If a Core Data relationship has an inverse relationship do you only need to set one of the relationships objects then the corresponding relationship is setup?
In the past I have set both relationships but when looking at iPhone Core Data Recipes it seems they only set 1 of the relationships?
Thanks
James
I found the answer in the documentation:
Since Core Data takes care of the object graph consistency maintenance for you, you only need to change one end of a relationship and all other aspects are managed for you. This applies to to-one, to-many, and many-to-many relationships. Consider the following examples.
So the answer is YES you only need to set one side of the relationship.
More information here:
https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CoreData/Articles/cdRelationships.html#//apple_ref/doc/uid/TP40001857-CJBDBHCB
Related
In my app I have two entities, User and Meetings. What I want is a list of User who have meetings today.
Also, I haven't added relationship between both the entities. Is there any way through which I can query both the entities in a single fetch request. Or is there any other way.
Please help me to solve this in best possible way
Thanks in advance
Core Data tries to map objects from the OOP-world into tables and rows from the rDBMS-world and back. This is called a object-relational mapper (ORM). Even this looks very easy, because concepts seems to be similar, it is a difficult task. One called it the "Vietnam of information technology".
However, at some point things do not go together. This is called the object-relational impedance mismatch (ORIM). At this point one has to decide, whether he takes the OOP-way or the rDBMS-way. Resolving relationships is one of this points.
Core Data decided to do this the OOP-way: Relationships are treated as relationships between "usual" objects. This has two consequences:
You do not join anything. In OOP objects are not joined. So in Core data objects are not joined. (However, they have some features in a fetch request with dictionaries, but this is not the usual way to access data in Core Data.)
To do the job, Core Data needs to know the relationships between objects. You have to set the relationships.
Here above shows the ER-Diagram.
It's easy to implement to-many binary relationship using core data. But it confuse me how to implement this kind of multiple relationship.
Hope that someone could give a hand.
I am not sure if I understood your problem correctly, but wouldn't two
one-to-many relationships (from Course to TA and from Student to TA) describe
your model? Each TA has exactly one Student and one Course, but each Student and
each Course can be related to many TAs.
I have category entity and subcategory entity. I need to get subcategory data to that related categoryId of category entity.
My category entity contains these attributes: "categoryId","categoryName"
subcategory contains: "subcategoryId","subcategoryName", "categoryId".
So can any one please guide me how can i put the relationship between this two entity ??
Thanks for advance.
Very first you are mixing the concepts of MySQL or SQLite with Core data. Unlike them Core Data does not have primary-foreign Key concept to relate entities(for easy understanding tables in MySQL). Just create relationship between those entities and you can fetch data their data.
Now about your Entities you have are category and subcategory. So you have to create relationship between them. If one category have many subcategories you have to check To many relationship option from Data Model Inspector..Otherwise One to One relationship would be the one you should go for..Have a look at screenshot how you can make your relationship one to many..
This is a good tutorial link for one to many relationship. You can refer if you do not know how to implement.
Also this for tutorial simple relationship in Core Data..
if you are beginner with Core data you can go with Quick Tutorial Start by Apple and you will get basic idea of Core data.
I asked this question earlier, but I'm missing one important thing. I have a NoteObject entity and every note, without Core Data, just has an arrayOfTags (which is an array of NSStrings). I decided to use to-many relationships to store the tags instead of an array. So I created a new "Tags" entity, and set up a to-many relationship from my NoteObject to Tags. This works great when every tag is related to only one note, but I'm unsure how I would go about linking one tag with multiple notes. How would I set up that relationship?
In you data modeler just make the Tags -> Notes relationship a to-many relationship. If you have already generated the Entity subclasses you will have to redo those. You will also have to be careful when creating tags to ensure uniqueness, but other than that it should be fairly straightforward. myTag.notes should work just as well as myNote.tags.
Apple's documentation suggests the use of inverse relationships when modelling data models in CoreData.
I have the following example:
A Book (Entity) has several "pages" and one "frontCover" and one "backCover".
A Page (Entity) is in one "book" (so "book" is the inverse of "pages").
OK so far, that's the standard case...BUT now, my problem:
I only have one class Cover (Entity). A Cover (Entity) is on one "book". On this "book" the Cover is EITHER the "frontCover" OR the "backCover". So, the inverse of "book" is EITHER "frontCover" OR "backCover".
This cannot be modelled in CoreData. A relationship can only be the inverse of one relationship, but not of EITHER this OR that relationship.
What is the best way to model this? Unidirectional relationships (no invers)?
Thanks for your answers,
Chris
You could do something like the model below (first image). This would leaving of Cover's inverse relationships as nil. This doesn't feel right to me, though.
Another option (second image) would be to give Book a 'covers' relationship which references 2 Cover objects, and give Cover an isFront boolean attribute. This would allow for inverse relationship called 'book'.
One way to do it could be to create Cover as an "abstract" entity with two sub entities - FrontCover & BackCover. Then you could create the relationship & inverse to each of those.
I just want to add.
One way is to have 2 subentities. But that's useful only if FrontCover and BackCover differs a lot.
If they are exactly the same object, you should instead use an enum in the entities.
That enum differentiate whether the cover is frontCover or BackCover.
Then you set only 1 "to many" relationship from book to cover.
The purpose of coredata is to save your data. Your logic should be in the code anyway.
Also creating two subEntities is essensially the same with westSider's answer. Sub entities simply add another relationship on the original one.