I am using realm and I am not sure about this.
I have two option to do in Post. One is to save entire owner object and another is to save only user object ID. If I save entire object, is it object by reference? I don't want to increase database size.
The reason why I want to store entire object is that it is easy to access. It is like Post.User.email. I don't need to query. I read through realm though. https://realm.io/docs/objc/latest/
Thanks in advance.
No need to go by ID. In Realm, when you add a property of another RealmObject type, that will store the reference, not the entire object. So it will not take more space on disk than what you are expecting.
I am not sure if your diagram reflects your actual types, but note that when using collections, you want them to be of type RealmList<>, not Array<> (I am assuming you are writing C# as you referred to the Xamarin docs).
Related
If a Realm Object is included in a Realm.add(_:update:) with update set to true, its index is included in the resultant RealmCollectionChange, even when none of its properties changed.
In my case, I'm parsing JSON, which is then turned into my specific Object. Some of these will have been changed, some not, and some will be entirely new. So when I realm.add(possiblyNewOrUpdatedObjects, update: true), Objects whose values didn't change at all are included in the modifications index array.
Is there something I'm missing about this behavior? Why are non-modified Objects considered modified?
At the moment, that's the accepted functionality. If you set a property on an Object, even if it's the same value as before, that still counts as a change modification.
There's an issue on the Realm Cocoa repo discussing this functionality and if it should be considered a bug that needs fixing. Issue priorities at Realm are determined by how many users ask about it, so please add your support to that issue. :)
When using Realm.add(_:update:), Realm tries to verify that the object is managed and make sure that the object you are adding is managing by the same Realm. Then it will try to get or create row (fetch or create an object) before populate all properties.
Basically Realm doesn't know what are the value of each properties you are trying to update. Realm only sees them as a new object.
I'm using realm to cache certain network data coming down which is specific to the last search. I'd also like to be able to star or pin this data to save it for future usage. The data objects are exactly the same and I can't find anything in the documentation that allows me to save them in separate tables in Realm.
The easiest way to go about doing this would be to simply create a subclass of your model object with a different name (e.g., MyDataObject and its subclass MySavedDataObject). This will create a new table in the Realm database file with the same schema and will let you distinguish between the two types of objects.
You can then create a copy of a normal object as a saved object as simply as the following:
let myNewSavedObject = MySavedDataObject(value: myDataObject)
That all being said, instead of duplicating data, I would personally recommend being a bit more efficient with the existing data set. Surely simply adding an additional boolean property to the schema named something like saved would let you simply mark objects that you wish to keep without needing a whole second table. :)
I'm making a simple bank account tracker, for self-instructional purposes. I'm using Core Data to store three entities, related as in the screenshot:
WMMGTransaction objects are simply stored as they are recorded, and extracted as needed to feed tableviews and detail views. This will be done via NSFetchedResultsController and a predicate. I'm using MagicalRecord to access Core Data, if that matters.
My question is this:
When I pass WMMGAccount data from one VC to another, such as when creating a new account, or when selecting one from a list (via delegation as a rule), does it matter if I pass a reference to the entire entity, or can I just use an NSString bearing the .name of the account and identify the account when required with a predicate and an NSFetchedResultsController? I guess this is a strategy question, and may generate discussion, rather than having a cut and dried answer, but I'm wrestling with it, so I thought I'd ask.
It sounds like you're asking if you should pass an object to the code that needs it, or if you should pass information that could be used to look up the same object again.
Unless you need to use the managed object on a different thread or queue, you should always pass the actual object. No sense re-fetching an object you already have. It's extra work and code complexity that (unless there are some unusual extenuating details you didn't mention) won't help in any way.
If you are needing to use the object on a different queue or thread, passing information that can be used to look it up is the correct approach. But in that case-- don't pass the value of one of the properties. Use the managed object ID.
Core Data won't force name values to be unique, while the object's managedObjectID is unique. It's also faster when retrieving the object, because you can use objectForID: or existingObjectForID: instead of performing a fetch.
I am learning a bit on NSCoreData and before introducing it some existing projects I have, I would like to validate my good understanding of the core principles.
