Question

I am trying to create a generic method that will retrieve an item by its id:

public T GetByID(int id)
{
    return (T) context.GetObjectByKey(
        new System.Data.EntityKey(context.DefaultContainerName + "." +
             context.CreateObjectSet<T>().EntitySet.Name, 
             "ProductID", id));
}

Basically I am able to infer the entity name from T, however I have no idea how to figure out what the primary key is for an entity?

Answer 1

I ended up creating my own attribute and modifying the T4 template to place that attribute above the primary key column. Here are the steps I took:

1. Add the following above the [DataMember] attribute in the T4 template:

   <#if (ef.IsKey(edmProperty)) {#>    
   [PrimaryKeyAttribute]
   <#}#>
   

2. Create the PrimaryKeyAttribute:

   [AttributeUsage(AttributeTargets.Property)]
   public class PrimaryKeyAttribute : Attribute
   {}
   

3. Introduce a helper method to determine the primary key of an entity:

   private string GetPrimaryKey<K>()
   {
       string primaryKey = string.Empty;
   
       PropertyInfo[] entityProperties = typeof(K).GetProperties();
   
       foreach (PropertyInfo prop in entityProperties)
       {
           object[] attrs = prop.GetCustomAttributes(false);
           foreach (object obj in attrs)
           {
               if (obj.GetType() == typeof(PrimaryKeyAttribute))
               {
                   primaryKey = prop.Name;
                   break;
               }
           }
       }
   
       if (string.IsNullOrEmpty(primaryKey))
           throw new Exception("Cannot determine entity's primary key");
   
       return primaryKey;
   }
   

4. Finally write the generic GetByID as such:

   public T GetByID(int id)
   {
       return (T)context.GetObjectByKey(new EntityKey(context.DefaultContainerName 
                                           + "." + context.CreateObjectSet<T>().EntitySet.Name
                                           , GetPrimaryKey<T>(), id));            
   }