.NetCore中EFCore的使用整理（二）-關聯表查詢

時間 2019-11-06

標籤 netcore efcore 使用整理關聯查詢简体版

原文原文鏈接

EF經常使用處理關聯加載的方式有3中：延遲加載(Lazy Loading)、貪婪加載 (Eager Loading)以及顯示加載。html

1、EF Core 1.1sql

1.當前的版本，還不支持延遲加載（Lazy Loading），不未來是否支持數據庫

2.目前支持貪婪加載：使用Include加載 關聯表的數據，這種方式使用一條Join的 Sql語句進行查詢。ide

3. 貪婪加載的優點在於僅執行1次SQL查詢即返回所須要的結果。但使用JOIN查詢在數據庫記錄條數較多時，多條簡單的SQL查詢每每比一條複雜的JOIN查詢效率要好。post

Include語句能夠在一次查詢中使用屢次：ui

ctx.Categories
    .Include(c => c.Products)
    .Include(c => c.News);

4.限制加載的方式暫時忽略使用。this

2、EF Core 中主外鍵設置url

1.使用數據註釋，DataAnnotations模式，這種方式適合Code First或者說手寫實體類和自定義主外鍵關聯。spa

Menu表.net

[Table("Menu")]
public partial class Menu
{
    [Key]
    public int MenuID { get; set; }
    public string MenuName { get; set; }


    public string LinkUrl { get; set; }
    public DateTime AddTime { get; set; }
    public int SortNumber { get; set; }

    public int ModelID { get; set; }

    [ForeignKey("ModelID")]
    public virtual Model Model { get; set; }
}

View Code

Model表

[Table("Model")]
public partial class Model
{

    public Model()
    {
        this.Menus = new HashSet<Menu>();
    }

    [Key]
    public int ModelID { get; set; }
    public string ModelName { get; set; }
    public int SortNumber { get; set; }

    public DateTime AddTime { get; set; }

    public virtual ICollection<Menu> Menus { get; set; }

}

View Code

上下文類

    public class MenuModelContext : DbContext
    {

        public virtual DbSet<Menu> Menus { get; set; }
        public virtual DbSet<Model> Models { get; set; }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseSqlServer(@"Server=(localdb)\mssqllocaldb;database=MenuModel;Trusted_Connection=True;");
        }
    }

View Code

2.使用Fluent API模式，這種方式，是自動DBFirst自動生成實體層的默認方式

Menu表

public partial class Menu
{
    public int MenuId { get; set; }
    public string MenuName { get; set; }
    public string LinkUrl { get; set; }
    public DateTime AddTime { get; set; }
    public int SortNumber { get; set; }
    public int ModelId { get; set; }

    public virtual Model Model { get; set; }
}

View Code

Model表

public partial class Model
{
    public Model()
    {
        Menu = new HashSet<Menu>();
    }

    public int ModelId { get; set; }
    public string ModelName { get; set; }
    public int SortNumber { get; set; }
    public DateTime AddTime { get; set; }

    public virtual ICollection<Menu> Menu { get; set; }
}

View Code

上下文類：

public partial class MenuModelContext : DbContext
{
    public virtual DbSet<Menu> Menu { get; set; }
    public virtual DbSet<Model> Model { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlServer(@"Server=(localdb)\mssqllocaldb;database=MenuModel;Trusted_Connection=True;");
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Menu>(entity =>
        {
            entity.Property(e => e.MenuId).HasColumnName("MenuID");

            entity.Property(e => e.AddTime).HasColumnType("datetime");

            entity.Property(e => e.LinkUrl).HasMaxLength(200);

            entity.Property(e => e.MenuName)
                .IsRequired()
                .HasMaxLength(10);

            entity.Property(e => e.ModelId).HasColumnName("ModelID");

            entity.HasOne(d => d.Model)
                .WithMany(p => p.Menu)
                .HasForeignKey(d => d.ModelId)
                .OnDelete(DeleteBehavior.Restrict)
                .HasConstraintName("FK__Menu__ModelID__25869641");
        });

        modelBuilder.Entity<Model>(entity =>
        {
            entity.Property(e => e.ModelId).HasColumnName("ModelID");

            entity.Property(e => e.AddTime).HasColumnType("datetime");

            entity.Property(e => e.ModelName)
                .IsRequired()
                .HasMaxLength(10);
        });
    }
}

View Code

3、使用Include 獲取關聯表數據實例

注：使用Include 方式獲取的數據爲對應類型的對象，而不是動態類型Dynamic_xxxx

MenuModelContext _Context = new MenuModelContext();
List<Menu> list = _Context.Menus
    .Include(q => q.Model) //手動指定關聯表查詢,一對一
    .ToList();
foreach (var item in list)
{
    Console.WriteLine(item.MenuName);
    Console.WriteLine(item.Model);
}

MenuModelContext _Context = new MenuModelContext();
List<Model> list = _Context.Models
    .Include(q => q.Menus) //手動指定關聯表查詢,一對多
    .ToList();
foreach (var item in list)
{
    Console.WriteLine(item.ModelName);
    Console.WriteLine(item.Menus.Count);
}