SQLite EntityFramework多对多Db首先

Question

I have three SQLite tables:

1. Categories with two columns: Id (PK), Name
2. Elements with four columns: Id (PK), Name, Description, ImgUrl,
3. ElementsCategories with two columns: ElementId (FK), CategoryId (FK)

**PK = Primary Key, FK = Foreign Key

I created ADO.NET model and I wanted to get all the Categories from the database. My Category class looks like this:

class Category
{
    public long Id { get; set; }
    public string Name { get; set; }

    public virtual ICollection<Element> Elements{ get; set; }
}

I created function:

public List<Category> GetCategories()
{
    List<Category> categories;

    using (var db = new MyDb())
    {
        categories= db.Categories.Select(p => new Category
        {
            Id = p.Id,
            Name = p.Name,
            Elements = ????
        }).ToList();
    }

    return categories;
}

and I can't get list of Elements that belong to category. I can only get list of elements from ElementsCategories table. How can I set list of Elements to Elements field in Category object?

Answer 1

According to what you describe you have mapped the junction table ( ElementsCategories ) as an entity, in that case your query would be like this:

public List<Category> GetCategories()
{
    List<Category> categories;

    using (var db = new MyDb())
    {
        categories= db.Categories.Select(p => new Category
        {
            Id = p.Id,
            Name = p.Name,
            Elements = p.ElementsCategories.Select(ec=>ec.Element)
        }).ToList();
    }

    return categories;
}

Now, this is not going to work if Category is an entity type. I'm supposing that Category class you are showing is a DTO .