Properties

Properties for a model are not derived from a database structure, but instead explicitly declared inside your model class definitions. These properties then map (or, if using automigrate, generate) fields in your repository/database.

If you are coming to DataMapper from another ORM framework, such as ActiveRecord, this may be a fundamental difference in thinking to you. However, there are several advantages to defining your properties in your models:

• information about your model is centralized in one place: rather than having to dig out migrations, xml or other configuration files.

• use of mixins can be applied to model properties: better code reuse

• having information centralized in your models, encourages you and the developers on your team to take a model-centric view of development.

• it provides the ability to use Ruby's access control functions.

• and, because DataMapper only cares about properties explicitly defined in your models, DataMapper plays well with legacy databases, and shares databases easily with other applications.

Declaring Properties

Inside your class, you 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.

class Post
  include DataMapper::Resource

  property :title,   String,  :required => true  # Cannot be null
  property :publish, Boolean, :default => false  # Default value for new records is false
end

By default, DataMapper supports the following primitive (Ruby) types also called core properties:

• Boolean

• Class (datastore primitive is the same as String. Used for Inheritance)

• Date

• DateTime

• Decimal

• Float

• Integer

• Object (marshalled out during serialization)

• String (default length is 50)

• Text (limit of 65k characters by default)

• Time

Limiting Access

Property access control is uses the same terminology Ruby does. Properties are public by default, but can also be declared private or protected as needed (via the :accessor option).

class Post
 include DataMapper::Resource

  property :title, String, :accessor => :private    # Both reader and writer are private
  property :body,  Text,   :accessor => :protected  # Both reader and writer are protected
end

Access control is also analogous to Ruby attribute readers and writers, and can be declared using :reader and :writer, in addition to :accessor.

class Post
  include DataMapper::Resource

  property :title, String, :writer => :private    # Only writer is private
  property :tags,  String, :reader => :protected  # Only reader is protected
end

Overriding Accessors

The reader/writer for any property can be overridden in the same manner that Ruby attr readers/writers can be. After the property is defined, just add your custom reader or writer:

class Post
  include DataMapper::Resource

  property :title, String

  def title=(new_title)
    raise ArgumentError if new_title != 'Lee is l337'
    super(new_title)
  end
end

Calling super ensures that any validators defined for the property are kept active.

Lazy Loading

By default, some properties are not loaded when an object is fetched in DataMapper. These lazily loaded properties are fetched on demand when their accessor is called for the first time (as it is often unnecessary to instantiate -every- property -every- time an object is loaded). For instance, DataMapper::Property::Text fields are lazy loading by default, although you can over-ride this behavior if you wish:

Example:

class Post
  include DataMapper::Resource

  property :title, String  # Loads normally
  property :body,  Text    # Is lazily loaded by default
end

If you want to over-ride the lazy loading on any field you can set it to a context or false to disable it with the :lazy option. Contexts allow multiple lazy properties to be loaded at one time. If you set :lazy to true, it is placed in the :default context

class Post
  include DataMapper::Resource

  property :title,   String                                    # Loads normally
  property :body,    Text,   :lazy => false                    # The default is now over-ridden
  property :comment, String, :lazy => [ :detailed ]            # Loads in the :detailed context
  property :author,  String, :lazy => [ :summary, :detailed ]  # Loads in :summary & :detailed context
end

Delaying the request for lazy-loaded attributes even applies to objects accessed through associations. In a sense, DataMapper anticipates that you will likely be iterating over objects in associations and rolls all of the load commands for lazy-loaded properties into one request from the database.

Example:

Widget.get(1).components
  # loads when the post object is pulled from database, by default

Widget.get(1).components.first.body
  # loads the values for the body property on all objects in the
  # association, rather than just this one.

Widget.get(1).components.first.comment
  # loads both comment and author for all objects in the association
  # since they are both in the :detailed context

Keys

Properties can be declared as primary or natural keys on a table. You should a property as the primary key of the table:

Examples:

property :id,        Serial                # auto-incrementing key
property :legacy_pk, String, :key => true  # 'natural' key

This is roughly equivalent to ActiveRecord's set_primary_key, though non-integer data types may be used, thus DataMapper supports natural keys. When a property is declared as a natural key, accessing the object using the indexer syntax Class[key] remains valid.

