A singleton class to manage a MacRuby application's data storage requirements.

Datastore.rb

#
#  Datastore.rb
#  A singleton class to manage a MacRuby application's data storage requirements.
#
#  Chris Powell, cpowell@prylis.com, http://cbpowell.wordpress.com
#
#  This work is licensed under a Creative Commons Attribution 3.0 Unported License.
#  http://creativecommons.org/licenses/by/3.0/
#
#  For usage and discussion, see http://cbpowell.wordpress.com/category/macruby/
#
class Datastore
    # Required initialization of class variables
    @@managedObjectModel = nil
    @@managedObjectContext = nil
    @@persistentStoreCoordinator = nil

    # Just to make sure no one instantiates me :)
    private_class_method :new

    # Returns the support folder for the application, used to store the Core Data
    # store file.  This code uses a folder named "YOUR_APP_NAME" for
    # the content, either in the NSApplicationSupportDirectory location or (if the
    # former cannot be found), the system's temporary directory.
    #
    def self.applicationSupportFolder
        paths = NSSearchPathForDirectoriesInDomains(NSApplicationSupportDirectory, NSUserDomainMask, true)
        basePath = (paths.count > 0) ? paths[0] : NSTemporaryDirectory()
        return basePath.stringByAppendingPathComponent("YOUR_APP_NAME")
    end
              
    # The Managed Object Model (or MOM) keeps track of objects and their relationships. 
    # It doesn't care about actual persistency which is the MOC's job.
    #
    # We designed our model by editing a xcdatamodeld file. When we compile our app, this is
    # compiled into a mom file and stored into a folder with the momd extension 
    # (as in mom deployment directory). This is how the application knows about the data 
    # structures and relationships to use.
    #
    # Returns the managed object model for the application, creating if necessary.
    def self.mom
        unless @@managedObjectModel
            model_url = NSBundle.mainBundle.URLForResource("YOUR_APP_NAME", withExtension:"momd")
            @@managedObjectModel = NSManagedObjectModel.alloc.initWithContentsOfURL(model_url)
        end

        @@managedObjectModel
    end

    # The managed object context acts as a bridge between the MOM and the actual persistence mechanism.
    #
    # You can think of a managed object context as an intelligent scratch pad. 
    # When you fetch objects from a persistent store, you bring temporary copies 
    # onto the scratch pad where they form an object graph (or a collection of 
    # object graphs). You can then modify those objects however you like. Unless 
    # you actually save those changes, however, the persistent store remains unaltered.
    #
    # Returns the managed object context (MOC) for the application (which is already
    # bound to the persistent store coordinator for the application.)
    #
    def self.moc
        unless @@managedObjectContext
            coordinator = self.psc
            
            unless coordinator
                dict = {
                    NSLocalizedDescriptionKey => "Failed to initialize the store",
                    NSLocalizedFailureReasonErrorKey => "There was an error building up the data file."
                }
                error = NSError.errorWithDomain("YOUR_ERROR_DOMAIN", code:9999, userInfo:dict)
                NSApplication.sharedApplication.presentError(error)
                return nil
            end
        
            @@managedObjectContext = NSManagedObjectContext.alloc.init
            @@managedObjectContext.setPersistentStoreCoordinator(coordinator)
        end
        
        @@managedObjectContext
    end

    # A PSC is the actual persistence mechanism, and is connected to a MOC. 
    # When you fetch objects, the MOC asks the coordinator to return those objects that match 
    # the fetch request.
    #
    # Returns the persistent store coordinator (PSC) for the application. This implementation 
    # creates and returns a coordinator, having added the store for the application to it. 
    # (The directory for the store is created, if necessary.)
    #
    def self.psc
        unless @@persistentStoreCoordinator
            error = Pointer.new_with_type('@')
            
            fileManager = NSFileManager.defaultManager
            applicationSupportFolder = self.applicationSupportFolder
            
            unless fileManager.fileExistsAtPath(applicationSupportFolder, isDirectory:nil)
                fileManager.createDirectoryAtPath(applicationSupportFolder, attributes:nil)
            end
            
            url = NSURL.fileURLWithPath(applicationSupportFolder.stringByAppendingPathComponent("YOUR_APP_NAME.sql"))
            
            @@persistentStoreCoordinator = NSPersistentStoreCoordinator.alloc.initWithManagedObjectModel(self.mom)
            
            # Could be NSBinaryStoreType, NSXMLStoreType or NSSQLiteStoreType
            unless @@persistentStoreCoordinator.addPersistentStoreWithType(NSSQLiteStoreType, 
                                                                           configuration:nil, 
                                                                           URL:url, 
                                                                           options:nil, 
                                                                           error:error)
                NSApplication.sharedApplication.presentError(error[0])
            end
        end
        
        @@persistentStoreCoordinator
    end

    # Retrieve objects from the database.
    # Credit to Matt Gallagher of 'Cocoa with Love' for inspiring this: http://cocoawithlove.com/2008/03/core-data-one-line-fetch.html
    #
    # * *Args*    :
    #   - +entity_name+ -> the Core Data entity to retrieve, e.g. "Person"
    #   - +withLimit+ -> a limit on the query results, nil if no limit desired
    #   - +withOrder+ -> a string or NSSortDescriptor object to order the results, nil if none required
    #   - +withPredicate+ -> a string or NSPredicate object to constrain the results, nil if none required
    # * *Returns* :
    #   - an array of objects
    # * *Raises* :
    #   - +RuntimeError+ -> if there is a fetch error
    #
    def self.find_by_entity_name(entity_name, withLimit:limit, withOrder:string_or_sort_descriptor, withPredicate:string_or_predicate, *args)
        request = NSFetchRequest.new
        request.entity = self.mom.entitiesByName[entity_name.to_sym]
        
        if string_or_predicate
            if string_or_predicate.is_a?(NSString)
                pred = NSPredicate.predicateWithFormat(string_or_predicate, argumentArray:args)
            elsif string_or_predicate.is_a?(NSPredicate)
                pred = string_or_predicate
            end
        end
        
        if string_or_sort_descriptor
            if string_or_sort_descriptor.is_a?(NSString)
                order_by = NSSortDescriptor.alloc.initWithKey(string_or_sort_descriptor, ascending:true)
            elsif string_or_sort_descriptor.is_a?(NSSortDescriptor)
                order_by = string_or_sort_descriptor
            end
        end
                      
        request.setPredicate(pred) if pred
        request.setFetchLimit(limit) if limit
        request.sortDescriptors = [order_by] if order_by
                
        fetch_error = Pointer.new_with_type('@')
        results = self.moc.executeFetchRequest(request, error:fetch_error)

        if ((fetch_error[0] != nil) || (results == nil))
            msg = (fetch_error[0].localizedDescription ? fetch_error[0].localizedDescription : "Unknown")
            raise "Error fetching entity #{entity_name} because #{msg}"
        end
        
        return results
    end
end