I'm designing my first project using Core Data (for iPhone) and Im having some issues that might be related with my design approach.
I'm doing an application that allows the user to create an Order (let's say for a restaurant).
I'm using the graphic designer to model my persistence object (i.e. OrdeMO). I add MO to the ead of each name to indicate its a Managed Object.
I use XCode to create the Managed Object Class automatically.
I have created some "DAO" classes that allows you to search or create a new object in the Managed Context.
Now to my problem.
I want to create an OrderMO object to store the order the user is creating, BUT I don't want it to be part of the context until the user actually places it.
I tried creating the object with [OrderMO alloc] but the object I get is "incomplete" and when I try to set any of its attribute I get an error.
I'm assuming the problem is that I need to create the order IN the context in order to use it. Is that so?
I have considered various options:
Create the object in the context and the user rollback if the user discards the order. The problem is that the user might save other context object during the process (like his prefs) so this doesn't work. Is there a way to create the object "inside a separate transaction" of sorts?
Create a wrapper object that will hold the same data as the MO, and then only create the MO when the user place the order. The downside of this is that I have to maintain a new class.
Create an attribute in the MO, such as "placed", and use to filter my searches in the context. The problem with this one is that I will end up with "trash" objects in the domain (i.e. unplaced orders) and I will have to do some cleanup from time to time...
Do I have any other choice?
Any suggestion is appreciated.
Thanks (for reading this long post!)
Gonso
You should create the OrderMO object in the managed object context and then delete it if the user decides not to place the order.
If the context is saved before the object is deleted, the "trash" object will be deleted from the persistent store on the next save (if the context wasn't saved, the "trash" object will never be saved to the persistent store).
The flag to determine if the order was placed or not does not have to live in the OrderMO object as you suggest in option 3. It could be in the view controller that is tracking the order(s) that are being edited. And, again, you won't have "trash" objects because they will have been deleted.
Related
NSIncrementalStore has a required method "obtainPermanentIDsForObjects".
To get the ID's I have to create new row's in the database. Then to save the data in
executeRequest:withContext:error:, I have to hit the DB again.
Is there a reason why I shouldn't get new rowID's and save the data at the same time in obtainPermanentIDsForObjects?
The answer is probably not, if your entities have relationships.
To save a relationship, you need to uniquely identify the corresponding objects. Identifying an object requires your internal identifier which can be obtain via the permanent objectID (NSManagedObjectID).
But (some of) your objects won't have a permanent and internal ID yet because they haven't been handled yet in "obtainPermanentIDsForObjects"
In other words first you need to assign permanentID's (and thus also internalID's) before you can save references to them.
is there a simple and efficient/fast way to query a managedobjectcontext to get an array of all the managedobjects in the context that have not yet been added to the persistent store?
i ask this because i would like to be able to save nsmanagedobjects that have been added to the MOC only if they conform to certain criteria. basically i want to be able to do this so that if some unexpected event happened before my managed object attributes were properly populated, i can catch this fact and purge the object(s) before saving the context. given the complexity of the navigation possible in the app, i'd like to have a look at the data to be sure they are good before i save.
i suppose i could also do this with some kind of validation rule and a flag field that doesn't get set until i am sure the user has added all the data to the record, but i don't yet know how to implement this...
any help much appreciated.
The insertedObjects method of NSManagedObjectContext
returns the set of objects that have been inserted into the context but not yet saved in a persistent store.
It seems controllerDidChangeContent: is being called as soon as I create a new managed object in my context. The documentation seems to suggest this method is called only once you save: the context.
This "bug" if it is one, is causing my application to crash because as part of my table view cell, I need to load other managed objects that don't exist at the time of creating the main managed object.
Someone seems to have spotted this too, please check out the following link and I would love to hear your opinions on this: http://openradar.appspot.com/10207615
More information
Although the link I added to this post showcases an example using two NSManagedObjectContext, my application is using one context, but the controllerDidChangeContent: is being messaged none the less as soon as an object is created in the one and only context, and controllerDidChangeContent: is being called a second time when I save: this context. It is to my understanding that this method should only be messaged when the context is saved.
The solution is to avoid dealing with more than one managedObjectContext. If your cell needs to load other managed objects, it should still use the same managed object context as the main managed object.
I have yet to see a use case where it is absolutely unavoidable to use more than one managed object context referring to the same model active at the same time.
This question covncerns my lack of understanding of how to use the core data undo manager and how to restore a NSManagedObject to its state before editing was done.
I am just learning my way around Core Data. I have my NSManagedObject classes set up with their dynamic accessors. I perform a fetch that returns several NSManagedObject entity results. Content from each of these entity results (first name, last name) get put into a table view, and then the user picks one out of the table for detailed view and then editing.
The detail view controller receives a pointer to the selected NSManagedObject entity. As the user edits the fields, the corresponding property value in the NSManagedObject entity is updated. This seemed like the cleanest way to manage these changes.
Now, rather than committing the changes using save, I want to provide a cancel-editing feature that rolls back to what is in the data base for that entity. I really only want to restore the one entity and not perform the entire refetch.
I tried rollback and I tried NSUndoManager (with beginUndoGrouping and endUndoGrouping), and that is not working. I don't think I understand what rollback is really supposed to do.
But in any case, I still want to restore the property values in just that single entity (taking the lazy approach to only fetch what is needed, which is the one entity) so that my detail view controller can refill its view with the correct information. Right now it is using the NSManagedObject entity values, which contain the edited values, which were cancelled.
I suppose I could just start the edit process by creating a copy of the NSManagedObject. If the cancel-editing button is pressed, I could copy it back into the original. (I might even be able to just replace the original with the copy by moving the pointer. But since the pointer has actually been passed through several objects, I'm not sure how to manage the retain number on the copy.)
Does anyone have any other suggestions?
Thanks
Using rollback should accomplish what you want and I'm not sure what it doesn't. It is probably an implementation detail error.
You can find the specific managed object/s that were updated but not yet saved by calling the context's updatedObjects.
I want to follow the DDD philosophy and not access entity objects of an aggregate directly. So, i have to call the root object to get the associated entity. But In other cases I dont always want every associated entity to load when the root is called. Is that the purpose of lazy loading?
How do I access entity objects through the root without loading all the associated objects everytime if i disable lazyloading feature of linq?
EDIT:
For example, If I have a Person as the Root Entity, and the Person has Name, Addresses and OwnedProperties. If I want to get a list of People so that I could display their names, I dont necvessarily want to load up Owned Properties every time on the call to the Repository. Conversely, on another page I may want to show a list of OwnedProperties, but do not want the other information to load with the call. what is the simple way of just calling the Person without the owned property entity other than creating a new person object without that owned properties?
I don't thinks that's possible without lazy loading.
Getting all data at once: Eager Loading
Getting data when accessed: Lazy Loading
According to your edit:
What I do in these situations, is create a 'View' class or a 'DTO' class which just contains the properties that I'm interested in.
For instance, I could have a 'PersonView' class which just has a Name property for instance.
Then, using my OR/M mapper (I use NHibernate), I create a HQL query (or Criteria query) which works on my 'Person' entity. Before I execute the query, I tell NHibernate that I want 'PersonView' objects as a result (I specify a projection). Then, NHibernate is smart enough to execute a query that only retrieves the columns that are necessary to populate the PersonView instances.
One way to avoid lazy loading is just using the object 'id'