From what I have understood, NSCoreData make it easier to manage local storage of object (+retrieval after that) by subclassing our Model class from NSManagedObject rather than from NSObject.
Right ?
I have a few questions then. Let's consider I am building a real estate application with as core model object the class Property that can represent an appartment, a house, and all related information. Currently it is managed in my app as a subclass of NSObject.
1) I retrieve the properties from the server through a search query, and have written a initWithJson : method to populate each instance.
Now if I subclass Property from NSManagedObject, I will create my instances by using
+(id)insertNewObjectForEntityForName:(NSString *)entityName
inManagedObjectContext:(NSManagedObjectContext *)context
and I will be still be able to add a populateWithJson: to my class to fill in the properties.
Then I will create a lot of Property instances in the current managedObjectContext, and if I do a save, they will be stored at the physical layer.
If I call again the same webservice, and retrieve the same JSON content, I will recreate the identical managed objects.
How to avoid redundancy with the [managedObjectContext save:&error] call and not to store physically several time the representation of a single real life property ?
2) Let's say I want to store physically only some properties, for instance only the one the user want to have as favorites.
[managedObjectContext save:&error] will save all created / modified / deleted managed objects from the context to the physical layer, and not only the one I want.
How to achieve that ?
Am I supposed to declare another context (managedObjectContext2), move the instance I want to store in that context, and do the save in that one ?
(I mean, I will have a context just to manipulate the object, create instances from the JSON and represents them in UI ... and a second one to actually do the storage)
Or am I supposed to stores all the objects, and add a isFavorite BOOL property , and then fetching using a predicate on that property ?
3) The app has a common navigation pattern : the UITableView lists Properties instance with the minimum information required, and going on a detail view call a webservice to request more information on a specific Property instance (images, full text description).
Is it a good practice for instance to call the webservice only if the property.fullDescription is nil, and then update the object and store it locally with all detailed information, and the next time only to fetch it locally with a predicate on the property.id ?
What about object that might be updated server-side after they have been created?
Thanks for your lights
1) Retrieve the server data into a temporary form (array of dictionaries?), then for each possible property in the array, check to see if you already have an object in Core Data that matches. If you do, either ignore it or update any changed attributes; if not, create a Property object.
2) Decide which things you want to persist in order to support your app's functions. There's no point in creating a managed object for something you don't want to save. Note, though, that Core Data supports sub-classes if you want both Property and FavoriteProperty.
3) Entirely up to your "business rules"…. How often do you need local data to be updated? The only technical consideration might be the guideline to not keep large files locally that can be re-created on demand.
I am new to db4o.
I have this question in mind:
when the object are retrieved from DAL, maybe it will update in Business layer, then we lost it's original property, so when it comes to updating how can I find which one is the original object in the database to update?
You need to be more precise about "the object". If you modify the object instance's properties, simply storing it again will perform an update:
MyClass someInstance = ObjectContainer.Query<MyClass>().FirstOrDefault();
someInstance.Name = "NewName";
someInstance.PhoneNumber = 12132434;
ObjectContainer.Store(someInstance); // This is the update call
[This is just pseudo-code]
So you don't need to match objects to each other as you would have to when using an RDBMS.
However, you need to make sure you are not using a different instance of ObjectContainer, because a different container will not know these objects are the same instance (since there is no ID field in them).
Your application architecture should help to do this for most workflows, so there should be really only one IObjectContainer around. Only if timespans are really long (e.g. you need to store a reference to the object in a different database and process it somehow) it'd use the UUID. AS you already pointed out, that requires to store the ID somewhere else and hence complexifies your architecture.
If you however intend to create a new object and 'overwrite' the old object, things get somewhat more complicated because of other objects that might refer to it. However, this is a somehwat pathological case and should typically be handled within the domain model itself, e.g. by copying object data from one object to another.
You should load the object via its ID:
objectContainer.get().ext().getByID(id);
or via its UUID:
objectContainer.get().ext().getByUUID(uuId);
See the docs for the latter one. For an explanation see the answer here or the docs here. In short use uuid only for long term referencing.