Migration failed while trying to create a many to many relationship - entity-framework-core

Question

I am trying to connect two tables with a code first migration. I thought EF would create many to many relationship table itself but I get error "build failed". While building whole project everything works fine. It's just the migration.
Following are my models -
Task:
[Key]
public int Id { get; set; }
public DateTime? CreatedAt { get; set; }
public DateTime? EndedAt { get; set; }
[Column(TypeName = "text")]
public string CreatedBy { get; set; }
[Required]
[Column(TypeName = "text")]
public string Title { get; set; }
[Required]
[Column(TypeName = "text")]
public string Description { get; set; }
public virtual TaskGroups TaskGroup { get; set; }
public string Status { get; set; }
[Column(TypeName = "text")]
public string WantedUser { get; set; }
TaskGroup:
[Required]
public int Id { get; set; }
[Required]
public string GroupName { get; set; }
public virtual Tasks Tasks { get; set; }
At first I've tried with ICollection<> but I got the same error.
My project is .Net Core 3.
Any ideas?
Edit
Tasks
[Key]
public int Id { get; set; }
public DateTime? CreatedAt { get; set; }
public DateTime? EndedAt { get; set; }
[Column(TypeName = "text")]
public string CreatedBy { get; set; }
[Required]
[Column(TypeName = "text")]
public string Title { get; set; }
[Required]
[Column(TypeName = "text")]
public string Description { get; set; }
public string Status { get; set; }
[Column(TypeName = "text")]
public string WantedUser { get; set; }
public IList<TaskGroupTask> TaskGroupTask { get; set; }
TaskGroups
[Key]
public int Id { get; set; }
[Required]
public string GroupName { get; set; }
public IList<TaskGroupTask> { get; set; }
TaskGroupTask
public int TaskId { get; set; }
public int TaskGroupId { get; set; }
public Tasks Tasks { get; set; }
public TaskGroups TaskGroups { get; set; }
DbContext
protected override void OnModelCreating(ModelBuilder builder)
{
base.OnModelCreating(builder);
builder.Entity<TaskGroupTask>(e =>
{
e.HasKey(p => new { p.TaskId, p.TaskGroupId });
e.HasOne(p => p.Tasks).WithMany(t =>
t.TaskGroupTask).HasForeignKey(p => p.TaskId);
e.HasOne(p => p.TaskGroups).WithMany(tg =>
tg.TaskGroupTask).HasForeignKey(p => p.TaskGroupId);
});
}
public DbSet<TaskGroupTask> TaskGroupTask { get; set; }

Answer 1

You will need to create a joining entity, like MyJoiningEntity or TaskGroupTask, whose sole purpose is to create a link between Task and TaskGroup. Following models should give you the idea -
public class Task
{
public int Id { get; set; }
public string Description { get; set; }
public IList<JoiningEntity> JoiningEntities { get; set; }
}
public class TaskGroup
{
public int Id { get; set; }
public string GroupName { get; set; }
public IList<JoiningEntity> JoiningEntities { get; set; }
}
// this is the Joining Entity that you need to create
public class JoiningEntity
{
public int TaskId { get; set; }
public int TaskGroupId { get; set; }
public Task Task { get; set; }
public TaskGroup TaskGroup { get; set; }
}
Then you can configure the relation in the OnModelCreating method of your DbContext class, like -
modelBuilder.Entity<JoiningEntity>(e =>
{
e.HasKey(p => new { p.TaskId, p.TaskGroupId });
e.HasOne(p => p.Task).WithMany(t => t.JoiningEntities).HasForeignKey(p => p.TaskId);
e.HasOne(p => p.TaskGroup).WithMany(tg => tg.JoiningEntities).HasForeignKey(p => p.TaskGroupId);
});
This will define a composite primary key on JoiningEntity table based on the TaskId and TaskGroupId properties. Since this table's sole purpose is to link two other tables, it doesn't actually need it's very own Id field for primary key.
Note: This approach is for EF versions less than 5.0. From EF 5.0 you can create a many-to-many relationship in a more transparent way.

Answer 2

Since I have some time, I've decided to pull all pices of the code in one place. I think it would be very usefull sample how to create code-first many-to-many relations for database tables. This code was tested in Visual Studio and a new database was created without any warnings:
public class Task
{
public Task()
{
TaskTaskGroups = new HashSet<TaskTaskGroup>();
}
[Key]
public int Id { get; set; }
public DateTime? CreatedAt { get; set; }
public DateTime? EndedAt { get; set; }
[Column(TypeName = "text")]
public string CreatedBy { get; set; }
[Required]
[Column(TypeName = "text")]
public string Title { get; set; }
[Required]
[Column(TypeName = "text")]
public string Description { get; set; }
public string Status { get; set; }
[Column(TypeName = "text")]
public string WantedUser { get; set; }
[InverseProperty(nameof(TaskTaskGroup.Task))]
public virtual ICollection<TaskTaskGroup> TaskTaskGroups { get; set; }
}
public class TaskGroup
{
public TaskGroup()
{
TaskTaskGroups = new HashSet<TaskTaskGroup>();
}
[Required]
public int Id { get; set; }
[Required]
public string GroupName { get; set; }
[InverseProperty(nameof(TaskTaskGroup.TaskGroup))]
public virtual ICollection<TaskTaskGroup> TaskTaskGroups { get; set; }
}
public class TaskTaskGroup
{
[Key]
public int Id { get; set; }
public int TaskId { get; set; }
[ForeignKey(nameof(TaskId))]
[InverseProperty(nameof(TaskTaskGroup.Task.TaskTaskGroups))]
public virtual Task Task { get; set; }
public int TaskGroupId { get; set; }
[ForeignKey(nameof(TaskGroupId))]
[InverseProperty(nameof(TaskTaskGroup.Task.TaskTaskGroups))]
public virtual TaskGroup TaskGroup { get; set; }
}
public class TaskDbContext : DbContext
{
public TaskDbContext()
{
}
public TaskDbContext(DbContextOptions<TaskDbContext> options)
: base(options)
{
}
public DbSet<Task> Tasks { get; set; }
public DbSet<TaskGroup> TaskGroups { get; set; }
public DbSet<TaskTaskGroup> TaskTaskGroups { get; set; }
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
optionsBuilder.UseSqlServer(#"Server=localhost;Database=Task;Trusted_Connection=True;");
}
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<TaskTaskGroup>(entity =>
{
entity.HasOne(d => d.Task)
.WithMany(p => p.TaskTaskGroups)
.HasForeignKey(d => d.TaskId)
.OnDelete(DeleteBehavior.ClientSetNull)
.HasConstraintName("FK_TaskTaskGroup_Task");
entity.HasOne(d => d.TaskGroup)
.WithMany(p => p.TaskTaskGroups)
.HasForeignKey(d => d.TaskGroupId)
.HasConstraintName("FK_TaskTaskGroup_TaskCroup");
});
}
}