Class DataMapper::Property
In: lib/dm-core/property.rb
lib/dm-core/property/boolean.rb
lib/dm-core/property/typecast/time.rb
lib/dm-core/property/typecast/numeric.rb
lib/dm-core/property/class.rb
lib/dm-core/property/date.rb
lib/dm-core/property/date_time.rb
lib/dm-core/property/text.rb
lib/dm-core/property/time.rb
lib/dm-core/property/float.rb
lib/dm-core/property/decimal.rb
lib/dm-core/property/discriminator.rb
lib/dm-core/property/lookup.rb
lib/dm-core/property/binary.rb
lib/dm-core/property/string.rb
lib/dm-core/property/object.rb
lib/dm-core/property/integer.rb
lib/dm-core/property/serial.rb
lib/dm-core/property/numeric.rb
Parent: Object

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:

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.

Methods

Included Modules

DataMapper::Assertions Subject

Classes and Modules

Module DataMapper::Property::Lookup
Module DataMapper::Property::PassThroughLoadDump
Module DataMapper::Property::Typecast
Class DataMapper::Property::Binary
Class DataMapper::Property::Boolean
Class DataMapper::Property::Class
Class DataMapper::Property::Date
Class DataMapper::Property::DateTime
Class DataMapper::Property::Decimal
Class DataMapper::Property::Discriminator
Class DataMapper::Property::Float
Class DataMapper::Property::Integer
Class DataMapper::Property::Numeric
Class DataMapper::Property::Object
Class DataMapper::Property::Serial
Class DataMapper::Property::String
Class DataMapper::Property::Text
Class DataMapper::Property::Time

Constants

PRIMITIVES = [ TrueClass, ::String, ::Float, ::Integer, ::BigDecimal, ::DateTime, ::Date, ::Time, ::Class
OPTIONS = [ :accessor, :reader, :writer, :lazy, :default, :key, :field, :index, :unique_index, :unique, :allow_nil, :allow_blank, :required
VISIBILITY_OPTIONS = [ :public, :protected, :private ].to_set.freeze   Possible :visibility option values
INVALID_NAMES = (Resource.instance_methods + Resource.private_instance_methods + Query::OPTIONS.to_a   Invalid property names

Attributes

allow_blank  [R] 
allow_nil  [R] 
default  [R] 
index  [R]  Returns index name if property has index.

@return [Boolean, Symbol, Array]

  returns true if property is indexed by itself
  returns a Symbol if the property is indexed with other properties
  returns an Array if the property belongs to multiple indexes
  returns false if the property does not belong to any indexes

@api public

instance_variable_name  [R] 
model  [R] 
name  [R] 
options  [R] 
primitive  [R] 
reader_visibility  [R] 
repository_name  [R] 
required  [R] 
unique_index  [R]  Returns true if property has unique index. Serial properties and keys are unique by default.

@return [Boolean, Symbol, Array]

  returns true if property is indexed by itself
  returns a Symbol if the property is indexed with other properties
  returns an Array if the property belongs to multiple indexes
  returns false if the property does not belong to any indexes

@api public

writer_visibility  [R] 

Public Class methods

@api public

@api semipublic

@api semipublic

@api private

Gives all the options set on this property

@return [Hash] with all options and their values set on this property

@api public

Public Instance methods

Returns whether or not the property can be a blank value

@return [Boolean]

  whether or not the property can be blank

@api public

Returns whether or not the property can accept ‘nil’ as it‘s value

@return [Boolean]

  whether or not the property can accept 'nil'

@api public

A hook to allow properties to extend or modify the model it‘s bound to. Implementations are not supposed to modify the state of the property class, and should produce no side-effects on the property instance.

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

@return [String] name of field in data-store

@api semipublic

Standardized reader method for the property

@param [Resource] resource

  model instance for which this property is to be loaded

@return [Object]

  the value of this property for the provided instance

@raise [ArgumentError] "resource should be a Resource, but was .…"

@api private

Fetch the ivar value in the resource

@param [Resource] resource

  model instance for which this property is to be unsafely loaded

@return [Object]

  current @ivar value of this property in +resource+

@api private

Returns a concise string representation of the property instance.

@return [String]

  Concise string representation of the property instance.

@api public

Returns whether or not the property is a key or a part of a key

@return [Boolean]

  true if the property is a key or a part of a key

@api public

Returns whether or not the property is to be lazy-loaded

@return [Boolean]

  true if the property is to be lazy-loaded

@api public

Loads lazy columns when get or set is called.

@param [Resource] resource

  model instance for which lazy loaded attribute are loaded

@api private

Check if the attribute corresponding to the property is loaded

@param [Resource] resource

  model instance for which the attribute is to be tested

@return [Boolean]

  true if the attribute is loaded in the resource

@api private

Test a value to see if it matches the primitive type

@param [Object] value

  value to test

@return [Boolean]

  true if the value is the correct type

@api semipublic

@api private

Returns whether or not the property must be non-nil and non-blank

@return [Boolean]

  whether or not the property is required

@api public

Returns whether or not the property is "serial" (auto-incrementing)

@return [Boolean]

  whether or not the property is "serial"

@api public

Provides a standardized setter method for the property

@param [Resource] resource

  the resource to get the value from

@param [Object] value

  the value to set in the resource

@return [Object]

  +value+ after being typecasted according to this property's primitive

@raise [ArgumentError] "resource should be a Resource, but was .…"

@api private

Set the ivar value in the resource

@param [Resource] resource

  the resource to set

@param [Object] value

  the value to set in the resource

@return [Object]

  the value set in the resource

@api private

@api semipublic

Returns true if property is unique. Serial properties and keys are unique by default.

@return [Boolean]

  true if property has uniq index defined, false otherwise

@api public

Test the value to see if it is a valid value for this Property

@param [Object] loaded_value

  the value to be tested

@return [Boolean]

  true if the value is valid

@api semipulic

Protected Instance methods

Assert given visibility value is supported.

Will raise ArgumentError if this Property‘s reader and writer visibilities are not included in VISIBILITY_OPTIONS.

@return [undefined]

@raise [ArgumentError] "property visibility must be :public, :protected, or :private"

@api private

[Validate]