ChangeProcessor.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Text;
using System.Diagnostics;

namespace System.Data.Linq {
    using System.Data.Linq.Mapping;
    using System.Data.Linq.Provider;

    /// <summary>
    /// Describes the type of change the entity will undergo when submitted to the database.
    /// </summary>
    public enum ChangeAction {
        /// <summary>
        /// The entity will not be submitted.
        /// </summary>
        None = 0,
        /// <summary>
        /// The entity will be deleted.
        /// </summary>
        Delete,
        /// <summary>
        /// The entity will be inserted.
        /// </summary>
        Insert,
        /// <summary>
        /// The entity will be updated.
        /// </summary>
        Update
    }

    internal class ChangeProcessor {
        CommonDataServices services;
        DataContext context;
        ChangeTracker tracker;
        ChangeDirector changeDirector;
        EdgeMap currentParentEdges;
        EdgeMap originalChildEdges;
        ReferenceMap originalChildReferences;

        internal ChangeProcessor(CommonDataServices services, DataContext context) {
            this.services = services;
            this.context = context;
            this.tracker = services.ChangeTracker;
            this.changeDirector = services.ChangeDirector;
            this.currentParentEdges = new EdgeMap();
            this.originalChildEdges = new EdgeMap();
            this.originalChildReferences = new ReferenceMap();
        }

        internal void SubmitChanges(ConflictMode failureMode) {
            this.TrackUntrackedObjects();
            // Must apply inferred deletions only after any untracked objects
            // are tracked
            this.ApplyInferredDeletions();
            this.BuildEdgeMaps();

            var list = this.GetOrderedList();

            ValidateAll(list);

            int numUpdatesAttempted = 0;
            ChangeConflictSession conflictSession = new ChangeConflictSession(this.context);
            List<ObjectChangeConflict> conflicts = new List<ObjectChangeConflict>();
            List<TrackedObject> deletedItems = new List<TrackedObject>();
            List<TrackedObject> insertedItems = new List<TrackedObject>();
            List<TrackedObject> syncDependentItems = new List<TrackedObject>();
            
            foreach (TrackedObject item in list) {
                try {
                    if (item.IsNew) {
                        if (item.SynchDependentData()) {
                            syncDependentItems.Add(item);
                        }
                        changeDirector.Insert(item);
                        // store all inserted items for post processing
                        insertedItems.Add(item);
                    }
                    else if (item.IsDeleted) {
                        // Delete returns 1 if the delete was successfull, 0 if the row exists
                        // but wasn't deleted due to an OC conflict, or -1 if the row was
                        // deleted by another context (no OC conflict in this case)
                        numUpdatesAttempted++;
                        int ret = changeDirector.Delete(item);
                        if (ret == 0) {
                            conflicts.Add(new ObjectChangeConflict(conflictSession, item, false));
                        }
                        else {
                            // store all deleted items for post processing
                            deletedItems.Add(item);
                        }
                    }
                    else if (item.IsPossiblyModified) {
                        if (item.SynchDependentData()) {
                            syncDependentItems.Add(item);
                        }
                        if (item.IsModified) {
                            CheckForInvalidChanges(item);
                            numUpdatesAttempted++;
                            if (changeDirector.Update(item) <= 0) {
                                conflicts.Add(new ObjectChangeConflict(conflictSession, item));
                            }
                        }
                    }
                }
                catch (ChangeConflictException) {
                    conflicts.Add(new ObjectChangeConflict(conflictSession, item));
                }
                if (conflicts.Count > 0 && failureMode == ConflictMode.FailOnFirstConflict) {
                    break;
                }
            }

            // if we have accumulated any failed updates, throw the exception now
            if (conflicts.Count > 0) {
                // First we need to rollback any value that have already been auto-[....]'d, since the values are no longer valid on the server
                changeDirector.RollbackAutoSync();
                // Also rollback any dependent items that were [....]'d, since their parent values may have been rolled back
                foreach (TrackedObject syncDependentItem in syncDependentItems) {
                    Debug.Assert(syncDependentItem.IsNew || syncDependentItem.IsPossiblyModified, "SynchDependent data should only be rolled back for new and modified objects.");
                    syncDependentItem.SynchDependentData();
                }
                this.context.ChangeConflicts.Fill(conflicts);
                throw CreateChangeConflictException(numUpdatesAttempted, conflicts.Count);
            }
            else {
                // No conflicts occurred, so we don't need to save the rollback values anymore
                changeDirector.ClearAutoSyncRollback();
            }

