entity framework - Use join-table of many-to-many relationship as source for another relation

Question

I have two entities, Project and Tag, defined as below

// Project.cs

namespace ProjectResume.Data.Entities
{
    public class Project
    {

        public Project()
        {
            this.Tags = new List<Tag>();
        }

        [Key]
        [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
        public int Id { get; set; }
        [Required]
        [MinLength(3)]
        [MaxLength(128)]
        public string Name { get; set; }
        [RegularExpression("([a-z0-9\-])+")]
        public string Slug { get; set; }
        public DateTime? StartDate { get; set; }
        public DateTime? EndDate { get; set; }
        [Required]
        [MinLength(3)]
        [MaxLength(512)]
        public string ShortDescription { get; set; }
        public string DetailedDescription { get; set; }

        public virtual ICollection<Tag> Tags { get; set; }
    }

}

// Tag.cs

namespace ProjectResume.Data.Entities
{
    public class Tag
    {
        public Tag()
        {
            this.Projects = new List<Project>();
        }

        [Key]
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int Id { get; set; }
        [Required]
        [MinLength(3)]
        [MaxLength(32)]
        public string Name { get; set; }
        [RegularExpression("([a-z0-9\-])+")]
        public string Slug { get; set; }
        [RegularExpression("#([a-fA-F0-9]{6}|[a-fA-F0-9]{3})")]
        public string Color { get; set; }

        public virtual ICollection<Project> Projects { get; set; }
    }
}

// DefaultDbContext.cs

namespace ProjectResume.Data
{
    public class DefaultDbContext : DbContext
    {
        // ...

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            base.OnModelCreating(modelBuilder);

            modelBuilder.Entity<Project>()
                .HasMany(p => p.Tags)
                .WithMany(t => t.Projects);

        }
    }
}

My goal is to create a 3rd entity, ProjectTagHighlight, which should store one or more highlights per project per tag. I'd assume the table needs at least an Id, ProjectId, TagId, Description, but I'm unsure on how to set up this relation given the code I already have, given the fact that I don't specify a ProjectTag entity myself.

The goal is to be able to access data in two different ways, either

Tag -> Project[] -> ProjectTagHighlight[] or Project -> Tag[] -> ProjectTagHighlight[]

Answer 1

Because you have used the third-part ProjectTagHighlight. You can access data as

ProjectTagHighlight -> Tag , ProjectTagHighlight -> Project

They form a many-to-many relationship through ProjectTagHighlight.

        modelBuilder.Entity<ProjectTagHighlight>()
            .HasOne(x => x.project)
            .WithOne(y => y.projectTagHighlight)
            .HasForeignKey<ProjectTagHighlight>(f => f.ProjectId);
        modelBuilder.Entity<ProjectTagHighlight>()
            .HasOne(x => x.tag)
            .WithOne(y => y.projectTagHighlight)
            .HasForeignKey<ProjectTagHighlight>(f => f.TagId);