Detached objects, or objects that are created outside of Entity Framework(EF), don’t have automatic tracking enabled, and updating database from detached objects is not hard, but requires extra knowledge of EF. With this post I’d like to spell out different ways of doing it.
Before doing anything else, we must first check the state of an entity. If you are using Code First, an entity would be your POCO class that is mapped to the database table.
To get information about an entity in question, and it’s relation to the current DbContext, you call a DbContext.Entry<TEntity> method. It will return a DbEntityEntry<TEntity> object which can be used to check the state and perform additional actions on the entity.
When you create a new instance of a class outside of EF, it will have a detached state because context is not tracking that object.
var customer = new Customer();
var customerEntry = _context.Entry<T>(customer);
Debug.Write(customerEntry.State) // EntityState.Detached
If your object is in any state other than EntityState.Detached, that means EF knows about it and tracks it, so you don’t need to do anything else, DbContext.SaveChanges() would take care of persisting differences into the database.
When at first I was trying to update database from a detached object I thought that I can simply attach it and set it to be modified.
public override void Update(T entity)
{
var entry = _context.Entry<T>(entity);
if (entry.State == EntityState.Detached)
{
_context.Set<T>().Attach(entity);
entry.State = EntityState.Modified;
}
}
And that would work, but only if the object with the same key is not already present in the context. And if it does exist, you get a nice error:
An object with the same key already exists in the ObjectStateManager. The ObjectStateManager cannot track multiple objects with the same key.
So what do you do when an object is already being tracked by EF and you have a detached object, how do you merge them together?
Fortunately, EF does provide a way to update an existing object from a detached object. There are couple different ways you can do it, and it all depends on the context, and what you know about your object.
Here is the first way you can do it:
Querying Database First And Updating Tracked Object
using(var db = new StoreDbContext())
{
var existingCart = db.Carts.Find(cartId);
if(existingCart != null)
{
//entity is already in the context
var attachedEntry = db.Entry(existingCart);
attachedEntry.CurrentValues.SetValues(newCart);
}
else
{
//Since we don't have it in db, this is a simple add.
db.Cart.Add(newCart);
}
}
After querying a database and finding a record, we know that EF is already tracking the existing object. We then get an entry object of the tracked entity and call attachedEntry.CurrentValues.SetValues to update it with new values. This method works like an auto mapper and updates scalar properties from the passed in object. Also if the property values is different from the original, it sets the property state to be modified.
That works, but requires an extra database call to get an existing record. There is also a way to do that without an extra query.
Not Performing An Extra Database Call And Checking Local Context
There are times when you know for fact an entity is already in the database. An example would be when a database generated id of your object is not set to zero or default. In that situation you can save an extra call by querying the local context first. If an entity does exist in the local context you perform a similar update with attachedEntry.CurrentValues.SetValues, and if it does not exist, you can modify a state of your detached object to modified, which would attach the object and update the database.
For example, when I know that the cart id is not zero, that means it already exists in the database:
if(newCart.Id > 0) // Id already assigned, need to update.
{
//We query local context first to see if it's there.
var attachedEntity = db.Carts.Local.Find(newCart.Id);
//We have it in the context, need to update.
if (attachedEntity != null) {
var attachedEntry = _context.Entry(attachedEntity);
attachedEntry.CurrentValues.SetValues(newCart);
}
else
{
//If it's not found locally, we can attach it by setting state to modified.
//This would result in a SQL update statement for all fields
//when SaveChanges is called.
var entry = db.Entry(newCart);
entry.State = EntityState.Modified;
}
}
else
{
//This is a simple add since we don't have it in db.
db.Cart.Add(newCart);
}
There is a small difference when you query the database first. EF knows which values have been modified an only those specific value. On the other hand, when you are blindly attaching modified entity, EF doesn’t know what fields have changes and performs an update on all fields.
It Works, But Not Fully…
When you call CurrentValues.SetValues(newCart) it will update all scalar properties on your newCart object and set them to modified. However, navigation properties would not get the same respect. As of today EF does not support of full object graph merging, and leaves that for you to manage on your own. So if you have newCart.Customer navigational property it would not get updated. It’s the second most requested feature for EF at the moment, so I think they would add it in the future release.
So for now you have to manually call SetValues on all your navigational properties in order for them to be updated. There are also other solutions that people have written that might help with updating a full graph.
Finally, if you object graph is not very large, you can get away with getting DbEntityEntry and calling SetValues on each navigational property. But if you do have a large graph and want something automatic I would try something like GraphDiff first.
