DBDesignEF.cs

using Microsoft.EntityFrameworkCore;
using DBDesign.Model;

namespace DBDesign
{

    public class DBDesignEF : DbContext
    {
        public DbSet<User> Users { get; set; }
        public DbSet<Friend> Friends { get; set; }
        public DbSet<UserMessage> UserMessages { get; set; }
        public DbSet<Group> Groups { get; set; }
        public DbSet<UserGroup> UserGroups { get; set; }
        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            const string _connectionString = "Your connection string here";
            optionsBuilder.UseMySql(_connectionString, ServerVersion.AutoDetect(_connectionString));
        }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Friend>()
                .HasKey(f => new { f.User1Id, f.User2Id });

            modelBuilder.Entity<UserGroup>()
                .HasKey(ug => new { ug.UserId, ug.GroupId });
            modelBuilder.Entity<UserGroup>()
                .HasOne(ug => ug.User)
                .WithMany(u => u.UserGroups)
                .HasForeignKey(ug => ug.UserId);
            modelBuilder.Entity<UserGroup>()
                .HasOne(ug => ug.Group)
                .WithMany(g => g.UserGroups)
                .HasForeignKey(ug => ug.GroupId);

        }
    }
}