Added flag prohibiting GraphDiff from ever removing collection elements ...

GraphDiff/GraphDiff.Tests/GraphDiff.Tests.csproj

@@ -72,6 +72,7 @@
     <Compile Include="Tests\AttributeMappingBehaviours.cs" />
     <Compile Include="Tests\ComplexTypeBehaviours.cs" />
     <Compile Include="Tests\GuidKeyBehaviors.cs" />
+    <Compile Include="Tests\NeverRemoveItemsBehaviors.cs" />
     <Compile Include="Tests\OwnedCollectionBehaviours.cs" />
     <Compile Include="Tests\OneMemberBehaviours.cs" />
     <Compile Include="Tests\OwnedEntityBehaviours.cs" />

GraphDiff/GraphDiff.Tests/Tests/NeverRemoveItemsBehaviors.cs

@@ -0,0 +1,86 @@
+using System.Collections.Generic;
+using System.Data.Entity;
+using System.Linq;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using RefactorThis.GraphDiff.Tests.Models;
+
+namespace RefactorThis.GraphDiff.Tests.Tests
+{
+    [TestClass]
+    public class NeverRemoveItemsBehaviors
+    {
+        [TestMethod]
+        public void ShouldNeverRemoveFromAssociatedCollection()
+        {
+            var node = new TestNode
+            {
+                Title = "New Node",
+                OneToManyAssociated = new List<OneToManyAssociatedModel>
+                {
+                    new OneToManyAssociatedModel { Title = "First One" }
+                }
+            };
+
+            using (var context = new TestDbContext())
+            {
+                context.Nodes.Add(node);
+                context.SaveChanges();
+            } // Simulate detach
+
+            node.OneToManyAssociated.Remove(node.OneToManyAssociated.Single());
+
+            GraphDiffConfiguration.NeverRemoveFromCollections = true;
+
+            using (var context = new TestDbContext())
+            {
+                node = context.UpdateGraph(node, map => map.AssociatedCollection(p => p.OneToManyAssociated));
+                context.SaveChanges();
+
+                Assert.AreEqual(1, node.OneToManyAssociated.Count);
+
+                var nodeReloaded = context.Nodes.Include(p => p.OneToManyAssociated).Single(p => p.Id == node.Id);
+                Assert.IsNotNull(nodeReloaded);
+                Assert.AreEqual(1, nodeReloaded.OneToManyAssociated.Count);
+            }
+        }
+
+        [TestMethod]
+        public void ShouldNeverRemoveFromOwnedCollection()
+        {
+            var node = new TestNode
+            {
+                Title = "New Node",
+                OneToManyOwned = new List<OneToManyOwnedModel>
+                {
+                    new OneToManyOwnedModel { Title = "Hello" },
+                    new OneToManyOwnedModel { Title = "Hello2" },
+                    new OneToManyOwnedModel { Title = "Hello3" }
+                }
+            };
+
+            using (var context = new TestDbContext())
+            {
+                context.Nodes.Add(node);
+                context.SaveChanges();
+            } // Simulate detach
+
+            node.OneToManyOwned.Remove(node.OneToManyOwned.First());
+            node.OneToManyOwned.Remove(node.OneToManyOwned.First());
+            node.OneToManyOwned.Remove(node.OneToManyOwned.First());
+
+            GraphDiffConfiguration.NeverRemoveFromCollections = true;
+
+            using (var context = new TestDbContext())
+            {
+                node = context.UpdateGraph(node, map => map.OwnedCollection(p => p.OneToManyOwned));
+                context.SaveChanges();
+
+                Assert.AreEqual(3, node.OneToManyOwned.Count);
+
+                var reloadedNode = context.Nodes.Include(p => p.OneToManyOwned).Single(p => p.Id == node.Id);
+                Assert.IsNotNull(reloadedNode);
+                Assert.AreEqual(3, reloadedNode.OneToManyOwned.Count);
+            }
+        }
+    }
+}

GraphDiff/GraphDiff/GraphDiffConfiguration.cs

@@ -16,5 +16,11 @@ public static class GraphDiffConfiguration
         /// to ensure the EF local cache has the latest state.
         /// </summary>
         public static bool ReloadAssociatedEntitiesWhenAttached { get; set; }
+
+        /// <summary>
+        /// If GraphDiff encounters an entity in a database collection that is no longer part of the graph it is 
+        /// currently attaching, it will remove that element from the database as well unless this flag is set.
+        /// </summary>
+        public static bool NeverRemoveFromCollections { get; set; }
     }
 }

GraphDiff/GraphDiff/Internal/Graph/CollectionGraphNode.cs

@@ -101,6 +101,9 @@ private void UpdateElement<T>(IChangeTracker changeTracker, IEntityManager entit
 
         private void RemoveElement(IChangeTracker changeTracker, object dbItem, object dbCollection)
         {
+            if (GraphDiffConfiguration.NeverRemoveFromCollections)
+                return;
+
             dbCollection.GetType().GetMethod("Remove").Invoke(dbCollection, new[] { dbItem });
             changeTracker.AttachRequiredNavigationProperties(dbItem, dbItem);