A model's properties are not introspected from the fields in the data-store; In fact the reverse happens. You declare the properties for a model inside it's class definition, which is then used to generate the fields in the data-store.

This has a few advantages. First it means that a model's properties are documented in the model itself, not a migration or XML file. If you've ever been annoyed at having to look in a schema file to see the list of properties and types for a model, you'll find this particularly useful. There's no need for a special annotate rake task either.

Second, it lets you limit access to properties using Ruby's access semantics. Properties can be declared public, private or protected. They are public by default.

Finally, since DataMapper only cares about properties explicitly defined in your models, DataMapper plays well with legacy data-stores and shares them easily with other applications.

Declaring Properties

Inside your class, call the property method for each property you want to add. The only two required arguments are the name and type, everything else is optional.

1 class Post
2   include DataMapper::Resource
4   property :id,        Serial                       # primary serial key
5   property :title,     String,  :required => true   # Cannot be nil
6   property :published, Boolean, :default  => false  # Default value for new records is false
7 end


Primary Keys

Primary keys are not automatically created for you, as with ActiveRecord. You MUST configure at least one key property on your data-store. More often than not, you'll want an auto-incrementing integer as a primary key, so DM has a shortcut:

1   property :id, Serial

Natural Keys

Anything can be a key. Just pass :key => true as an option during the property definition. Most commonly, you'll see String as a natural key:

1   property :slug, String, :key => true  # any Type is available here

Natural Keys are protected against mass-assignment, so their setter= will need to be called individually if you're looking to set them.

Fair warning: Using Boolean, Discriminator, and the time related types as keys may cause your DBA to hunt you down and "educate" you. DM will not be held responsible for any injuries or death that may result.

Composite Keys

You can have more than one property in the primary key:

1 class Post
2   include DataMapper::Resource
4   property :old_id, Integer, :key => true
5   property :new_id, Integer, :key => true
6 end

Setting default values

Defaults can be set via the :default key for a property. They can be static values, such as 12 or "Hello", but DataMapper also offers the ability to use a Proc to set the default value. The property becomes whatever the Proc returns, which will be called the first time the property is used without having first set a value. The Proc itself receives two arguments: The resource the property is being set on, and the property itself.

1 class Image
2   include DataMapper::Resource
4   property :id,     Serial
5   property :path,   FilePath, :required => true
6   property :md5sum, String,   :length => 32, :default => lambda { |r, p| Digest::MD5.hexdigest( if r.path }
7 end

When creating the resource, or the first time the md5sum property is accessed, it will be set to the hex digest of the file referred to by path.

Fair Warning: A property default must not refer to the value of the property it is about to set, or there will be an infinite loop.

Setting default options

If you find that you're setting the same default options over and over again, you can specify them once and have them applied to all properties you add to your models.

 1 # set all String properties to have a default length of 255
 2 DataMapper::Property::String.length(255)
 4 # set all Boolean properties to not allow nil (force true or false)
 5 DataMapper::Property::Boolean.allow_nil(false)
 7 # set all properties to be required by default
 8 DataMapper::Property.required(true)
10 # turn off auto-validation for all properties by default
11 DataMapper::Property.auto_validation(false)
13 # set all mutator methods to be private by default
14 DataMapper::Property.writer(:private)

You can of course still override these defaults by specifying any option explicitly when defining a specific property.

Lazy Loading

Properties can be configured to be lazy loaded. A lazily loaded property is not requested from the data-store by default. Instead it is only loaded when it's accessor is called for the first time. This means you can stop default queries from being greedy, a particular problem with text fields. Text fields are lazily loaded by default, which you can over-ride if you need to.

1 class Post
2   include DataMapper::Resource
4   property :id,    Serial
5   property :title, String
6   property :body,  Text                    # Is lazily loaded by default
7   property :notes, Text,   :lazy => false  # Isn't lazy, will load by default
8 end

Lazy Loading can also be done via contexts, which let you group lazily loaded properties together, so that when one is fetched, all the associated ones will be as well, cutting down on trips to the data-store.

 1 class Post
 2   include DataMapper::Resource
 4   property :id,       Serial
 5   property :title,    String
 6   property :subtitle, String   :lazy => [ :show ]
 7   property :body,     Text     :lazy => [ :show ]
 8   property :views,    Integer, :lazy => [ :show ]
 9   property :summary,  Text
10 end

In this example, only the title (and the id, of course) will be loaded from the data-store on a Post.all. But as soon as the value for subtitle, body or views are called, all three will be loaded at once, since they're members of the :show group. The summary property on the other hand, will only be fetched when it is asked for.

Available Types

DM-Core supports the following 'primitive' data-types.

If you include DM-Types, the following data-types are supported:

You are encouraged to have a quick glance at the implementation of the various properties below. It's really easy to create a DataMapper property that encapsulates data that is suitable for use in high level application code while at the same time being able to be persisted in all kinds of datastores.

Limiting Access

Access for properties is defined using the same semantics as Ruby. Accessors are public by default, but you can declare them as private or protected if you need to. You can set access using the :accessor option. For demonstration, we'll reopen our Post class.

1 class Post
2   property :title, String, :accessor => :private    # Both reader and writer are private
3   property :body,  Text,   :accessor => :protected  # Both reader and writer are protected
4 end

You also have more fine grained control over how you declare access. You can, for example, have a public reader and private writer by using the :writer and :reader options. (Remember, the default is Public)

1 class Post
2   property :title, String, :writer => :private    # Only writer is private
3   property :tags,  String, :reader => :protected  # Only reader is protected
4 end

Over-riding Accessors

When a property has declared accessors for getting and setting, it's values are added to the model. Just like using attr_accessor, you can over-ride these with your own custom accessors. It's a simple matter of adding an accessor after the property declaration. Reopening the Post class....

1 class Post
2   property :slug, String
4   def slug=(new_slug)
5     raise ArgumentError if new_slug != 'DataMapper is Awesome'
6     super  # use original method instead of accessing @ivar directly
7   end
8 end