            // Only after all updates have been sucessfully processed do we want to make
            // post processing modifications to the objects and/or cache state.
            PostProcessUpdates(insertedItems, deletedItems);
        }

        private void PostProcessUpdates(List<TrackedObject> insertedItems, List<TrackedObject> deletedItems) {
            // perform post delete processing
            foreach (TrackedObject deletedItem in deletedItems) {
                // remove deleted item from identity cache
                this.services.RemoveCachedObjectLike(deletedItem.Type, deletedItem.Original);
                ClearForeignKeyReferences(deletedItem);
            }

            // perform post insert processing
            foreach (TrackedObject insertedItem in insertedItems) {
                object lookup = this.services.InsertLookupCachedObject(insertedItem.Type, insertedItem.Current);
                if (lookup != insertedItem.Current) {
                    throw new DuplicateKeyException(insertedItem.Current, Strings.DatabaseGeneratedAlreadyExistingKey);
                }
                insertedItem.InitializeDeferredLoaders();
            }
        }

        /// <summary>
        /// Clears out the foreign key values and parent object references for deleted objects on the child side of a relationship.
        /// For bi-directional relationships, also performs the following fixup:
        ///   - for 1:N we remove the deleted entity from the opposite EntitySet or collection
        ///   - for 1:1 we null out the back reference
        /// </summary>
        private void ClearForeignKeyReferences(TrackedObject to) {
            Debug.Assert(to.IsDeleted, "Foreign key reference cleanup should only happen on Deleted objects.");
            foreach (MetaAssociation assoc in to.Type.Associations) {
                if (assoc.IsForeignKey) {
                    // If there is a member on the other side referring back to us (i.e. this is a bi-directional relationship),
                    // we want to do a cache lookup to find the other side, then will remove ourselves from that collection.
                    // This cache lookup is only possible if the other key is the primary key, since that is the only way items can be found in the cache.
                    if (assoc.OtherMember != null && assoc.OtherKeyIsPrimaryKey) {
                        Debug.Assert(assoc.OtherMember.IsAssociation, "OtherMember of the association is expected to also be an association.");
                        // Search the cache for the target of the association, since
                        // it might not be loaded on the object being deleted, and we
                        // don't want to force a load.
                        object[] keyValues = CommonDataServices.GetForeignKeyValues(assoc, to.Current);
                        object cached = this.services.IdentityManager.Find(assoc.OtherType, keyValues);

                        if (cached != null) {
                            if (assoc.OtherMember.Association.IsMany) {
                                // Note that going through the IList interface handles 
                                // EntitySet as well as POCO collections that implement IList 
                                // and are not FixedSize.
                                System.Collections.IList collection = assoc.OtherMember.MemberAccessor.GetBoxedValue(cached) as System.Collections.IList;
                                if (collection != null && !collection.IsFixedSize) {
                                    collection.Remove(to.Current);
                                    // Explicitly clear the foreign key values and parent object reference
                                    ClearForeignKeysHelper(assoc, to.Current);
                                }
                            }
                            else {
                                // Null out the other association.  Since this is a 1:1 association,
                                // we're not concerned here with causing a deferred load, since the
                                // target is already cached (since we're deleting it).
                                assoc.OtherMember.MemberAccessor.SetBoxedValue(ref cached, null);
                                // Explicitly clear the foreign key values and parent object reference
                                ClearForeignKeysHelper(assoc, to.Current);
                            }
                        }
                        // else the item was not found in the cache, so there is no fixup that has to be done
                        // We are explicitly not calling ClearForeignKeysHelper because it breaks existing shipped behavior and we want to maintain backward compatibility
                    }
                    else {
                        // This is a unidirectional relationship or we have no way to look up the other side in the cache, so just clear our own side
                        ClearForeignKeysHelper(assoc, to.Current);
                    }                    
                }
                // else this is not the 1-side (foreign key) of the relationship, so there is nothing for us to do
            }
        }

