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Ramblings of a curious coder

Seeding CoreData Databases With Ruby

If you’re writing an iOS app that uses Core Data then you may well want to ship it with an initial database (which potentially gets over the air updates later on).

On iOS, CoreData stores always use sqlite3 as their backend. You could create a sqlite database directly, but you’d have to reverse engineer the way apple uses sqlite, ensure that you use the same name manging for table and column names, generate the same meta data used for persistent store migration etc. Too brittle for my liking.

Luckily both RubyCocoa and MacRuby allow you to access the Core Data framework from a ruby script. The former is bundled with Mac OS X since 10.5 (you will need to use the system ruby). RubyCocoa has some clunkier syntax because it doesn’t have the benefit of the extensions to ruby, however at the moment MacRuby doesn’t quite work with Active Record, which is where I was getting my seed data from. If your data is coming from else where, this may not be a problem. Other than the slight syntax differences around the handling of Objective-C’s sort-of-named-arguments the code is the same

The RubyCocoa version looks like this

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require 'rubygems'
require 'osx/cocoa'

OSX.require_framework 'CoreData'

class CoreDataStore
  def create_entity name, props={}, relationships={}
    entity = OSX::NSEntityDescription.insertNewObjectForEntityForName_inManagedObjectContext(name, context)
    props.each do |k,v|
      entity.setValue_forKey v, k
    end
    relationships.each do |k, objects|
      collection = entity.mutableSetValueForKey(k)
      objects.each {|o| collection.addObject o}
    end
    entity
  end

  def initialize(data_store_path, mom_path)
    @data_store_path = data_store_path
    @mom_path = mom_path
  end

  def context
    @context ||= OSX::NSManagedObjectContext.alloc.init.tap do |context|
      model = OSX::NSManagedObjectModel.alloc.initWithContentsOfURL(
          OSX::NSURL.fileURLWithPath(@mom_path))
      coordinator = OSX::NSPersistentStoreCoordinator.alloc.initWithManagedObjectModel(model)

      result, error = coordinator.addPersistentStoreWithType_configuration_URL_options_error(
         OSX::NSSQLiteStoreType, nil, OSX::NSURL.fileURLWithPath(@data_store_path), nil)
      if !result
        raise "Add persistent store failed: #{error.description}"
      end
      context.setPersistentStoreCoordinator coordinator
    end
  end

  def save
    res, error = context.save_
    if !res
      raise "Save failed: #{error.description}"
    end
    res
  end
end

Having done this, you use it like this

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store = CoreDataStore.new('seedData.sqlite', 'yourmodel.mom')
blog = store.create_entity 'Blog', 'title' => 'Hello world', 'body' => "it's a fine day"
store.create_entity 'Comment', 'body' => 'I Agree', 'blog' => blog
store.save

The too arguments are the path to the file you want to store the data in, and the path to your model file.

Your Mom’s a data model

You may be wondering what a .mom file is. In XCode you work with a .xcdatamodel file. When you build your app, this is compiled down into a .mom file. You can also do it your self by running /Developer/usr/bin/momc my_model.xcdatamodel my_model.mom