In core data,
I want to use relationship. But I have a doubt in my mind. Apologies but I didn't find clear answers on various websites and blogs. Other things like Add, Fetch, Delete queries are clear to me. But I have some questions in core data which are still not clear.
I have made two Entities:
Photographer [Attributes : name and camera]
Photo [Attributes: zoner and photographerName] .
1) I have connected them with relations. So if I connect this two entities with relations then I should remove that photographerName from Photo?
2) As I have connected these two entities with the relation then how can I use the photographer name with the photo Entity?
3) How can I add values in this if I use it with relations? [Now it is showing me Null in the relation from sqlite browser]
So if I connect this two entities with relations then I should remove that PhotographerName from Photo ?
Generally, yes, it is redundant.
how can I use the photographer name with the photo Entity ?
self.photographer.name
(assuming you are in the Photo class and the relationship names is photographer)
How can I add values in this if I use it with relations
I guess you mean how can I set the relationship value. Create an instance (or find an existing instance) or each entity and then:
photo.photographer = photographer;
Notes:
Ensure that the relationship has an inverse
Names the relationship ends photographer and photos (1 to many)
Try to set the photographer of a photo, or use the relationship methods auto-generated in the Photographer class to add photos
Related
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.
in my IOS10, swift 3, xcode8 application, I have Movie in coreData.
Movie: {id:Int32, name:String, genre: [ {Genre} ] }
Genre: {id:Int32, name:String}
the way I did it, is by creating a movie entity, and a genre entity, and set up many-to-many relationship between them...
is this correct ? cause one movie can have many genres, like action, comedy
and a genre, like romantic, can be associated with many movies.
my second question is, when I am saving a new movie to CoreData, How will the Genre entity know what happened, is there like a CoreData Feature that will do that automatically? or I should add a movie to the genre Set, and add a genre set for movie ?
my end goal is to be able to search for movies by titles, or by genre.
This is fairly basic, so I would recommend researching it: CoreData Documentation
But a basic rundown is when you create a many<->many relationship CoreData creates a connections table between the two models. The way coreData works is with managed objects, meaning every time you try and access an attribute of an NSManagedObject it will be the value in the saved database.
When you add something to a many-many relationship, the connections table will create the connection, and when access it from either model, the results will be based on that connections table.
TLDR, in CoreData if you have a relationship between two models, you only need to add the connection to one of the models.
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 am using Core Data in my app and have some entities that have many-to-many relationships. Is there a way to configure the relationships in such way that all entities that don't have at least one entry in some cross-reference table get removed?
Simplified entities:
User:
- songs
- albums
Song
- users
Album
- users
For example, when I remove all users from some album, I want that album to get removed. I know this can be done by checking the number of remaining users, but is there a simpler way?
You can set the delete rule of the relationships in both directions to "Cascade" in the model editor. Core Data should then delete the corresponding entity instances automatically.
You should not follow any advice recommending intermediate "join" tables. This is bad and redundant practice within the Core Data framework and only necessary if you need to store additional information about each relationship itself.
I have two entities: patient and checkpoint.
Patient has attributes such as DOB, name, ID, etc.
Checkpoint has attributes such as dateRecorded, height, weight, etc.
You probably get the idea- I want there to be a set of patients, and then each patient can have checkpoints associated with that patient.
On both entities, how should I set the settings? The settings are:
I looked at the documentation for this, and I was still confused. I think what I want is a one to many relationship (for patient), but then I'm not sure how to set the inverses for either of them, or the delete rule and the other stuff. THANK YOU!!
I just got started with Core Data this week. Great question!
Relationships:
Since one patient can have many checkpoints, the Patient to Checkpoint relationship is a One to Many relationship. The concept of an "inverse relationship" is essentially this: You've got a relationship going one way (Patient to Checkpoint) - now go ahead and look at it from the inverse, the Checkpoint's perspective. A checkpoint can apply to only a single patient. Therefore, the Checkpoint to Patient relationship is a One to One relationship.
Inverse Relationships:
To handle the inverse relationship, simply create each relationship, ignoring the inverse. Then, after you have the relationship on each object, go ahead and define the inverse as the relationship on the other entity.
In other words, a relationship points to another entity or a group of entities. An inverse relationship points to a relationship on another entity.
Delete Rules:
As far as delete rules are concerned, it's fairly simple. When trying to delete a patient which has checkpoints...
Deny: Core Data won't let you delete the Patient.
Cascade: Core Data will delete the Entity (Patient), as well as cascading through relationships and deleting those objects as well. (In other words, Core Data will delete the Checkpoint objects too.)
Nullify: Core Data will delete the patient but first remove the relationship. The Checkpoint will remain intact.
For the Patient entity might want either deny or cascade, depending on how you want to manage your data. Based on your usage case, you probably don't want nullify, since Checkpoints are dependent upon Patient entities.
You want nullify for the Checkpoint, since the Cascade would prevent you from deleting a checkpoint without deleting the entire patient, and Deny would effectively force the same.
Based on the scenario mentinoed, it looks like a one to many relationship between patient and checkpoint tables.
Now add a relationship from “Patient” to “Checkpoint”, and also set the inverse between the tables.
Also, set the delete rule for both relationships to “cascade”. This means that if you delete one object with Patient, the corressponding Coredata will delete the associated object as well.