handle add column request (#1272)

* handle add column request

* comments

* rename data to ViewModel

test/Microsoft.SqlTools.ServiceLayer.UnitTests/TableDesigner/TableColumnCollectionTest.cs

@@ -0,0 +1,30 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using System;
+using Microsoft.SqlTools.ServiceLayer.TableDesigner.Contracts;
+using NUnit.Framework;
+using Newtonsoft.Json;
+
+namespace Microsoft.SqlTools.ServiceLayer.UnitTests.TableDesigner
+{
+    public class TableColumnCollectionTest
+    {
+        [Test]
+        public void AutoNameForNewItemTest()
+        {
+            var collection = new TableColumnCollection();
+            collection.AddNew();
+            Assert.AreEqual(1, collection.Data.Count, "The item count should be 1");
+            Assert.AreEqual("column1", collection.Data[0].Name.Value);
+            collection.Data.Add(new TableColumnViewModel() { Name = new InputBoxProperties() { Value = "column3" } });
+            Assert.AreEqual(2, collection.Data.Count, "The item count should be 2");
+            collection.AddNew();
+            Assert.AreEqual(3, collection.Data.Count, "The item count should be 3");
+            // the name that is not yet used should be picked
+            Assert.AreEqual("column2", collection.Data[2].Name.Value);
+        }
+    }
+}