        // Ensure the the member and foreign keys are nulled so that after trackedInstance is deleted,
        // the object does not appear to be associated with the other side anymore. This prevents the deleted object
        // from referencing objects still in the cache, but also will prevent the related object from being implicitly loaded
        private static void ClearForeignKeysHelper(MetaAssociation assoc, object trackedInstance) {            
            Debug.Assert(assoc.IsForeignKey, "Foreign key clearing should only happen on foreign key side of the association.");
            Debug.Assert(assoc.ThisMember.IsAssociation, "Expected ThisMember of an association to always be an association.");
            
            // If this member is one of our deferred loaders, and it does not already have a value, explicitly set the deferred source to
            // null so that when we set the association member itself to null later, it doesn't trigger an implicit load.
            // This is only necessary if the value has not already been assigned or set, because otherwise we won't implicitly load anyway when the member is accessed.
            MetaDataMember thisMember = assoc.ThisMember;

            if (thisMember.IsDeferred &&
                !(thisMember.StorageAccessor.HasAssignedValue(trackedInstance) || thisMember.StorageAccessor.HasLoadedValue(trackedInstance)))
            {
                // If this is a deferred member, set the value directly in the deferred accessor instead of going 
                // through the normal member accessor, so that we don't trigger an implicit load.                                            
                thisMember.DeferredSourceAccessor.SetBoxedValue(ref trackedInstance, null);
            }

            // Notify the object that the relationship should be considered deleted.
            // This allows the object to do its own fixup even when we can't do it automatically.
            thisMember.MemberAccessor.SetBoxedValue(ref trackedInstance, null);

            // Also set the foreign key values to null if possible
            for (int i = 0, n = assoc.ThisKey.Count; i < n; i++) {
                MetaDataMember thisKey = assoc.ThisKey[i];
                if (thisKey.CanBeNull) {
                    thisKey.StorageAccessor.SetBoxedValue(ref trackedInstance, null);
                }
            }
        }

        private static void ValidateAll(IEnumerable<TrackedObject> list) {
            foreach (var item in list) {
                if (item.IsNew) {
                    item.SynchDependentData();
                    if (item.Type.HasAnyValidateMethod) {
                        SendOnValidate(item.Type, item, ChangeAction.Insert);
                    }
                } else if (item.IsDeleted) {
                    if (item.Type.HasAnyValidateMethod) {
                        SendOnValidate(item.Type, item, ChangeAction.Delete);
                    }
                } else if (item.IsPossiblyModified) {
                    item.SynchDependentData();
                    if (item.IsModified && item.Type.HasAnyValidateMethod) {
                        SendOnValidate(item.Type, item, ChangeAction.Update);
                    }
                }
            }
        }

        private static void SendOnValidate(MetaType type, TrackedObject item, ChangeAction changeAction) {
            if (type != null) {
                SendOnValidate(type.InheritanceBase, item, changeAction);

                if (type.OnValidateMethod != null) {
                    try {
                        type.OnValidateMethod.Invoke(item.Current, new object[] { changeAction });
                    } catch (TargetInvocationException tie) {
                        if (tie.InnerException != null) {
                            throw tie.InnerException;
                        }

                        throw;
                    }
                }
            }
        }

        internal string GetChangeText() {
            this.ObserveUntrackedObjects();
            // Must apply inferred deletions only after any untracked objects
            // are tracked
            this.ApplyInferredDeletions();
            this.BuildEdgeMaps();

            // append change text only
            StringBuilder changeText = new StringBuilder();
            foreach (TrackedObject item in this.GetOrderedList()) {
                if (item.IsNew) {
                    item.SynchDependentData();
                    changeDirector.AppendInsertText(item, changeText);
                }
                else if (item.IsDeleted) {
                    changeDirector.AppendDeleteText(item, changeText);
                }
                else if (item.IsPossiblyModified) {
                    item.SynchDependentData();
                    if (item.IsModified) {
                        changeDirector.AppendUpdateText(item, changeText);
                    }
                }
            }
            return changeText.ToString();
        }

        internal ChangeSet GetChangeSet() {
            List<object> newEntities = new List<object>();
            List<object> deletedEntities = new List<object>();
            List<object> changedEntities = new List<object>();

