I am trying to learn osx and i would like to find out more about core data. I have read some parts of the documentation and some books, and I am now experimenting with core data in general.
Trying to make trivial mac app, i run in the database design issue:
Let say that in RDBMS you would have ids, primaryKeys and foreignKeys like this
table products
productID
categoryID
etc....
table Categories
categoryID
etc.....
My questions are:
What is the equivalent of primary key in core data?
What is the eqivalent of the foreign key in core data?
How do you ensure that record is unique in the table?
Can anyone clarify the design concept in core data database?
Any links about core data (aside from Core Data Programming Guide from apple) will be appreciated.
Regards, John
First of all, you must understand that Core Data is a object oriented persistence layer and not simply a database. Here you must switch your thinking from records, joints and queries to object oriented design.
Here I'm trying to answer your questions:
1. What is the equivalent of primary key in core data?
Core Data has its own primary key system. Each object is identified by a unique "ObjectID" that will be used as primary key internally.
You can define as many attributes in your entities and use them as "primary keys". However you won't be able to make your "record" (object) unique as it has no sense that a "object with a specific property is unique". This kind of logic is you who must add it programatically.
2. What is the eqivalent of the foreign key in core data?
A foreign key is represented in Core Data by a relationship. There are one-to-one relationships and one-to-many relationships. This means that an object can have a pointer to another object (therefore, in the database you would have a "foreign key") or an object have a collection that contains other objects (an array for example) (therefore, in the database you would have an extra table to represent this structure).
3. How do you ensure that record is unique in the table?
As mentioned above, you can't. In CoreData you have unique objects (NSManagedObject) per each context (NSManagedObjectContext) but you cannot control that an object with an specific attribute is unique among your other objects.
4. Can anyone clarify the design concept in core data database?
As said in the introduction, here you should not think in terms of databases. Core Data is an oriented object design persistence framework.
Core Data is not a simple thing that is fast learned. You need to spend some time to understand what you can do and most important, why and in which cases you should do it.
I highly recommend you to read the Core Data Programming Guide:
https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/CoreData/cdProgrammingGuide.html
Hoping to be helpful,
Joan
If you read the Apple Core Data Programming Guide you can get an idea and clarify your doubts
https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/CoreData/Articles/cdTechnologyOverview.html
Related
I'm trying to figure out how to use Core Data in my App. I already have in mind what the object graph would be like at runtime:
An Account object owns a TransactionList object.
A TransactionList object contains all the transactions of the account. Rather than being a flat list, it organizes transactions per day. So it contains a list of DailyTransactions objects sorted by date.
A DailyTransactions contains a list of Transaction objects which occur in a single day.
At first I thought Core Data was an ORM so I thought I might just need two tables: Account table and Transaction table which contained all transactions and set up the above object graph (i.e., organizing transactions per date and generating DailyTransactions objects, etc.) using application code at run time.
When I started to learn Core Data, however, I realized Core Data was more of an object graph manager than an ORM. So I'm thinking about using Core Data to implement above runtime object relationship directly (it's not clear to me what's the benefit but I believe Core Data must have some features that will be helpful).
So I'm thinking about a data model in Core Data like the following:
Acount <--> TransactionList -->> DailyTransactions -->> Transaction
Since I'm still learning Core Data, I'm not able to verify the design yet. I suppose this is the right way to use Core Data. But doesn't this put too many implementation details, instead of raw data, in persistent store? The issue with saving implementation details, I think, is that they are far more complex than raw data and they may contain duplicate data. To put it in another way, what exactly does the "data" in data model means, raw data or any useful runtime objects?
An alternative approach is to use Core Data as ORM by defining a data model like:
Account <-->> Transactions
and setting up the runtime object graph using application code. This leads to more complex application code but simpler database design (I understand user doesn't need to deal with database directly when using Core Data, but still it's good to have a simpler system). That said, I doubt this is not the right way to use Cord Data.
A more general question. I did little database programming before, but I had the impression that there was usually a business object layer above plain old data object layer in server side programming framework like J2EE. In those architectures, objects that encapsulate application business are not same as the objects loaded from database. It seems that's not the case with Core Data?
Thanks for any explanations or suggestions in advance.
(Note: the example above is an simplification. A transaction like transfer involves two accounts. I ignore that detail for simplification.)
