1 files changed, 30 insertions, 0 deletions
diff --git a/Timeline/Services/DatabaseExtensions.cs b/Timeline/Services/DatabaseExtensions.cs
new file mode 100644
index 00000000..a37cf05b
--- /dev/null
+++ b/Timeline/Services/DatabaseExtensions.cs
@@ -0,0 +1,30 @@
+using Microsoft.EntityFrameworkCore;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using Timeline.Entities;
+
+namespace Timeline.Services
+{
+    public static class DatabaseExtensions
+    {
+        /// <summary>
+        /// Check the existence and get the id of the user.
+        /// </summary>
+        /// <param name="username">The username of the user.</param>
+        /// <returns>The user id.</returns>
+        /// <exception cref="ArgumentException">Thrown if <paramref name="username"/> is null or empty.</exception>
+        /// <exception cref="UserNotExistException">Thrown if user does not exist.</exception>
+        public static async Task<long> CheckAndGetUser(DbSet<User> userDbSet, string username)
+        {
+            if (string.IsNullOrEmpty(username))
+                throw new ArgumentException("Username is null or empty.", nameof(username));
+
+            var userId = await userDbSet.Where(u => u.Name == username).Select(u => u.Id).SingleOrDefaultAsync();
+            if (userId == 0)
+                throw new UserNotExistException(username);
+            return userId;
+        }
+    }
+}