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Core Data Query for RubyMotion

Merge pull request #100 from y8/scope_clashing_fix

Bug: scoped defined in CDQManagedObject subclass are clashing
latest commit e3e5cc3aa5
@kemiller kemiller authored

Streamlined Core Data for RubyMotion

Core Data Query (CDQ) is a library to help you manage your Core Data stack while using RubyMotion. It uses a data model file, which you can generate in XCode, or you can use ruby-xcdm.

Dependency Status Build Status Gem Version

Get Started

  1. Introducing CDQ
  2. Greenfield Quick Start Tutorial
  3. Cheat Sheet
  4. API docs

Introducing CDQ

CDQ began its life as a fork of MotionData, but it became obvious I wanted to take things in a different direction, so I cut loose and ended up rewriting almost everything. If you pay attention, you can still find the genetic traces, so thanks to @alloy for sharing his work and letting me learn so much.

CDQ aims to streamline the process of getting you up and running Core Data, while avoiding too much abstraction or method pollution on top of the SDK. While it borrows many ideas from ActiveRecord (especially AREL), it is designed to harmonize with Core Data's way of doing things first.

I am actively developing and improving CDQ (updated February 2015) so if you have trouble or find a bug, please open a ticket!

Why use a static Data Model?

By using a real data model file that gets compiled and included in your bundle, you can take advantage of automatic migration, which simplifies managing your schema as it grows, if you can follow a few simple rules.


$ gem install cdq
$ motion create my_app # if needed
$ cd my_app
$ cdq init

This way assumes you want to use ruby-xcdm. Run cdq -h for list of more generators.

Using Bundler:

gem 'cdq'

If you want to see bleeding-edge changes, point Bundler at the git repo:

gem 'cdq', git: 'git://'

Setting up your stack

You will need a data model file. If you've created one in XCode, move or copy it to your resources file and make sure it's named the same as your RubyMotion project. If you're using ruby-xcdm (which I highly recommend) then it will create the datamodel file automatically and put it in the right place.

Now include the setup code in your app_delegate.rb file:

class AppDelegate
  include CDQ

  def application(application, didFinishLaunchingWithOptions:launchOptions)

That's it! You can create specific implementation classes for your entities if you want, but it's not required. You can start running queries on the console or in your code right away.


The best way to use CDQ is together with ruby-xcdm, which is installed as a dependency. For the full docs, see its github page, but here's a taste. Schema files are found in the "schemas" directory within your app root, and they are versioned for automatic migrations, and this is what they look like:

  schema "0001 initial" do

    entity "Article" do

      string    :body,        optional: false
      integer32 :length
      boolean   :published,   default: false
      datetime  :publishedAt, default: false
      string    :title,       optional: false

      belongs_to :author

    entity "Author" do
      float :fee
      string :name, optional: false
      has_many :articles


Ruby-xcdm translates these files straight into the XML format that Xcode uses for datamodels.

Boolean Values

Since CoreData stores boolean values as an NSNumber, cdq provides helper methods to allow you to get the boolean value of the property. Take the Article model from above with the boolean:published. If you call published directly you'll get the NSNumber 0 or 1. If you call published? you'll get a boolean true or false

article_1 = Article.create(published: true)
article_2 = Article.create(published: false)

article_1.published # => 1
article_2.published # => 0

article_1.published? # => true
article_2.published? # => false

Context Management

Managing NSManagedObjectContext objects in Core Data can be tricky, especially if you are trying to take advantage of nested contexts for better threading behavior. One of the best parts of CDQ is that it handles contexts for you relatively seamlessly. If you have a simple app, you may never need to worry about contexts at all.

Nested Contexts

For a great discussion of why you might want to use nested contexts, see here.

CDQ maintains a stack of contexts (one stack per thread), and by default, all operations on objects use the topmost context. You just call and it saves the whole stack. Or you can get a list of all the contexts in order with cdq.contexts.all and do more precise work.

Settings things up the way you want is easy. Here's how you'd set it up for asynchronous saves:


This pushes a private queue context onto the bottom of the stack, then a main queue context on top of it. Since the main queue is on top, all your data operations will use that. then saves the main context, and schedules a save on the root context.

In addition, since these two contexts are globally important, it makes them available at cdq.contexts.main and cdq.contexts.root.