            this.ObserveUntrackedObjects();
            // Must apply inferred deletions only after any untracked objects
            // are tracked
            this.ApplyInferredDeletions();

            foreach (TrackedObject item in this.tracker.GetInterestingObjects()) {
                if (item.IsNew) {
                    item.SynchDependentData();
                    newEntities.Add(item.Current);
                }
                else if (item.IsDeleted) {
                    deletedEntities.Add(item.Current);
                }
                else if (item.IsPossiblyModified) {
                    item.SynchDependentData();
                    if (item.IsModified) {
                        changedEntities.Add(item.Current);
                    }
                }
            }

            return new ChangeSet(newEntities.AsReadOnly(), deletedEntities.AsReadOnly(), changedEntities.AsReadOnly());
        }

        // verify that primary key and db-generated values have not changed
        private static void CheckForInvalidChanges(TrackedObject tracked) {
            foreach (MetaDataMember mem in tracked.Type.PersistentDataMembers) {
                if (mem.IsPrimaryKey || mem.IsDbGenerated || mem.IsVersion) {
                    if (tracked.HasChangedValue(mem)) {
                        if (mem.IsPrimaryKey) {
                            throw Error.IdentityChangeNotAllowed(mem.Name, tracked.Type.Name);
                        }
                        else {
                            throw Error.DbGeneratedChangeNotAllowed(mem.Name, tracked.Type.Name);
                        }
                    }
                }
            }
        }

        /// <summary>
        /// Create an ChangeConflictException with the best message
        /// </summary>       
        static private ChangeConflictException CreateChangeConflictException(int totalUpdatesAttempted, int failedUpdates) {
            string msg = Strings.RowNotFoundOrChanged;
            if (totalUpdatesAttempted > 1) {
                msg = Strings.UpdatesFailedMessage(failedUpdates, totalUpdatesAttempted);
            }
            return new ChangeConflictException(msg);
        }

        internal void TrackUntrackedObjects() {
            Dictionary<object, object> visited = new Dictionary<object, object>();

            // search for untracked new objects
            List<TrackedObject> items = new List<TrackedObject>(this.tracker.GetInterestingObjects());
            foreach (TrackedObject item in items) {
                this.TrackUntrackedObjects(item.Type, item.Current, visited);
            }
        }
        
        internal void ApplyInferredDeletions() {
            foreach (TrackedObject item in this.tracker.GetInterestingObjects()) {
                if (item.CanInferDelete()) {
                    // based on DeleteOnNull specifications on the item's associations,
                    // a deletion can be inferred for this item.  The actual state transition
                    // is dependent on the current item state.
                    if (item.IsNew) {
                        item.ConvertToRemoved();
                    }
                    else if (item.IsPossiblyModified || item.IsModified) {
                        item.ConvertToDeleted();
                    }
                }
            }
        }

        private void TrackUntrackedObjects(MetaType type, object item, Dictionary<object, object> visited) {
            if (!visited.ContainsKey(item)) {
                visited.Add(item, item);
                TrackedObject tracked = this.tracker.GetTrackedObject(item);
                if (tracked == null) {
                    tracked = this.tracker.Track(item);
                    tracked.ConvertToNew();
                }
                else if (tracked.IsDead || tracked.IsRemoved) {
                    // ignore
                    return;
                }

                // search parents (objects we are dependent on)
                foreach (RelatedItem parent in this.services.GetParents(type, item)) {
                    this.TrackUntrackedObjects(parent.Type, parent.Item, visited);
                }

                // synch up primary key
                if (tracked.IsNew) {
                    tracked.InitializeDeferredLoaders();

                    if (!tracked.IsPendingGeneration(tracked.Type.IdentityMembers)) {
                        tracked.SynchDependentData();
                        object cached = this.services.InsertLookupCachedObject(tracked.Type, item);
                        if (cached != item) {
                            TrackedObject cachedTracked = this.tracker.GetTrackedObject(cached);
                            Debug.Assert(cachedTracked != null);
                            if (cachedTracked.IsDeleted || cachedTracked.CanInferDelete()) {
                                // adding new object with same ID as object being deleted.. turn into modified
                                tracked.ConvertToPossiblyModified(cachedTracked.Original);
                                // turn deleted to dead...
                                cachedTracked.ConvertToDead();

