How to store many-to-many data?

Question

ASP.NET

Unable to save data in many-to-many relationship. There are three tables: Person, Project and ProjectPerson (an intermediate table).
Here is the implementation of the Project model:

public partial class Project
    {
        public Project()
        {
            ProjectPerson = new HashSet<ProjectPerson>();
        }
        public int Id { get; set; }
        public string Name { get; set; }
        public string Description { get; set; }
        public DateTime IncomeDate { get; set; }
        public int? LeaderId { get; set; }
        public Person Leader { get; set; }
        public ICollection<ProjectPerson> ProjectPerson { get; set; }
    }
}

Implementation of the Person model:

public partial class Person
    {
        public Person()
        {
            Project = new HashSet<Project>();
            ProjectPerson = new HashSet<ProjectPerson>();
        }

        public int Id { get; set; }
        public string FullName { get; set; }
        public string PhoneNumber { get; set; }
        public string Email { get; set; }
        public string Vk { get; set; }
        public string Instagram { get; set; }
        public string Facebook { get; set; }
        public string Twitter { get; set; }
        public ICollection<Project> Project { get; set; }
        public ICollection<ProjectPerson> ProjectPerson { get; set; }
    }
}

Implementation of the ProjectPerson model:

public partial class ProjectPerson
    {
        public int ProjectId { get; set; }
        public int PersonId { get; set; }
        public Person Person { get; set; }
        public Project Project { get; set; }
    }

Optional DBcontext:

modelBuilder.Entity<Person>(entity =>
            {
                entity.ToTable("person");

                entity.Property(e => e.Id).HasColumnName("ID");

                entity.Property(e => e.Email)
                    .HasColumnName("email")
                    .HasMaxLength(200);

                entity.Property(e => e.Facebook)
                    .HasColumnName("facebook")
                    .HasMaxLength(200);

                entity.Property(e => e.FullName)
                    .IsRequired()
                    .HasColumnName("full_name")
                    .HasMaxLength(200);

                entity.Property(e => e.Instagram)
                    .HasColumnName("instagram")
                    .HasMaxLength(200);

                entity.Property(e => e.PhoneNumber)
                    .HasColumnName("phone_number")
                    .HasMaxLength(50);

                entity.Property(e => e.Twitter)
                    .HasColumnName("twitter")
                    .HasMaxLength(200);

                entity.Property(e => e.Vk)
                    .HasColumnName("vk")
                    .HasMaxLength(200);
            });
 modelBuilder.Entity<Project>(entity =>
            {
                entity.ToTable("project");

                entity.Property(e => e.Id).HasColumnName("id");

                entity.Property(e => e.Description)
                    .IsRequired()
                    .HasColumnName("description")
                    .HasColumnType("text");

                entity.Property(e => e.IncomeDate)
                    .HasColumnName("incomeDate")
                    .HasColumnType("date");

                entity.Property(e => e.LeaderId).HasColumnName("leader_id");

                entity.Property(e => e.Name)
                    .IsRequired()
                    .HasColumnName("name")
                    .HasColumnType("nchar(50)");

                entity.HasOne(d => d.Leader)
                    .WithMany(p => p.Project)
                    .HasForeignKey(d => d.LeaderId)
                    .HasConstraintName("FK_project_person");
            });
modelBuilder.Entity<ProjectPerson>(entity =>
            {
                entity.HasKey(e => new { e.ProjectId, e.PersonId });

                entity.ToTable("project_person");

                entity.Property(e => e.ProjectId).HasColumnName("project_id");

                entity.Property(e => e.PersonId).HasColumnName("person_id");

                entity.HasOne(d => d.Person)
                    .WithMany(p => p.ProjectPerson)
                    .HasForeignKey(d => d.PersonId)
                    .OnDelete(DeleteBehavior.ClientSetNull)
                    .HasConstraintName("FK_project_person_person");

                entity.HasOne(d => d.Project)
                    .WithMany(p => p.ProjectPerson)
                    .HasForeignKey(d => d.ProjectId)
                    .OnDelete(DeleteBehavior.ClientSetNull)
                    .HasConstraintName("FK_project_person_project");
            });

Here is the controller:

[HttpGet]
        public IActionResult Create()
        {
            ViewBag.Persons = new SelectList(_context.Person.ToList(), "Id", "FullName");
            return View();
        }
[HttpPost]
        public IActionResult Create(Project project)
        {
            _context.Project.Add(project);
            _context.SaveChanges();
            return RedirectToAction("Index");
        }

And here is the view:

<form class="form" asp-action="Create" asp-controller="Project" method="post" enctype="multipart/form-data">
                    <div class="form-group row">
                        <label class="col-md-12" asp-for="Name">Название проекта</label>
                        <div class="col-md-12">
                            <input type="text" class="form-control"  asp-for="Name">
                        </div>
                    </div>
                    <div class="form-group row">
                        <label class="col-md-12" asp-for="Description">Описание</label>
                        <div class="col-md-12">
                            <textarea class="form-control" rows="4" asp-for="Description"></textarea>
                        </div>
                    </div>
                    <div class="form-group row">
                        <label class="col-sm-12" asp-for="LeaderId">Лидер</label>
                        <div class="col-sm-12">
                            <select class="custom-select col-12" id="inlineFormCustomSelect" asp-for="LeaderId" asp-items="ViewBag.Persons"><option selected="selected" disabled="disabled">Выберите руководителя</option></select>
                        </div>
                    </div>

                    <div class="form-group row">
                        <label class="col-sm-12" asp-for="ProjectPerson">Команда</label>
                        <div class="col-sm-12">
                            <select class="custom-select col-12" id="inlineFormCustomSelect" asp-for="ProjectPerson" asp-items="ViewBag.Persons" multiple><option selected="selected" disabled="disabled">Выберите руководителя</option></select>
                        </div>
                    </div>
                   <input type="submit" value="Добавить" class="btn btn-common">
                </form>

I want to note that the models and the context were not written by me, they are created by themselves from the MSQL database

Answer 1

In your case - you just need to add a ProjectPerson with appropriate values ​​to project or person. And save.
In general, you need to remove it and configure the model like this:

modelBuilder.Entity<Project>()
                .HasMany<Person>(s => s.Persons)
                .WithMany(c => c.Projects)
                .Map(cs =>
                        {
                            cs.MapLeftKey("project_id");
                            cs.MapRightKey("person_id");
                            cs.ToTable("project_person");
                        });

Additionally:

• A general example is www.entityframeworktutorial.net/code-first/configu...
  
• Approximately your case, only Code First: https://stackoverflow.com/questions/7050404/create...
  
• Approximately your case: https://stackoverflow.com/questions/35527175/entit...
  

Important: All code examples are for EF6 only, EF Core cannot work without an intermediate entity.

Answer 2

EF itself creates intermediate tables in a many-to-many relationship. You just need to specify this relationship between the Project and Person tables. There is no need to make them by hand.