Class DataMapper::Property
In: lib/dm-core/adapters/oracle_adapter.rb
lib/dm-core/property.rb
Parent: Object

Quick Links

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,  :nullable => false  # 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 types:

  • Boolean
  • String (default length is 50)
  • Text (limit of 65k characters by default)
  • Float
  • Integer
  • BigDecimal
  • DateTime
  • Date
  • Time
  • Object (marshalled out during serialization)
  • Class (datastore primitive is the same as String. Used for Inheritance)

Other types are known as custom types.

For more information about available Types, see DataMapper::Type

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 != 'Luke is Awesome'
     @title = new_title
   end
 end

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::Types::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 multipule 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
     # => infers 'validates_length :title,
            :minimum => 0, :maximum => 250'

   property :title, String, :nullable => false
     # => infers 'validates_present :title

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

   property :title, String, :length => 255, :nullable => false
     # => 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, part of the dm-more bundle. 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 => { |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

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

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

 :key                 name of the key associated with this property.

 :serial              if true, field value is auto incrementing

 :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 type.

 All other keys you pass to +property+ method are stored and available
 as options[:extra_keys].

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

custom?   default?   default_for   field   get   get!   hash   index   inspect   key?   lazy?   lazy_load   lazy_load_properties   length   loaded?   new   nullable?   primitive?   properties   serial?   set   set!   set_original_value   typecast   unique?   unique_index   valid?   value  

Included Modules

Extlib::Assertions

Constants

OPTIONS = [ :accessor, :reader, :writer, :lazy, :default, :nullable, :key, :serial, :field, :size, :length, :format, :index, :unique_index, :auto_validation, :validates, :unique, :precision, :scale, :min, :max   NOTE: PLEASE update OPTIONS in DataMapper::Type when updating them here
PRIMITIVES = [ TrueClass, String, Float, Integer, BigDecimal, DateTime, Date, Time, Object, Class, ].to_set.freeze
VISIBILITY_OPTIONS = [ :public, :protected, :private ].to_set.freeze   Possible :visibility option values
DEFAULT_LENGTH = 50
DEFAULT_PRECISION = 10
DEFAULT_SCALE_BIGDECIMAL = 0
DEFAULT_SCALE_FLOAT = nil
DEFAULT_NUMERIC_MIN = 0
DEFAULT_NUMERIC_MAX = 2**31-1

Attributes

default  [R] 
instance_variable_name  [R] 
max  [R] 
min  [R] 
model  [R] 
name  [R] 
options  [R] 
precision  [R] 
primitive  [R] 
reader_visibility  [R] 
repository_name  [R] 
scale  [R] 
type  [R] 
writer_visibility  [R] 

Public Class methods

TODO: document @api semipublic

Public Instance methods

Returns whether or not the property is custom (not provided by dm-core)

@return [Boolean]

  whether or not the property is custom

@api public

Returns true if the property has a default value

@return [Boolean]

  true if the property has a default value

@api semipublic

Returns a default value of the property for given resource.

When default value is a callable object, it is called with resource and property passed as arguments.

@param [Resource] resource

  the model instance for which the default is to be set

@return [Object]

  the default value of this property for +resource+

@api semipublic

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 the hash of the property name

This is necessary to allow comparisons between different properties in different models, having the same base model

@return [Integer]

  the property name hash

@api semipublic

Returns index name if property has index.

@return [true, Symbol, Array, nil]

  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 nil if the property does not belong to any indexes

@api public

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

TODO: document @api private

Returns maximum property length (if applicable). This usually only makes sense when property is of type Range or custom type.

@return [Integer, NilClass]

  the maximum length of this property

@api semipublic

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

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

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

TODO: document @api private

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

Sets original value of the property on given resource. When property is set on DataMapper resource instance, original value is preserved. This makes possible to track dirty attributes and save only those really changed, and avoid extra queries to the data source in certain situations.

@param [Resource] resource

  model instance for which to set the original value

@param [Object] original

  value to set as original value for this property in +resource+

@api private

typecasts values into a primitive (Ruby class that backs DataMapper property type). If property type can handle typecasting, it is delegated. How typecasting is perfomed, depends on the primitive of the type.

If type‘s primitive is a TrueClass, values of 1, t and true are casted to true.

For String primitive, to_s is called on value.

For Float primitive, to_f is called on value but only if value is a number otherwise value is returned.

For Integer primitive, to_i is called on value but only if value is a number, otherwise value is returned.

For BigDecimal primitive, to_d is called on value but only if value is a number, otherwise value is returned.

Casting to DateTime, Time and Date can handle both hashes with keys like :day or :hour and strings in format methods like Time.parse can handle.

@param [to_s, to_f, to_i, to_d, Hash] value

  the value to typecast

@return [rue, String, Float, Integer, BigDecimal, DateTime, Date, Time, Class]

  The typecasted +value+

@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

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

@return [true, Symbol, Array, nil]

  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 nil if the property does not belong to any indexes

@api public

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

@param [Object] value

  the value to be tested

@return [Boolean]

  true if the value is valid

@api semipulic

Returns given value unchanged for core types and uses dump method of the property type for custom types.

@param [Object] value

  the value to be converted into a storeable (ie., primitive) value

@return [Object]

  the primitive value to be stored in the repository for +val+

@api semipublic

[Validate]