                                this.services.RemoveCachedObjectLike(tracked.Type, item);
                                this.services.InsertLookupCachedObject(tracked.Type, item);
                            }
                            else if (!cachedTracked.IsDead) {
                                throw new DuplicateKeyException(item, Strings.CantAddAlreadyExistingKey);
                            }
                        }
                    }
                    else {
                        // we may have a generated PK, however we set the PK on this new item to 
                        // match a deleted item
                        object cached = this.services.GetCachedObjectLike(tracked.Type, item);
                        if (cached != null) {
                            TrackedObject cachedTracked = this.tracker.GetTrackedObject(cached);
                            Debug.Assert(cachedTracked != null);
                            if (cachedTracked.IsDeleted || cachedTracked.CanInferDelete()) {
                                // adding new object with same ID as object being deleted.. turn into modified
                                tracked.ConvertToPossiblyModified(cachedTracked.Original);
                                // turn deleted to dead...
                                cachedTracked.ConvertToDead();

                                this.services.RemoveCachedObjectLike(tracked.Type, item);
                                this.services.InsertLookupCachedObject(tracked.Type, item);
                            }
                        }
                    }
                }

                // search children (objects that are dependent on us)
                foreach (RelatedItem child in this.services.GetChildren(type, item)) {
                    this.TrackUntrackedObjects(child.Type, child.Item, visited);
                }
            }
        }
        
        internal void ObserveUntrackedObjects() {
            Dictionary<object, object> visited = new Dictionary<object, object>();

            List<TrackedObject> items = new List<TrackedObject>(this.tracker.GetInterestingObjects());
            foreach (TrackedObject item in items) {
                this.ObserveUntrackedObjects(item.Type, item.Current, visited);
            }
        }

        private void ObserveUntrackedObjects(MetaType type, object item, Dictionary<object, object> visited) {
            if (!visited.ContainsKey(item)) {
                visited.Add(item, item);
                TrackedObject tracked = this.tracker.GetTrackedObject(item);
                if (tracked == null) {
                    tracked = this.tracker.Track(item);
                    tracked.ConvertToNew();
                } else if (tracked.IsDead || tracked.IsRemoved) {
                    // ignore
                    return;
                }

                // search parents (objects we are dependent on)
                foreach (RelatedItem parent in this.services.GetParents(type, item)) {
                    this.ObserveUntrackedObjects(parent.Type, parent.Item, visited);
                }

                // synch up primary key unless its generated.
                if (tracked.IsNew) {
                     if (!tracked.IsPendingGeneration(tracked.Type.IdentityMembers)) {
                        tracked.SynchDependentData();
                     } 
                }

                // search children (objects that are dependent on us)
                foreach (RelatedItem child in this.services.GetChildren(type, item)) {
                    this.ObserveUntrackedObjects(child.Type, child.Item, visited);
                }
            }
        }

        private TrackedObject GetOtherItem(MetaAssociation assoc, object instance) {
            if (instance == null)
                return null;
            object other = null;
            // Don't load unloaded references
            if (assoc.ThisMember.StorageAccessor.HasAssignedValue(instance) ||
                assoc.ThisMember.StorageAccessor.HasLoadedValue(instance)
                ) {
                other = assoc.ThisMember.MemberAccessor.GetBoxedValue(instance);
            }
            else if (assoc.OtherKeyIsPrimaryKey) {
                // Maybe it's in the cache, but not yet attached through reference.
                object[] foreignKeys = CommonDataServices.GetForeignKeyValues(assoc, instance);
                other = this.services.GetCachedObject(assoc.OtherType, foreignKeys);
            }
            // else the other key is not the primary key so there is no way to try to look it up
            return (other != null) ? this.tracker.GetTrackedObject(other) : null;
        }