Now that I read more about the Core Data, I'll try to answer my own question since no one did it. I hope this may help other people who have the same confusion as I did. Note the answer is based on my current (limited) understanding.
1. Core Data is an object graph manager for data to be persistently stored
There are a lot articles on the net emphasizing that Core Data manages object graph and it's not an ORM or database. While they might be technically correct, they unfortunately cause confusion to beginner like me. In my opinion, it's equally important to point out that objects managed by Core Data are not arbitrary runtime objects but those that are suitable for being saved in database. By suitable it means all these objects conform to principles of database schema design.
So, what' a proper data model is very much a database design question (it's important to point out this because most articles try to ask their readers to forget about database).
For example, in the account and transactions example I gave above, while I'd like to organize transactions per day (e,g., putting them in a two-level list, first by date, then by transaction timestamp) at runtime. But the best practice in database design is to save all transactions in a single table and generating the two-level list at runtime using application code (I believe so).
So the data model in Core Data should be like:
Account <->> Transaction
The question left is where I can add the code to generate the runtime structure (e.g., two-level list) I'd like to have. I think it's to extend Account class.
2. Constraints of Core Data
The fact that Core Data is designed to work with database (see 1) explains why it has some constraints on the data model design (i.e., attribute can't be of an arbitrary type, etc.).
While I don't see anyone mentioned this on the net, personally I think relationship in Core Data is quite limited. It can't be of a custom type (e.g, class) but has to be a variable (to-one) or an array (to-many) at run time. That makes it far less expressive. Note: I guess it's so due to some technical reason. I just hope it could be a class and hence more flexible.
For example, in my App I actually have complex logic between Account and its Transaction and want to encapsulate it into a single class. So I'm thinking to introduce an entity to represent the relationship explicitly:
Account <->> AccountTranstionMap <-> Transaction
I know it's odd to do this in Core Data. I'll see how it works and update the answer when I finish my app. If someone knows a better way to not do this, please let me know!
3. Benefits of Core Data
If one is writing a simple App, (for example, an App that data modal change are driven by user and hence occurs in sequence and don't have asynchronous data change from iCloud), I think it's OK to ignore all the discussions about object graph vs ORM, etc. and just use the basic features of Core Data.
From the documents I have read so far (there are still a lot I haven't finished), the benefits of Core Data includes automatic mutual reference establishment and clean up, live and automatically updated relationship property value, undo, etc. But if your App is not complex, it might be easier to implement these features using application code.
That said, it's interesting to learn a new technology which has limitation but at the same time can be very powerful in more complex situations. BTW, just curious, is there similar framework like Core Data on other platforms (either open source or commercial)? I don't think I read about similar things before.
I'll leave the question open for other answers and comments :) I'll update my answer when I have more practical experience with Core Data.
Core data is simple, efficient, and mind blowing if we know where to use and where not to.
I have been playing with Core data for a while.
I want to know Where not to use core data.
The question is simple, lets say, I want to use core data as a real "Database" like MySql.
Have primary key, search, sort (with one or more columns) etc..
As far as I tried I couldn't achieve it.
There is no useful documentation or help anywhere I can find for "havingPredicate".
Is choosing to use core data in the place of a DB is a potential liability?
or
If it is possible, is there a particular "core data model" in which it can be done or at least most of it can be done.. (with limitations which is distinct)?
Core Data provides some database-like features, but with an API that's different from using raw SQL statements. For example, Core Data entities are roughly equivalent to SQL tables, but its handling of relationships is very different from how SQL works. Whether it's a "potential liability" depends on what you need to accomplish in your app. Core Data was designed as a system that allows reading and writing of model objects directly, and while it has some features similar to SQL, thinking in terms of SQL is a good way to screw it up.
Have primary key, search, sort (with one or more columns) etc..
As far as I tried I couldn't achieve it.
Core Data uses its own unique ID system which serves as a primary key. Every instance of NSManagedObject has a property called objectID which is unique. You can add your own primary key fields but it's up to you to ensure uniqueness. It's typically not necessary, but again it really depends on what you need to accomplish. Searching and sorting are built in via NSPredicate and NSSortDescriptor. If you found this to be difficult or impossible, it would be a good idea to post a new question with details of what you tried and how it failed to do what you needed.
There is no useful documentation or help anywhere I can find for "havingPredicate".
