ASP.NET - How to do an inner join with entity framework

Question

I am using ASP.NET MVC with Entity Framework and I getting my data with SqlQuery like so:

dbBlog.Data.SqlQuery("SELECT bc.id, bc.title, bc.post, bc.date, bc.featuredImage, cc.category FROM BlogClasses bc INNER JOIN CategoriesClasses cc ON bc.category = cc.id WHERE cc.category = '" + id.Replace("_", " ") + "' ORDER BY bc.date desc").ToList();

What I am trying to do now and is an INNER JOIN without using SqlQuery, I have done some reading on Join, I am really struggling to understand it.

I have my categories defined:

private Categories dbCategories = new Categories();

I really have no clue on what the next step it and how .join works, can someone point me in the right direction?

Thanks,

Answer 1

Without knowing your schema I can't give a specific answer, but with Join() , you need to provide:

IEnumerable<TInner> inner - This is what you want to Join to .

Expression<Func<TOuter, TKey>> outerKeySelector - This is how to specify the field from whatever you are joining from .

Expression<Func<TInner, TKey>> innerKeySelector - This is how you specify the field you are joining to - something from TInner defined earlier.

Expression<Func<TOuter, TInner, TResult>> resultSelector - This is what you will be returning from the join.

Example, this SQL Query:

SELECT i.*
FROM Outer o JOIN Inner i ON o.SomeId = i.SomeId
WHERE o.SomeValue < 5

Can be written with Join() like:

var result = outers.Where(o => o.SomeValue < 5)
                 .Join(inners, o => o.SomeId, i => i.SomeId, (o, i) => i);

Answer 2

Entity framework uses configurations to reflect relationships between related entities. Given entities like:

public class BlogClass
{
   public int Id {get; set;}
   public string Title {get; set;}
   public string Post {get; set;}
   public DateTime Date {get; set;}
   public string FeaturedImage {get; set;} // Assuming a URL?

   public virtual ClassCategory Category {get;set;}
}

public class ClassCategory
{
   public int Id {get; set;}
   public string Category {get; set;}
   // Other properties...
}

We can set up our configurations:

public class BlogClassConfiguration : EntityTypeConfiguration<BlogClass>
{
   public BlogClassConfiguration()
   {
      ToTable("BlogClasses");
      HasKey(x => x.Id)
         .Property(x => x.Id)
         .HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity); // Assuming the ID's are set up as identity or have defaults.

      HasRequired(x => x.Category)
         .WithMany()
         .Map(x => x.MapKey("Category")); // This maps our CategoryClasses reference (Category) to a column on the BlogCategories table called "Category".
   }
}
Public class ClassCategoryConfiguration : EntityTypeConfiguration<ClassCategory>
{
   public ClassCategoryConfiguration()
   {
      ToTable("CategoryClasses");
      HasKey(x => x.Id)
         .Property(x => x.Id)
         .HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);
   }
}

We tell the DbContext to load these configurations when building its models:

public class BlogDbContext : DbContext
{
   public DbSet<BlogClass> BlogClasses {get; set;}

   public BlogDbContext(string connectionString)
      : base (connectionString)
   {}

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

 modelBuilder.Configurations.AddFromAssembly(typeof(TestDbContext).Assembly);
   }
}

Then EF manages the joins entirely.

var categoryName = category.Replace("_", " ");

using (var context = new BlogContext("BlogConnectionString"))
{
   var blogs = context.BlogClasses
      .Where(x => x.Category.Category == categoryName)
      .OrderByDescending(x => x.Date)
      .ToList(); // Assumes not too many items returned!
}

This example scopes a DbContext as an example. Ideally this should be managed by an IoC container and provided as a dependency.