I have two tables of data. One is table_A(id, x, x, b_id) and table_B(id, x).
I would like to add a relationship between b_id from table_A to id of table_B. I already have a JSON data like that, and I tried with Xcode to make so connection, but all I can make is a new relationship between those two.
I'm new to this, so would apreciate any help.
You are thinking of CoreData in terms of a DBMS which it is not. You don't need to set up foreign keys to make relationships in CoreData. If you want to assign a B table entity to a A you just create a relationship of between the two and you can set the attribute. The foreignKey and linking is all done by CoreData in the background.
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I have a question about core data. I have an Entity named User and this entity has 3 attributes: name, images, videos. How can I save an array of multiple images or videos to the attributes images or videos?
With relational databases, whenever you have one attribute that is going to have a bunch of values for a single entity, you have a one to many relationship and you actually need another entity. So in a strict relational database, you would create a new entity, Image, that had one column pointing to the image's data and a second column pointing to the user that that image is associated with, a foreign key. In CoreData, they represent these foreign key columns as a relationship. So you'll have to make a new entity for each 1-to-M(any) relationship you have and give them a relationship back to user, and in turn user to them.
Try this tutorial, it may help you.
I would like to create a model with two entities Orders and Products. They are linked by a relationship 'Contain' that has an attribute 'quantity'. How can I represent that in CoreData ? (Do not send me the ray tutorial, or any tutorial on youtube, I think I have done every thing). It is very important the relationship with attribute and not something general. (I know that it is not a database, but it is a Conceptual data model/Conceptual Schema as it is named by "entities" and "relationship" so if there is relationships, there must be a way to have relationships with attribute).
EDIT :
Am I doing the right thing by not adding id_order and id_product to the Contain entity ?
In CoreData, Contain would be another entity. It would have relationships to Orders and Products and a quantity attribute.
You cannot add attributes to a relationship in CoreData.
I'm using CoreData for the first time in one of my project. My table involves FOREIGN KEYS and since CoreData doesn't support FOREIGN KEYS, I'm having some issues.
Below is the structure of my tables.
My Problem is the establishment attribute.The establishment attribute is supposed to hold the name of a particular facility from the Facilities table. However, since it's a relationship, Xcode expects a Facility rather than just a name of a facility (NSString).
Is this possible, or am I just mixing up FOREIGN KEYS with RELATIONSHIPS in CoreData? How would I solve this problem?
Thanks in advance.
A relationship is not a property, so it does not have a type. In the model editor you add a relationship explicitly.
From your diagram, I see that you did not set the inverse relationship. There needs to be a corresponding relationship from name to the Assessors entity. (Set the "Destination" to Assessors in the model editor.)
I would also suggest to rename a few items.
First, use singular: Assessor, Facility. These are objects (comparable to classes), not tables.
Second, because your name attribute refers to an assessor, call it assessor, and similarly call its reverse relationship facilities (it is a to-many relationship in this direction, so the plural is appropriate).
If you need the name of the assessor of a facility, you use
facility.assessor.name
This should make it obvious why you do not need foreign keys. Indeed, I would urge you to think that it is not Core Data that does not support foreign keys, but that it is the traditional relational databases that do not support relationships!
I'm struggling with creating a suitable Core Data model for my app. I'm hoping someone here can provide some guidance.
I have two entities -- "Goals" and "Items". The Goals entity contains only a goal description, but any goal may have any number of subgoals, and these may extend multiple levels in a tree structure. Subgoals are to be contained within the same entity, so presumably the Goal entity will contain a pointer to "parent" which will be the parent goal of any subgoal.
There will also be an "Items" entity that contains a couple of text fields and a couple of binary items, and must be linked (ideally, by a unique identifier, perhaps objectID) to the particular goal or subgoal the item(s) are related to.
I am totally fumbling with how to set this model up. I know what attributes need to be in each entity, but the relationships, particularly between goals and "subgoals", has me stumped. I don't seem to be able to turn up any good examples of tree structures in Core Data on the Internet, and even the couple of books I have on Core Data don't seem to address it.
Can anyone here help an old SQL programmer get headed the right direction with these relationships in Core Data? Thanks.
Have you tried creating a one-to-many from Goal to itself, and a one-to-one from Goal to Item? The only thing I would worry about here is circular references.
Also, read Relationships and Fetched Properties in the CoreData Programming Guide.
Here is how it is done:
You set up a to-many relationship from Goal to Item in the model editor. Don't use any ids, foreign keys etc. This is old-fashioned database thinking - you can forget about it. Here we are only dealing with an object graph. The database layer is just an implementation detail for persisting the data.
Make two more relationships in entity Goal to itself: a to-one called parent, a to-many called subGoals. Make them the inverse of each other. Simple!
QED is correct, you can create a to many relationship on goal (call it subgoals) as well as a to-one relationship on goal (call it parentGoal) and set them as inverses to each other.
Then create another to many relationship (call it items) on the goal entity, with the inverse being a to one relationship on the item entity (call it goal). Then you're all set. You don't need to link items with a unique id, just add them to the items relationship.
Also note that if you did want to give items a unique id, do not use the objectID. The objectID should only be used as a temporary id as they are not guaranteed to remain the same. In fact they will change if you ever do a Core Data migration.
One way, though not really great, is to create a another entity, say subGoal, and each goal has one subGoal and each object of subGoal has many goal.
I'm currently learning to use Core Data on iOS , in my test application I have two entities with an inverse relation, the delete cascade is working fine but i wonder if it is possible to have a update or insert cascade as well? for example if I create a new instance of entity 1 i want some of its attributes to be copied onto a new object of entity 2.
Do I have to write some code for this or is there some built in solution?
searching the internet gave me no results.
(also since I'm new to Core Data i'm thinking in terms of tables as my persistent store is of SQLite so an insert into one table must essentially copy a few attributes into another table)
Try to think of it in a different way. If those two objects share those properties, perhaps it would be best to create another entity who contains those fields and entity 1 and entity 2 would both have a common relationship to. Having multiple copies of the same data just doesn't seem like a good idea where it can be avoided.
(You haven't mentioned multiplicity of the relationship, which could be important.)
Not sure if this directly addresses your question, but …
If you have A <--> B. (1-to-1 relationship)
Cascade rules:
A cascades: B
B nils: A
(this is an A "owns" B description)
(above A/B == entities, below A/B == instances of entities)
if A(1) -> B(2)
and then you set
A(3) -> B(2)
B(2)'s reverse relationship to A(1) is nil'd out before it's set to A(3)
A(1) is left with a nil value (if that's not valid in the data model description, you're now in trouble, otherwise, it's B-less)
A(1) -> <nil>