It serves the same purpose as in SQL via NSPredicate. The syntax is different, so you might want to consult Apple's Predicate Programming Guide to see how it works. As a simple example, the SQL fragment HAVING SUM(SaleAmount) > 1000 would be realized with something like #sum(saleAmount) > 1000).
I realize this may be common sense for a lot of people, so apologies if this seems like a stupid question.
I am trying to learn core data for iOS programming, and I have repeatedly read and heard it said that Core Data (CD) is not a relational database. But very little else is said about this, or why exactly it is important to know beyond an academic sense. I mean functionally at least, it seems you can use CD as though it were a database for most things - storing and fetching data, runnings queries etc. From my very rudimentary understanding of it, I don't really see how it differs from a database.
I am not questioning the fact that the distinction is important. I believe that a lot of smart people would not be wasting their time on this point if it weren't useful to understand. But I would like someone please to explain - ideally with examples - how CD not being a relational database affects how we use it? Or perhaps, if I were not told that CD isn't a relational database, how would this adversely impact my performance as an Objective-C/Swift programmer?
Are there things that one might try to do incorrectly if they treated CD as a relational database? Or, are there things which a relational database cannot do or does less well that CD is designed to do?
Thank you all for your collective wisdom.
People stress the "not a relational database" angle because people with some database experience are prone to specific errors with Core Data that result from trying to apply their experience too directly. Some examples:
Creating entities that are essentially SQL junction tables. This is almost never necessary and usually makes things more complex and error prone. Core Data supports many-to-many relationships directly.
Creating a unique ID field in an entity because they think they need one to ensure uniqueness and to create relationships. Sometimes creating custom unique IDs is useful, usually not.
Setting up relationships between objects based on these unique IDs instead of using Core Data relationships-- i.e. saving the unique ID of a related object instead of using ObjC/Swift semantics to relate the objects.
Core Data can and often does serve as a database, but thinking of it in terms of other relational databases is a good way to screw it up.
Core Data is a technology with many powerful features and tools such as:
Change tracking (undo/redo)
Faulting (not having to load entire objects which can save memory)
Persistence
The list goes on..
The persistence part of Core Data is backed by SQLite, which is a relational database.
One of the reasons I think people stress that Core Data is not a relational database is because is it so much more than just persistence, and can be taken advantage of without using persistence at all.
By treating Core Data as a relational database, I assume you mean that relationships between objects are mapped by ids, i.e. a Customer has a customerId and a product has a productId.
This would certainly be incorrect because Core Data let's you define powerful relationships between object models that make things easy to manage.
For example, if you want to have your customer have multiple products and want to delete them all when you delete the customer, Core Data gives you the ability to do that without having to manage customerIds/productIds and figuring out how to format complex SQL queries to match your in-memory model. With Core Data, you simply update your model and save your context, the SQL is done for you under the hood. (In fact you can even turn on debugging to print out the SQL Core Data is performing for you by passing '-com.apple.CoreData.SQLDebug 1' as a launch argument.
In terms of performance, Core Data does some serious optimizations under the hood that make accessing data much easier without having to dive deep into SQL, concurrency, or validation.
I THINK the point is that it is different from a relational database and that trying to apply relational techniques will lead the developer astray as others have mentioned. It actually operates at a higher level by abstracting the functionality of the relational database out of your code.
A key difference, from a programming standpoint, is that you don't need unique identifiers because core data just handles that. If you tried to create your own, you will come to see that they are redundant and a whole lot of extra trouble.
From the programmer's perspective, whenever you access an entity "record", you will have a pointer to any relationship -- perhaps a single pointer for a "to-one" relationship, or a set of pointers to the records in a "to-many" relationship. Core Data handles the retrieval of the actual "records" when you use one of the pointers.
Because Core Data efficiently handles faults (where the "record" (object) referenced by a pointer is not in memory) you do not have to concern yourself with their retrieval generally. When they're needed by your program Core Data will make them available.
At the end of the day, it provides similar functionality but under the hood it is different. It does require some different thinking in that ordinary SQL doesn't make sense in the context of Core Data as the SQL (in the case of a sqlite store) is handled for you.
The main adjustments for me in transitioning to Core Data were as noted -- getting rid of the concept of unique identifiers. They're going on behind the scenes but you never have to worry about them and should not try to define your own. The second adjustment for me was that whenever you need an object that is related to yours, you just grab it by using the appropriate pointer(s) in the entity object you already have.