        private bool HasAssociationChanged(MetaAssociation assoc, TrackedObject item) {
            if (item.Original != null && item.Current != null) {
                if (assoc.ThisMember.StorageAccessor.HasAssignedValue(item.Current) ||
                    assoc.ThisMember.StorageAccessor.HasLoadedValue(item.Current)
                    ) {
                    return this.GetOtherItem(assoc, item.Current) != this.GetOtherItem(assoc, item.Original);
                }
                else {
                    object[] currentFKs = CommonDataServices.GetForeignKeyValues(assoc, item.Current);
                    object[] originaFKs = CommonDataServices.GetForeignKeyValues(assoc, item.Original);
                    for (int i = 0, n = currentFKs.Length; i < n; i++) {
                        if (!object.Equals(currentFKs[i], originaFKs[i]))
                            return true;
                    }
                }
            }
            return false;
        }

        private void BuildEdgeMaps() {
            this.currentParentEdges.Clear();
            this.originalChildEdges.Clear();
            this.originalChildReferences.Clear();

            List<TrackedObject> list = new List<TrackedObject>(this.tracker.GetInterestingObjects());
            foreach (TrackedObject item in list) {
                bool isNew = item.IsNew;
                MetaType mt = item.Type;
                foreach (MetaAssociation assoc in mt.Associations) {
                    if (assoc.IsForeignKey) {
                        TrackedObject otherItem = this.GetOtherItem(assoc, item.Current);
                        TrackedObject dbOtherItem = this.GetOtherItem(assoc, item.Original);
                        bool pointsToDeleted = (otherItem != null && otherItem.IsDeleted) || (dbOtherItem != null && dbOtherItem.IsDeleted);
                        bool pointsToNew = (otherItem != null && otherItem.IsNew);

                        if (isNew || pointsToDeleted || pointsToNew || this.HasAssociationChanged(assoc, item)) {
                            if (otherItem != null) {
                                this.currentParentEdges.Add(assoc, item, otherItem);
                            }
                            if (dbOtherItem != null) {
                                if (assoc.IsUnique) {
                                    this.originalChildEdges.Add(assoc, dbOtherItem, item);
                                }
                                this.originalChildReferences.Add(dbOtherItem, item);
                            }
                        }
                    }
                }
            }
        }

        enum VisitState {
            Before,
            After
        }

        private List<TrackedObject> GetOrderedList() {
            var objects = this.tracker.GetInterestingObjects().ToList();

            // give list an initial order (most likely correct order) to avoid deadlocks in server
            var range = Enumerable.Range(0, objects.Count).ToList();
            range.Sort((int x, int y) => Compare(objects[x], x, objects[y], y));
            var ordered = range.Select(i => objects[i]).ToList();

            // permute order if constraint dependencies requires some changes to come before others
            var visited = new Dictionary<TrackedObject, VisitState>();
            var list = new List<TrackedObject>();
            foreach (TrackedObject item in ordered) {
                this.BuildDependencyOrderedList(item, list, visited);
            }
            return list;
        }

        private static int Compare(TrackedObject x, int xOrdinal, TrackedObject y, int yOrdinal) {
            // deal with possible nulls
            if (x == y) {
                return 0;
            }
            if (x == null) {
                return -1;
            }
            else if (y == null) {
                return 1;
            }
            // first order by action: Inserts first, Updates, Deletes last
            int xAction = x.IsNew ? 0 : x.IsDeleted ? 2 : 1;
            int yAction = y.IsNew ? 0 : y.IsDeleted ? 2 : 1;
            if (xAction < yAction) {
                return -1;
            }
            else if (xAction > yAction) {
                return 1;
            }
            // no need to order inserts (PK's may not even exist)
            if (x.IsNew) {
                // keep original order
                return xOrdinal.CompareTo(yOrdinal);
            }
            // second order by type
            if (x.Type != y.Type) {
                return string.CompareOrdinal(x.Type.Type.FullName, y.Type.Type.FullName);
            }
            // lastly, order by PK values
            int result = 0;
            foreach (MetaDataMember mm in x.Type.IdentityMembers) {
                object xValue = mm.StorageAccessor.GetBoxedValue(x.Current);
                object yValue = mm.StorageAccessor.GetBoxedValue(y.Current);
                if (xValue == null) {
                    if (yValue != null) {
                        return -1;
                    }
                }
                else {
                    IComparable xc = xValue as IComparable;
                    if (xc != null) {
                        result = xc.CompareTo(yValue);
                        if (result != 0) {
                            return result;
                        }
                    }
                }
            }
            // they are the same? leave in original order
            return xOrdinal.CompareTo(yOrdinal);
        }

