Consider an iPhone application that is a catalogue of animals. The application should allow the user to add custom information for each animal — let’s say a rating (on a scale of 1 to 5), as well as some notes they can enter in about the animal. However, the user won’t be able to modify the animal data itself. Assume that when the application gets updated, it should be easy for the (static) catalogue part to change, but we’d like the (dynamic) custom user information part to be retained between updates, so the user doesn’t lose any of their custom information.
We’d probably want to use Core Data to build this app. Let’s also say that we have a previous process already in place to read in animal data to pre-populate the backing (SQLite) store that Core Data uses. We can embed this database file into the application bundle itself, since it doesn’t get modified. When a user downloads an update to the application, the new version will include the latest (static) animal catalogue database, so we don’t ever have to worry about it being out of date.
But, now the tricky part: how do we store the (dynamic) user custom data in a sound manner?
My first thought is that the (dynamic) database should be stored in the Documents directory for the app, so application updates don’t clobber the existing data. Am I correct?
My second thought is that since the (dynamic) user custom data database is not in the same store as the (static) animal catalogue, we can’t naively make a relationship between the Rating and the Notes entities (in one database) and the Animal entity (in the other database). In this case, I would imagine one solution would be to have an “animalName” string property in the Rating/Notes entity, and match it up at runtime. Is this the best way to do it, or is there a way to “sync” two different databases in Core Data?
Here’s basically how I ended up solving this.
While Amorya’s and MHarrison’s answers were valid, they had one assumption: that once created, not only the tables but each row in each table would always be the same.
The problem is that my process to pre-populate the “Animals” database, using existing data (that is updated periodically), creates a new database file each time. In other words, I can’t rely on creating a relationship between the (static) Animal entity and a (dynamic) Rating entity in Core Data, since that entity may not exist the next time I regenerate the application. Why not? Because I have no control how Core Data is storing that relationship behind the scenes. Since it’s an SQLite backing store, it’s likely that it’s using a table with foreign key relations. But when you regenerate the database, you can’t assume anything about what values each row gets for a key. The primary key for Lion may be different the second time around, if I’ve added a Lemur to the list.
The only way to avoid this problem would require pre-populating the database only once, and then manually updating rows each time there’s an update. However, that kind of process isn’t really possible in my case.
So, what’s the solution? Well, since I can’t rely on the foreign key relations that Core Data makes, I have to make up my own. What I do is introduce an intermediate step in my database generation process: instead of taking my raw data (which happens to be UTF-8 text but is actually MS Word files) and creating the SQLite database with Core Data directly, I introduce an intermediary step: I convert the .txt to .xml. Why XML? Well, not because it’s a silver bullet, but simply because it’s a data format I can parse very easily. So what does this XML file have different? A hash value that I generate for each Animal, using MD5, that I’ll assume is unique. What is the hash value for? Well, now I can create two databases: one for the “static” Animal data (for which I have a process already), and one for the “dynamic” Ratings database, which the iPhone app creates and which lives in the application’s Documents directory. For each Rating, I create a pseudo-relationship with the Animal by saving the Animal entity’s hash value. So every time the user brings up an Animal detail view on the iPhone, I query the “dynamic” database to find if a Rating entity exists that matches the Animal.md5Hash value.
Since I’m saving this intermediate XML data file, the next time there’s an update, I can diff it against the last XML file I used to see what’s changed. Now, if the name of an animal was changed — let’s say a typo was corrected — I revert the hash value for that Animal in situ. This means that even if an Animal name is changed, I’ll still be able to find a matching Rating, if it exists, in the “dynamic” database.
This solution has another nice side effect: I don’t need to handle any migration issues. The “static” Animal database that ships with the app can stay embedded as an app resource. It can change all it wants. The “dynamic” Ratings database may need migration at some point, if I modify its data model to add more entities, but in effect the two data models stay totally independent.