I have a CoreDatabase model that is already used in the App Store, so I need to devise a way to migrate the model without losing the previous data. I have an entity that currently does not have a parent object, but I need to set its parent entity in the new model. However, when I try to do this, I get a crash that says that CoreData was unable to migrate the database model.
If anyone could provide an insight into how I would accomplish this, I'd really appreciate it!
EDIT: I get an Cannot migrate store in-place error with the underlying error of "SQLite error code:1, 'no such table: _T_ZCONTACT', reason=Failed to execute migration statements". The "Contact" is the name of the entity I need to change the parent for.
You may create a new entity having the same attributes as the old entity, set the new one's parent, then migrate the old entity to new one;
second, change all of invoke to the new entity.
Related
I'm trying to peform an update statement using WCF RIA Services, but everytime I update I keep getting "An entity with the same identity already exists in this EntitySet. Any insight on where I can start looking or figuring out what is wrong?
Step 1
LoadOperation<Analysis> AnalysisLP = ((App)Application.Current)._context.
Load(((App)Application.Current)._context.GetAnalysisQuery().
Where(o => o.ProjectID == Convert.ToInt32(((App)Application.Current).Project.ProjectID)));
Step 2
AnalysisLP.Completed += delegate
{
if (!AnalysisLP.HasError)
{
Analysis = AnalysisLP.Entities.FirstOrDefault();
};
Step 3
((App)Application.Current)._context.Analysis.Attach(Analysis);
((App)Application.Current)._context.SubmitChanges(OnSubmitCompleted, null);
Can anyone help me, what is it i'm doing wrong??
thanks
Your object Analysis comes from the EntitySet via a query but is still attached to that EntitySet.
You just need to change its properties and call SubmitChanges. Do not try to attach it again.
To avoid the “An Entity with the same identity already exists in the EntitySet” exception, Entities that are updated, modified or deleted must always be fully refreshed from server upon saving, there can be no references held in memory to the previous instances of the entities. To prevent orhpaned instances from hanging around, I follow these rules:
Entity instances should not have any property changed event handlers assigned directly to them, rather use OnCreated or OnPropertyNameChanged partial methods instead.
When entities are added to an EntitySet, do not assign parent Entity instance references, use the foreign key ID property instead (myEntity.ParentalID = SelectedParent.ParentalID rather than myEntity.Parent = SelectedParent) because the SelectedParent probably isn’t getting reloaded upon saving because it isn’t part of the unit of work, so that reference will be held after the save and refresh.
Any combo boxes that are used as populate sources for Entity properties of the Entity need to have their EntitySet reloaded after saving as well; otherwise those related Entities populating the combo will hold references to the previous entity instance.
If you use an object database, what happens when you need to change the structure of your object model?
For instance, I'm playing around with the Google App Engine. While I'm developing my app, I've realized that in some cases, I mis-named a class, and I want to change the name. And I have two classes that I think I need to consolidate.
However,I don't think I can, because the name of the class in intuitively tied into the datastore, and there is actual data stored under those class names.
I suppose the good thing about the "old way" of abstracting the object model from the data storage is that the data storage doesn't know anything about the object model --it's just data. So, you can change your object model and just load the data out of the datastore differently.
So, in general, when using a datastore which is intimate with your data model...how do you change things around?
If it's just class naming you're concerned about, you can change the class name without changing the kind (the identifier that is used in the datastore):
class Foo(db.Model):
#classmethod
def kind(cls):
return 'Bar'
If you want to rename your class, just implement the kind() method as above, and have it return the old kind name.
If you need to make changes to the actual representation of data in the datastore, you'll have to run a mapreduce to update the old data.
The same way you do it in relational databases, except without a nice simple SQL script: http://code.google.com/appengine/articles/update_schema.html
Also, just like the old days, objects without properties don't automatically get defaults and properties that don't exist in the schema still hang around as phantoms in the objects.
To rename a property, I expect you can remove the old property (the phantom hangs around) add the new name, populate the data with a copy from the old (phantom) property. The re-written object will only have the new property
You may be able to do it the way we are doing it in our project:
Before we update the object-model (schema), we export our data to a file or blob in json format using a custom export function and version tag on top. After the schema has been updated we import the json with another custom function which creates new entities and populates them with old data. Of course the import version needs to know the json format associated with each version number.
I have created a class that inherits for DomainService and have a Silverlight app that uses System.ServiceModel.DomainServices.Client to get a DomainContext. I have also created POCO DataContracts that are used in the DomainServices's Query, Update, Insert, and Delete methods. I also have a ViewModel that executes all the LoadOperations. And now I'm at the part of my app where I want to add new Entities to the generated EntitySets but am unsure about what's going to happen when one user creates new and sets the Key value; all while another user creates a similar entity with the same Key value.
I have seen in the documentation that an ObjectContext is used, but in my situation I was not able to use the EntityFramework model generator. So I had to create my datacontracts by hand.
So I guess my question is, is there any way I can force other silverlight apps to update on database change?
When you make a Save operation to your DomainContext, depending on the load behavior, it will automatically refresh.
TicketContext.Load(TicketContext.GetTicketByIdQuery(ticketId),
LoadBehavior.RefreshCurrent,
x =>
{
Ticket = x.Entities.First();
Ticket.Load();
((IChangeTracking) TicketContext.EntityContainer).AcceptChanges();
}, null);
Here I've set the LoadBehavior to RefreshCurrent. When you make a save, RIA will send the entity back across the wire to the client and merge the changes with the entity already cached on your client side context. I don't know if that quite answers your question or not however.
Any pointers on how I can update the data for a model on each access?
If I have code that accesses a Person object like so:
p = Person.objects.get(username='darkpixel')
I would like to fire off my own process to check an external site and possibly update the Person model before returning it.
I'm hesitant to override the get method. Is there a better way?
That code definitely belongs in your Manager. I wouldn't override get, but I would create a function named get_and_resync to do the job for you.
Turn the fields on the model into properties.
When I update an entity, the entity needs to change parent, is there any way to do that?
The clean way, and generally the only applicable one, is to make a new entity with the new parent and everything else copied, and delete the old one. Parents become part of an entity's key, so it's definitely NOT trivial to "change" that except by this kind of approach!