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This is a tool for generating the same xcdatamodeld files that Xcode does when designing a datamodel for Core Data. It is written in pure ruby, but it will be of particular interest to RubyMotion developers. It offers the essential features that Xcode does, plus a text-based workflow and some niceties, like automatic inverse relationships.

Dependency Status Build Status Gem Version

ruby-xcdm is maintained by Infinite Red, a web and mobile development company based in Portland, OR and San Francisco, CA.


Add this line to your application's Gemfile:

  gem 'ruby-xcdm'

And then execute:

  $ bundle

Or install it yourself as:

  $ gem install ruby-xcdm

Usage (RubyMotion)

  1. Make a directory called "schemas" inside your RubyMotion project
  2. Create one schema version per file within the directory
  3. To build the schema, run rake schema:build

If you want to build the schema every time you run the simulator, add this to your Rakefile:

task :"build:simulator" => :"schema:build"

You can override the name of the datamodel file, if you need to, using a config variable: = "custom"

Usage (Plain Ruby)

  1. Make a directory to hold your schemas (a.k.a. data model in XCode parlance)
  2. Create one schema version per file within the directory
  3. Run the command to generate a datamodel:
  xcdm MyApplicationName ./schema ./resources

Schema File Format

Here's a sample schema file:

  schema "001" do

    entity "Article" do

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

      belongs_to :author

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


All the built-in data types are supported:

  • integer16
  • integer32
  • integer64
  • decimal (See note below)
  • double
  • float
  • string
  • boolean
  • datetime
  • binary
  • transformable

NSDecimal is not well-supported in RubyMotion as of this writing. They are converted to floats and lose precision. HipByte is aware of the issue and intends to fix it, but until they do, you will need to use something else for storing currency. For an example, see here.

Inverse relationships are generated automatically. If the inverse relationship cannot be derived from the association name, you can use the :inverse option:

  entity "Game" do
    belongs_to :away_team, inverse: "Team.away_games"
    belongs_to :home_team, inverse: "Team.home_games"

  entity "Team" do
    has_many :away_games, inverse: "Game.away_team"
    has_many :home_games, inverse: "Game.home_team"

Many-to-many relationships are supported via the :plural_inverse option:

  entity "Person" do
    has_many :threads, plural_inverse: true

  entity "Thread" do
    has_many :people, plural_inverse: true

In this mode, Core Data will automatically create a relation table behind the scenes. If you want more control, you can make the intermediate table yourself:

  entity "Person" do
    has_many :postings

  entity "Thread" do
    has_many :postings

  entity "Posting" do
    belongs_to :person
    belongs_to :thread

    datetime :joined_at

You can also have symmetric one-to-one relationships via has_one:

  entity "Person" do
    has_one :ego

  entity "Ego" do
    has_one :person

Deletion rules can be easily set on relationships and the default rule is "Nullify":

  entity "Discussion" do
    has_many :messages, deletionRule: "Cascade"

  entity "Message" do
    belongs_to :discusion

  # Example:
  # Discussion.first.messages.count => 10
  # Messages.count => 10
  # Discussion.first.destroy
  # Messages.count => 0

Core Data has no equivalent of :through in ActiveRecord, so you'll need to handle that relation yourself.

If you need to set some of the more esoteric options on properties or relationships, you can include the raw parameters from NSEntityDescription and NSAttributeDescription, like renamingIdentifier or defaultValueString.


To create new versions, simply copy the old version, increase the version string (the last one in sort order is always interpreted to be the current version) and make your changes. So long as they conform to the automatic versioning rules, everything should work seamlessly.


  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request
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