User.get(1)
   # when :id is the primary key on the users table
User.get('bill')
   # when :name is the primary (natural) key on the users table

Indices

You can add indices for your properties by using the :index option. If you use true as the option value, the index will be automatically named. If you want to name the index yourself, use a symbol as the value.

property :last_name,  String, :index => true
property :first_name, String, :index => :name

You can create multi-column composite indices by using the same symbol in all the columns belonging to the index. The columns will appear in the index in the order they are declared.

property :last_name,  String, :index => :name
property :first_name, String, :index => :name
   # => index on (last_name, first_name)

If you want to make the indices unique, use :unique_index instead of :index

Inferred Validations

If you require the dm-validations plugin, auto-validations will automatically be mixed-in in to your model classes: validation rules that are inferred when properties are declared with specific column restrictions.

class Post
  include DataMapper::Resource

  property :title, String, :length => 250, :min => 0, :max => 250
  # => infers 'validates_length :title'

  property :title, String, :required => true
  # => infers 'validates_present :title'

  property :email, String, :format => :email_address
  # => infers 'validates_format :email, :with => :email_address'

  property :title, String, :length => 255, :required => true
  # => infers both 'validates_length' as well as 'validates_present'
  #    better: property :title, String, :length => 1..255
end

This functionality is available with the dm-validations gem. For more information about validations, check the documentation for dm-validations.

Default Values

To set a default for a property, use the :default key. The property will be set to the value associated with that key the first time it is accessed, or when the resource is saved if it hasn't been set with another value already. This value can be a static value, such as 'hello' but it can also be a proc that will be evaluated when the property is read before its value has been set. The property is set to the return of the proc. The proc is passed two values, the resource the property is being set for and the property itself.

property :display_name, String, :default => lambda { |resource, property| resource.login }

Word of warning. Don't try to read the value of the property you're setting the default for in the proc. An infinite loop will ensue.

Embedded Values (not implemented yet)

As an alternative to extraneous has_one relationships, consider using an EmbeddedValue.

Property options reference

:accessor            if false, neither reader nor writer methods are
                     created for this property

:reader              if false, reader method is not created for this property

:writer              if false, writer method is not created for this property

:lazy                if true, property value is only loaded when on first read
                     if false, property value is always loaded
                     if a symbol, property value is loaded with other properties
                     in the same group

:default             default value of this property

:allow_nil           if true, property may have a nil value on save

:key                 name of the key associated with this property.

:field               field in the data-store which the property corresponds to

:length              string field length

:format              format for autovalidation. Use with dm-validations plugin.

:index               if true, index is created for the property. If a Symbol, index
                     is named after Symbol value instead of being based on property name.

:unique_index        true specifies that index on this property should be unique

:auto_validation     if true, automatic validation is performed on the property

:validates           validation context. Use together with dm-validations.

:unique              if true, property column is unique. Properties of type Serial
                     are unique by default.

:precision           Indicates the number of significant digits. Usually only makes sense
                     for float type properties. Must be >= scale option value. Default is 10.

:scale               The number of significant digits to the right of the decimal point.
                     Only makes sense for float type properties. Must be > 0.
                     Default is nil for Float type and 10 for BigDecimal

Overriding default Property options

There is the ability to reconfigure a Property and it's subclasses by explicitly setting a value in the Property, eg:

# set all String properties to have a default length of 255
DataMapper::Property::String.length(255)

# set all Boolean properties to not allow nil (force true or false)
DataMapper::Property::Boolean.allow_nil(false)

# set all properties to be required by default
DataMapper::Property.required(true)

# turn off auto-validation for all properties by default
DataMapper::Property.auto_validation(false)

# set all mutator methods to be private by default
DataMapper::Property.writer(:private)

Please note that this has no effect when a subclass has explicitly defined it's own option. For example, setting the String length to 255 will not affect the Text property even though it inherits from String, because it sets it's own default length to 65535.

Misc. Notes

• Properties declared as strings will default to a length of 50, rather than 255 (typical max varchar column size). To overload the default, pass :length => 255 or :length => 0..255. Since DataMapper does not introspect for properties, this means that legacy database tables may need their String columns defined with a :length so that DM does not apply an un-needed length validation, or allow overflow.

• You may declare a Property with the data-type of Class. see SingleTableInheritance for more on how to use Class columns.