I have a class, which describe an object for parsing XML. This class contains NSStrings and NSMutableArrays of NSStrings.I want to store parsed data using CoreData. How should I write CoreData model to store such objects if there is no such data type like NSMutableArray? Is there any way?
You can create two entities, I'll call them Array and String. Array has one relationship, strings, which is a one-to-many relationship with the String entity. The String entity then has one attribute, I'll call it string too, which actually is a string and contains the value you need. This will allow you to store them as a set, if the order is important add another attribute to the String entity called order and make it a number.
There isnt an array data type, but u can box the string in a core data object (lets call it A for the remainder of the post). Then in the object (B) which you want to have the array of string (that persists) in just make a to-many relationship with the object A which contains the string, then youll have a Set of string on the object B which you can turn into an array and do with what you like. hope that helps
Related
I use a Transformable attribute named viewingDates on a Item entity.
Thus I can declare the property #NSManaged public var viewingDates: Set<Date> on the Item class.
But how to format a NSPredicate to fetch all Item where viewingDates is not empty?
Note: My database contains more than 15000 items and I would like if possible to avoid having to fetch them all before filtering them via items.filter { !$0.viewingDates.isEmpty }.
With transformable properties you can't filter fetch requests based on the property values, at all. Transformable properties are converted to Data when you save, and back when you fetch, but during fetch they're still Data. All you can really do is check whether they're null.
If this property were optional, you could check if it was null. But you cannot use a predicate that considers the contents of the property.
I've two different object with same father. I want to store them in the same collection, but I want to be able to retrieve each object separately.
for example if these are my objects:
I want to retrieve all of FirstChild objects without retrieving any SecondChild Object.
Is there any way other than adding a type field to the father object, to retrieve them?
Assuming first child and second child are different types stored in different fields of the father object (father is a composition of first and second child)
datastore.find(FatherObject.class).retrievedFields(false,"secondChildField")
will get everything except secondChildField or
datastore.find(FatherObject.class).retrievedFields(true,"firstChildField")
will bring only firstChildField.
When you create your query, pass in the class reference of the type you want: datastore.createQuery(SecondChild.class). Morphia, by default, tracks the class type of the document so it can filter by that type.
It has to be specified while creating the core data model in IB what Kind of attribute it will be, theres choices like String, Date, Integer, Decimal etc.
My question is, how do i store lets say an NSDate in the same attribute, and at some other time, add a new entity but this time with an NSNumber for that attribute.
In other words, i'd just want an equalent of id Object in coredata, where after fetching, i'd check if [[Object isKindOfClass[NSDate class]] or an NSNumber.
I've heard of transformable, but i'm not creating customized Objects to be stored.
Any light on this would be great
This is a very bad idea. Actually, dates are represented as numbers in SQLite, but Core Data could be using a different kind of store, so you are just making too many assumption of how things are going to work.
It would be much cleaner and easier to specify additional attributes for your entity, a number and a date. You could then easily check if any of them is nil or contains a value. Even introducing a third attribute to tell you if the object has a date or number would be preferable to your setup.
I have come across this scenario in the current project. If your values contain int, float, boolean you can use NSNumber as attribute and if it contains NSDate then attribute should be as a string in the entity.
Hope this help.
Suppose I have a list of books stored in Core Data. I want to search for a book by it's primary key ID.
I know the sqlite file created by Core Data has an ID column in each table, but this doesn't seem to be exposed to me in anyway.
Does anyone have any recommendations?
-[NSManagedObject objectID] is the unique ID for an object instance in Core Data. It can be serialized via -[NSManagedObjectID URIRepresentation]. You can retrieve the objectID from a persistent store coordinator with -[NSPersistentStoreCoordinator managedObjectIDForURIRepresentation:] and then get the object from a managed object context with -[NSManagedObjectContext objectWithID:].
BUT
You should keep in mind that Core Data is not an ORM. It is an object graph management framework. That is uses SQLite (and unique row IDs) as a backend is purely an implementation detail. The sooner you can get yourself out of the SQL/RDBMS mindset, the faster you will be happy with Core Data. Instead of trying to find an object from a stored ID, consider why you need that object and what object needs it. If an instance of class Foo needs to be able to get to an instance of class Bar, why not just create an association from the Foo to the Bar and set the appropriate Bar instance as the target of the association on the appropriate Foo instance. Let Core Data keep track of object IDs.
As Barry Wark said, remember always that Core Data is not an orm. Pure SQL details are not exposed to the user and every row is just an object. By the way, sometime you should need to access the "primary key", for example when you need to sync the coredata db with external sql databases (in my case I needed it in a callback function to change the state of an object after INSERT it with success in the remote db). In this case, you can use:
objectId = [[[myCoredataObject objectID] URIRepresentation] absoluteString]
that will return a string like: x-coredata://76BA122F-0BF5-4D9D-AE3F-BD321271B004/Object/p521 that is the unique id used by Core Data to identify that object.
If you want to get back an object with that unique id:
NSManagedObject *managedObject = [managedObjectContext objectWithID:[persistentStoreCoordinator managedObjectIDForURIRepresentation:[NSURL URLWithString:objectId]]];
NB: Remember that if the receiver has not yet been saved in the CoreData Context, the object ID is a temporary value that will change when the object is saved.
This is the way you can get the object id as String using Swift from a NSManagedObject:
entity.objectID.uriRepresentation().absoluteString
in Swift this will be done by getting ID of the row as URI then get last path of URI
entity.objectID.uriRepresentation().lastPathComponent
the output of last path will look like this
p12
this output is string so you can remove the p using:
trimmingCharacters()
// Like this
let id = entity.objectID.uriRepresentation().lastPathComponent.trimmingCharacters(in: ["p"])
Example: I read data from an XML file. This data has unique id elements. I want to store those objects with their original unique id. How would I do that?
I figured out I could ask the managed object for it's ID, like this:
NSManagedObjectID *moID = [managedObject objectID];
but here the problem is: The XML tells me with the id element which object this is, and I need to look up in the database of core data if this object already exists in there, or not. So is it the only option to make an id attribute in my managed object model for that entity and then query for that? Then I will have two id systems right?
Don't worry about the ObjectID of Core Data. This is an internal unique ID which is not guarantied to be constant during the object's life cycle (e.g. it will change when you save the object to sql store). Just create a new mandatory attribute in your model and flag it as indexed so retrieval will be fast.
In the entity associated to this kind of objects, simply add another attribute of type string, call it objectID or similar and declare it to be mandatory.