        private void BuildDependencyOrderedList(TrackedObject item, List<TrackedObject> list, Dictionary<TrackedObject, VisitState> visited) {
            VisitState state;
            if (visited.TryGetValue(item, out state)) {
                if (state == VisitState.Before) {
                    throw Error.CycleDetected();
                }
                return;
            }

            visited[item] = VisitState.Before;

            if (item.IsInteresting) {
                if (item.IsDeleted) {
                    // if 'item' is deleted
                    //    all objects that used to refer to 'item' must be ordered before item
                    foreach (TrackedObject other in this.originalChildReferences[item]) {
                        if (other != item) {
                            this.BuildDependencyOrderedList(other, list, visited);
                        }
                    }
                }
                else {
                    // if 'item' is new or changed
                    //   for all objects 'other' that 'item' refers to along association 'assoc'
                    //      if 'other' is new then 'other' must be ordered before 'item'
                    //      if 'assoc' is pure one-to-one and some other item 'prevItem' used to refer to 'other'
                    //         then 'prevItem' must be ordered before 'item'
                    foreach (MetaAssociation assoc in item.Type.Associations) {
                        if (assoc.IsForeignKey) {
                            TrackedObject other = this.currentParentEdges[assoc, item];
                            if (other != null) {
                                if (other.IsNew) {
                                    // if other is new, visit other first (since item's FK depends on it)
                                    if (other != item || item.Type.DBGeneratedIdentityMember != null) {
                                        this.BuildDependencyOrderedList(other, list, visited);
                                    }
                                }
                                else if ((assoc.IsUnique || assoc.ThisKeyIsPrimaryKey)) {
                                    TrackedObject prevItem = this.originalChildEdges[assoc, other];
                                    if (prevItem != null && other != item) {
                                        this.BuildDependencyOrderedList(prevItem, list, visited);
                                    }
                                }
                            }
                        }
                    }
                }

                list.Add(item);
            }

            visited[item] = VisitState.After;
        }

        class EdgeMap {
            Dictionary<MetaAssociation, Dictionary<TrackedObject, TrackedObject>> associations;

            internal EdgeMap() {
                this.associations = new Dictionary<MetaAssociation, Dictionary<TrackedObject, TrackedObject>>();
            }

            internal void Add(MetaAssociation assoc, TrackedObject from, TrackedObject to) {
                Dictionary<TrackedObject, TrackedObject> pairs;
                if (!associations.TryGetValue(assoc, out pairs)) {
                    pairs = new Dictionary<TrackedObject, TrackedObject>();
                    associations.Add(assoc, pairs);
                }
                pairs.Add(from, to);
            }

            internal TrackedObject this[MetaAssociation assoc, TrackedObject from] {
                get {
                    Dictionary<TrackedObject, TrackedObject> pairs;
                    if (associations.TryGetValue(assoc, out pairs)) {
                        TrackedObject to;
                        if (pairs.TryGetValue(from, out to)) {
                            return to;
                        }
                    }
                    return null;
                }
            }
            internal void Clear() {
                this.associations.Clear();
            }
        }

        class ReferenceMap {
            Dictionary<TrackedObject, List<TrackedObject>> references;

            internal ReferenceMap() {
                this.references = new Dictionary<TrackedObject, List<TrackedObject>>();
            }

            internal void Add(TrackedObject from, TrackedObject to) {
                List<TrackedObject> refs;
                if (!references.TryGetValue(from, out refs)) {
                    refs = new List<TrackedObject>();
                    references.Add(from, refs);
                }
                if (!refs.Contains(to))
                    refs.Add(to);
            }

            internal IEnumerable<TrackedObject> this[TrackedObject from] {
                get {
                    List<TrackedObject> refs;
                    if (references.TryGetValue(from, out refs)) {
                        return refs;
                    }
                    return Empty;
                }
            }

            internal void Clear() {
                this.references.Clear();
            }

            private static TrackedObject[] Empty = new TrackedObject[] { };
        }
    }
}