UniqueAttribute validation modifications based on this article: http://blogs.microsoft.co.il/blogs/shimmy/archive/2012/01/23/validationattribute-that-validates-a-unique-field-against-its-fellow-rows-in-the-database.aspx

attributeValidation.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.ComponentModel.DataAnnotations;
using System.Data.Entity;
using System.Reflection;
using System.Data.Entity.ModelConfiguration.Conventions;
using PcnWeb.Helpers;
using System.Data.Objects;
using System.Data.Entity.Infrastructure;
using System.Data.Metadata.Edm;

///
/// from: http://code.msdn.microsoft.com/windowsdesktop/CSASPNETUniqueConstraintInE-d357224a
///

namespace System.ComponentModel.DataAnnotations
{
	[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
	public class UniqueAttribute : ValidationAttribute
	{
		private readonly Type _ContextType;
		public Type ContextType
		{
			get
			{
				return _ContextType;
			}
		}

		//TODO: If placed in your domain, uncomment and replace MyDbContext with your domain's DbContext/ObjectContext class.
		public UniqueAttribute() : this(typeof(PcnWeb.Models.PCNWebContext)) { }


		/// <summary>
		/// Initializes a new instance of <see cref="UniqueAttribute"/>.
		/// </summary>
		/// <param name="contextType">The type of <see cref="DbContext"/> or <see cref="ObjectContext"/> subclass that will be used to search for duplicates.</param>
		public UniqueAttribute(Type contextType)
		{
			if (contextType == null)
				throw new ArgumentNullException("contextType");
			if (!contextType.IsSubclassOf(typeof(DbContext)) && !contextType.IsSubclassOf(typeof(ObjectContext)))
				throw new ArgumentException("The contextType Type must be a subclass of DbContext or ObjectContext.", "contextType");
			if (contextType.GetConstructor(Type.EmptyTypes) == null)
				throw new ArgumentException("The contextType type must declare a public parameterless consructor.");

			_ContextType = contextType;
		}


		/// <summary>
		/// Validates the value against the matching columns in the other rows of this table.
		/// Note that this method does not validate null or empty strings.
		/// </summary>
		/// <param name="value">The value to validate</param>
		/// <param name="validationContext">The context information about the validation operation.</param>
		/// <returns>An instance of the <see cref="ValidationResult"/> class.</returns>
		protected override ValidationResult IsValid(object value, ValidationContext validationContext)
		{
			if (value == null || (value is string && string.IsNullOrWhiteSpace((string)value))) return ValidationResult.Success;

			var type = validationContext.ObjectType;
			var property = type.GetProperty(validationContext.DisplayName);//MemberName
			type = property.DeclaringType;

			using (var dbcontext = (IDisposable)Activator.CreateInstance(_ContextType))
			{
				var context = dbcontext is DbContext ? ((IObjectContextAdapter)dbcontext).ObjectContext : (ObjectContext)dbcontext;
				var md = context.MetadataWorkspace;
				var entityType = md.GetItems<EntityType>(DataSpace.CSpace).SingleOrDefault(et => et.Name == type.Name);

				while (entityType.BaseType != null)
					entityType = (EntityType)entityType.BaseType;

				var objectType = typeof(object);
				var isInherited = false;
				var baseType = type;
				while (baseType.Name != entityType.Name && baseType.BaseType != objectType)
				{
					baseType = baseType.BaseType;
					isInherited = true;
				}

				var methodCreateObjectSet = typeof(ObjectContext).GetMethod("CreateObjectSet", Type.EmptyTypes).MakeGenericMethod(baseType);
				var baseObjectSet = (ObjectQuery)methodCreateObjectSet.Invoke(context, new object[] { });
				var objectSet = baseObjectSet;
				var setType = baseObjectSet.GetType();

				if (isInherited)
				{
					var ofType = setType.GetMethod("OfType");
					ofType = ofType.MakeGenericMethod(type);
					objectSet = (ObjectQuery)ofType.Invoke(baseObjectSet, null);
					setType = objectSet.GetType();
				}

				var methodWhere = setType.GetMethod("Where");

				var eSql = string.Format("it.{0} = @{0}", validationContext.DisplayName);

				var query = (ObjectQuery)methodWhere.Invoke(objectSet,
					new object[] { eSql, new[] { new ObjectParameter(validationContext.DisplayName, value) } });

				var result = query.Execute(MergeOption.NoTracking).Cast<object>();

				bool isValid = true;
				using (var enumerator = result.GetEnumerator())
				{
					if (enumerator.MoveNext())
					{
						var nameProperty = typeof(ObjectSet<>).MakeGenericType(baseType).GetProperty("EntitySet");
						var entitySet = (EntitySet)nameProperty.GetValue(baseObjectSet, null);
						var entitySetName = entitySet.Name;

						do
						{
							var current = enumerator.Current;
							var curKey = context.CreateEntityKey(entitySetName, current);
							var validatingKey = context.CreateEntityKey(entitySetName, validationContext.ObjectInstance);

							if (curKey != validatingKey)
							{
								isValid = false;
								break;
							}
						} while (enumerator.MoveNext());
					}
				}

				return isValid ?
					ValidationResult.Success :
					new ValidationResult(
					string.Format("There is already a '{0}' record that has its '{1}' field set to '{2}'.",
						validationContext.ObjectType.Name,
						validationContext.DisplayName,
						value),
					new[] { validationContext.DisplayName });
			}
		}
	}
}
namespace PcnWeb.Helpers
{
	/// <summary> 
	///  Try to analyze the ModelEntity by reflecting them one by one and then 
	///  fetch the properties with "unique" attribute and then do calling Sql statement to create database. 
	///  Based on :http://stackoverflow.com/a/10566485/429544
	/// </summary> 
	/// <typeparam name="T"></typeparam> 
	public class MyDbGenerator<T> : IDatabaseInitializer<T> where T : DbContext
	{
		public void InitializeDatabase(T context)
		{
			context.Database.Delete();
			context.Database.Create();
			//Fetch all the father class's public properties 
			var fatherPropertyNames = typeof(DbContext).GetProperties().Select(pi => pi.Name).ToList();
			//Loop each dbset's T 
			foreach (PropertyInfo item in typeof(T).GetProperties().Where(p => fatherPropertyNames.IndexOf(p.Name) < 0).Select(p => p))
			{
				//fetch the type of "T" 
				Type entityModelType = item.PropertyType.GetGenericArguments()[0];
				var allfieldNames = from prop in entityModelType.GetProperties()
									where prop.GetCustomAttributes(typeof(System.ComponentModel.DataAnnotations.UniqueAttribute), true).Count() > 0
									select prop.Name;
				foreach (string s in allfieldNames)
				{
					context.Database.ExecuteSqlCommand("alter table " + entityModelType.Name.pluralize() + " add unique(" + s + ")");
				}
			}
		}
	}
}

Entity Framework(v5) Unique Key Solution References.md

The above code is based on the following resources:

• This article explains how to create a new DataAttribute [Unique] whe can be used on properties of Entity Framework models. Upon Database initilization the models [Unique] properties are turned into unique indexes on the database. http://code.msdn.microsoft.com/windowsdesktop/CSASPNETUniqueConstraintInE-d357224a

• This article (with modified code attached) creates validators in the business logic based on the models [Unique] attribute which then provide a nice wrror message to the user. http://blogs.microsoft.co.il/blogs/shimmy/archive/2012/01/23/validationattribute-that-validates-a-unique-field-against-its-fellow-rows-in-the-database.aspx

System.Data already has a UniqueConstraint class.