I am new to iOS programming but have done SQL stuff for years. I am trying to use Core Data to build my model. Following the tutorials I have created a schema for my application that involves a number of one-to-many relationships that are not bi-directional.
For example I have a Games entity and a Player entity. A Game includes a collection of Players. Because a Player can be involved in more than one game, an inverse relationship does not make any sense and is not needed.
Yet when I compile my application, I get Consistency Error messages in two forms. One says.
Game.players does not have an inverse; this is an advanced setting.
Really? This is an "advanced" capability enough to earn a warning message? Should I just ignore this message or am I actually doing something wrong here that Core Data is not designed to do?
The other is of the form Misconfigured Property and logs the text:
Something.something should have an inverse.
So why would it think that?
I can't find any pattern to why it picks one error message over the other. Any tips for an iOS newb would be appreciated.
This is under Xcode 5.0.2.
Core Data is not a database. This is an important fact to grasp otherwise you will be fighting the framework for a long time.
Core Data is your data model that happens to persist to a database as one of its options. That is not its main function, it is a secondary function.
Core Data requires/recommends that you use inverse relationships so that it can keep referential integrity in check without costly maintenance. For example, if you have a one way between A and B (expressed A --> B) and you delete a B, Core Data may need to walk the entire A table looking for references to B so that it can clean them up. This is expensive. If you have a proper bi-directional relationship (A <-> B) then Core Data knows exactly which A objects it needs to touch to keep the referential integrity.
This is just one example.
Bi-directionality is not required but it is recommended highly enough that it really should be considered a requirement. 99.999% of the time you want that bi-directional relationship even if you never use it. Core Data will use it.
Why not just add the inverse relationship? It can be to-many as well, and you may well end up using it - often fetch requests or object graph navigation works faster or better coming from a different end of a relationship.
Core Data prefers you to define relationships in both directions (hence the warnings) and it costs you nothing to do so, so you may as well. Don't fight the frameworks - core data isn't an SQLLite "manager", it is an object graph and persistence tool, that can use SQL as a back end.
I'm working with Core Data and a many-to-many relationship: a building can have multiple departments, and a department can be in multiple buildings. Having worked with databases before, I was unsure of how to implement this in Core Data, but I found this in the Core Data Programming Guide:
If you have a background in database management and this causes you
concern, don't worry: if you use a SQLite store, Core Data
automatically creates the intermediate join table for you.
However, there's not really any data integrity. I've tried inserting a few building objects, which for now only have one attribute (number), and every time I set the department object (relationship) it relates to. This results in the database containing multiple building objects with the same building number, all relating to a different department object. Ideally, there would be one object per building number, with in it all the different departments that are located in it.
So, my question is: can Core Data maintain data integrity somehow, or should I check to see if a building object with that number already exists before inserting it? It looks like I'll have to manually check it, but it would be cool if Core Data could do this for me.
What melsam wrote is right. In addition to his answer I suggest you to use inverse relationships. About inverse, Apple says:
You should typically model relationships in both directions, and
specify the inverse relationships appropriately. Core Data uses this
information to ensure the consistency of the object graph if a change
is made (see “Manipulating Relationships and Object Graph Integrity”).
For a discussion of some of the reasons why you might want to not
model a relationship in both directions, and some of the problems that
might arise if you don’t, see “Unidirectional Relationships.”
A key point to understand is that when you work with Core Data, you work with objects. So, integrity criteria are resolved when you save the context or you explicity says to context to process also process pending changes (see processPendingChanges method).
About your question, I guess you have to create a fetch request and retrieve the object(s) you are looking for (e.g. you could provide to each object a specific id and set up a predicate with the id you want).
If the fetch request retrieve some objects, then you can update them. If not, create a new object with insertNewObjectForEntityForName:inManagedObjectContext:. Finally save the context.
I suggest you to read about Efficiently Importing Data.
Hope it helps.
Core Data maintains data integrity for you. I can assure you (from lots of experience with Core Data) that you do not have to manually check integrity. Doublecheck how your relationships and delete rules are set up in Xcode's Core Data Model Editor. I can't tell exactly what may be wrong with the details you've provided, but you'll find it if you poke around.