Temporary Contexts

From time to time, you may need to use a temporary context. For example, on importing a large amount of data from the network, it's best to process and load into a temporary context (possibly in a background thread) and then move all the data over to your main context all at once. CDQ makes that easy too:

  cdq.background do

    # Your work here

Object Lifecycle


  Author.create(name: "Le Guin", publish_count: 150, first_published: 1970)
  Author.create(name: "Shakespeare", publish_count: 400, first_published: 1550)
  Author.create(name: "Blake", publish_count: 100, first_published: 1778)


  author = Author.create(name: "Le Guin", publish_count: 150, first_published: 1970) # => "Le Guin"
  author.publish_count # => 150
  author.attributes # => { "name" => "Le Guin", "publish_count" => 150, "first_published" => 1970 }


  author = Author.first = "Ursula K. Le Guin"

NOTE Custom class methods will have to include CDQ in order to have access to the cdq object. If you're calling cdq from a class method, you also have to extend CDQ.


  author = Author.first


A quick aside about queries in Core Data. You should avoid them whenever possible in your production code. Core Data is designed to work efficiently when you hang on to references to specific objects and use them as you would any in-memory object, letting Core Data handle your memory usage for you. If you're coming from a server-side rails background, this can be pretty hard to get used to, but this is a very different environment. So if you find yourself running queries that only return a single object, consider rearchitecting. That said, queries are sometimes the only solution, and it's very handy to be able to use them easily when debugging from the console, or in unit tests.

All of these queries are infinitely daisy-chainable, and almost everything is possible to do using only chained methods, no need to drop into NSPredicate format strings unless you want to.

Here are some examples. See the cheat sheet for a complete list.


  Author.where(name: 'Shakespeare', publish_count: 15)
  Author.where("name LIKE %@", '*kesp*')
  Author.where("name LIKE %@", 'Shakespear?')

Sorts, Limits and Offsets

  Author.sort_by(:created_at, order: :descending)
  Author.sort_by(:created_at, case_insensitive: true)



  # Multiple comparisons against the same attribute

Nested Conjunctions





Like ActiveRecord, CDQ will not run a fetch until you actually request specific objects. There are several methods for getting at the data:

  • array
  • first
  • last
  • each
  • []
  • map
  • Anything else in Enumerable

Dedicated Models

If you're using CDQ in a brand new project, you'll probably want to use dedicated model classes for your entities. familiar-looking and natural syntax for queries and scopes:

  class Author < CDQManagedObject

Named Scopes

You can save up partially-constructed queries for later use using named scopes, even combining them seamlessly with other queries or other named scopes:

  class Author < CDQManagedObject
    scope :a_authors, where(:name).begins_with('A')
    scope :prolific, where(:publish_count).gt(99)


Using CDQ with a pre-existing model

If you have an existing app that already manages its own data model, you can still use CDQ, overriding its stack at any layer:

cdq.setup(context: App.delegate.mainContext) # don't set up model or store coordinator
cdq.setup(store: App.delegate.persistentStoreCoordinator) # Don't set up model
cdq.setup(model: App.delegate.managedObjectModel) # Don't load model

You cannot use CDQManagedObject as a base class when overriding this way, you'll need to use the master method, described below. If you have an existing model and want to use it with CDQManagedObject without changing its name, You'll need to use a cdq.yml config file. See CDQConfig.

Working without model classes using the master method

If you need or want to work without using CDQManagedObject as your base class, you can use the cdq()master method. This is a "magic" method, like rmq() in RubyMotionQuery or $() in jQuery, which will lift whatever you pass into it into the CDQ universe. The method is available inside all UIResponder classes (so, views and controllers) as well as in the console. You can use it anywhere else by including the model CDQ into your classes. To use an entity without a model class, just pass its name as a string into the master method, like so


Anything you can do with a model, you can also do with the master method, including defining and using named scopes:

  cdq('Author').scope :a_authors, cdq(:name).begins_with('A')
  cdq('Author').scope :prolific, cdq(:publish_count).gt(99)

NOTE: strings and symbols are NOT interchangeable. cdq('Entity') gives you a query generator for an entity, but cdq(:attribute) starts a predicate for an attribute.


As of version 0.1.10, there is some experimental support for iCloud, written by @katsuyoshi. Please try it out and let us know how it's working for you. To enable, initialize like this: true, container: "")

You can also set up iCloud in your cdq.yml file.


Things that are currently missing

  • There is no facility for custom migrations yet
  • There are no explicit validations (but you can define them on your data model)
  • Lifecycle Callbacks or Observers


If you need, you could watch SQL statements by setting the following launch argument through args environment variable:

$ rake args=' 3' takes a value between 1 and 3; the higher the value, the more verbose the output.

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