breeze - modify an entity, on the server, based upon it's navigational properties, before saving - breeze

Question

Is there any way to get an Entity's navigational property's "current" value in BeforeSaveEntity (or anywhere else before save) in breeze on the server side? By current, I mean what exists in the database, with any incoming changes merged in. This isn't for validation, rather I am computing a value for a parent property (that I don't want on the client) based upon both the parent fields and children fields...
For example,
public class Parent {
public ICollection<Child> Children{ get; set; }
}
. . .
protected override bool BeforeSaveEntity(EntityInfo entityInfo) {
if (entityInfo.Entity.GetType() == typeof(Parent) &&
(entityInfo.EntityState == EntityState.Added || entityInfo.EntityState == EntityState.Updated)) {
// Lazy load Parent's Children collection out of breeze's context
// so items are "current' (existing merged with changes)
Parent parent = (Parent)entityInfo.Entity;
Context.Entry(parent).Collection(p => p.Children).Load();
// this throws exception Member 'Load' cannot be called for property
// 'Children' because the entity of type 'Parent' does not exist in the context.
}
}
I'm thinking they are not in the DBContext yet. All I can think to do is to retrieve the existing children from the database, and hand merge the changes in BeforeSaveEntities, which is a hassle.

Answer 1

Lazy loading is not enable in the DbContext that Breeze uses for saving. The reason is detailed in this SO answer.
You should load any additional entities in a separate DbContext.
Here's an example of how I did it in a project. Perhaps the MergeEntities and DetachEntities methods should be included with Breeze to make it simpler to do this.
protected override Dictionary<Type, List<EntityInfo>> BeforeSaveEntities(Dictionary<Type, List<EntityInfo>> saveMap)
{
// create a separate context for computation, so we don't pollute the main saving context
using (var newContext = new MyDbContext(EntityConnection, false))
{
var parentFromClient = (Parent)saveMap[typeof(Parent)][0].Entity;
// Load the necessary data into the newContext
var parentFromDb = newContext.Parents.Where(p => p.ParentId == parentFromClient.ParentId)
.Include("Children").ToList();
// ... load whatever else you need...
// Attach the client entities to the ObjectContext, which merges them and reconnects the navigation properties
var objectContext = ((IObjectContextAdapter)newContext).ObjectContext;
var objectStateEntries = MergeEntities(objectContext, saveMap);
// ... perform your business logic...
// Remove the entities from the second context, so they can be saved in the original context
DetachEntities(objectContext, saveMap);
}
return saveMap;
}
/// Attach the client entities to the ObjectContext, which merges them and reconnects the navigation properties
Dictionary<ObjectStateEntry, EntityInfo> MergeEntities(ObjectContext oc, Dictionary<Type, List<EntityInfo>> saveMap)
{
var oseEntityInfo = new Dictionary<ObjectStateEntry, EntityInfo>();
foreach (var type in saveMap.Keys)
{
var entitySet = this.GetEntitySetName(type);
foreach(var entityInfo in saveMap[type])
{
var entityKey = oc.CreateEntityKey(entitySet, entityInfo.Entity);
ObjectStateEntry ose;
if (oc.ObjectStateManager.TryGetObjectStateEntry(entityKey, out ose))
{
if (ose.State != System.Data.Entity.EntityState.Deleted)
ose.ApplyCurrentValues(entityInfo.Entity);
}
else
{
oc.AttachTo(entitySet, entityInfo.Entity);
ose = oc.ObjectStateManager.GetObjectStateEntry(entityKey);
}
if (entityInfo.EntityState == Breeze.ContextProvider.EntityState.Deleted)
{
ose.Delete();
}
oseEntityInfo.Add(ose, entityInfo);
}
}
return oseEntityInfo;
}
/// Remove the entities in saveMap from the ObjectContext; this separates their navigation properties
static void DetachEntities(ObjectContext oc, Dictionary<Type, List<EntityInfo>> saveMap)
{
foreach (var type in saveMap.Keys)
{
foreach (var entityInfo in saveMap[type])
{
try
{
oc.Detach(entityInfo.Entity);
}
catch
{ // the object cannot be detached because it is not attached
}
}
}
}