First commit that builds.

code/MessengerBroker/Extensions/EntityExtensions.cs

@@ -0,0 +1,22 @@
+using System.Linq.Expressions;
+
+namespace MessengerBroker
+{
+    public static class EntityExtensions
+    {
+        public static IQueryable<Db.Model.User> GetUsers(this IQueryable<Db.Model.User> source, Guid[] guids)
+        {
+            return source.Where(x => guids.Any(g => g == x.Id));
+        }
+
+        public static IQueryable<Db.Model.User> GetUsersExcept(this IQueryable<Db.Model.User> source, IEnumerable<Guid> guids)
+        {
+            return source.Where(x => !guids.Any(g => g == x.Id));
+        }
+
+        public static bool GetUserExists(this IQueryable<Db.Model.User> source, Guid id)
+        {
+            return source.Any(x=> x.Id == id);
+        }
+    }
+}