsync(role).Result;
        }
        
        var user = new User()
        {
        UserName = "myapiuser",
        FirstName = "myapiuser",
        LastName = "webapi",
        Email = "gmartinezsan@gmail.com"
        };
        
        var userResult =  userMgr.CreateAsync(user, "MyP4ssw0rd!").Result;
        if (userResult.Succeeded)
        {
          userMgr.AddToRoleAsync(user, "Administrator").Wait();
        }
        else
        {
          throw new InvalidOperationException("Failed to build user or role");
        }
        ctx.SaveChanges();
      }
      
      if (!ctx.Books.Any() && !ctx.Categories.Any())
      {
        ctx.Add(_category);
        ctx.AddRange(_sample);
        ctx.SaveChanges();
      }
     
    }
  }
}