Ruby Syntax to Populate a new Object's Instance Variables [duplicate] - ruby

Question

I find myself using hash arguments to constructors quite a bit, especially when writing DSLs for configuration or other bits of API that the end user will be exposed to. What I end up doing is something like the following:
class Example
PROPERTIES = [:name, :age]
PROPERTIES.each { |p| attr_reader p }
def initialize(args)
PROPERTIES.each do |p|
self.instance_variable_set "##{p}", args[p] if not args[p].nil?
end
end
end
Is there no more idiomatic way to achieve this? The throw-away constant and the symbol to string conversion seem particularly egregious.

Answer 1

You don't need the constant, but I don't think you can eliminate symbol-to-string:
class Example
attr_reader :name, :age
def initialize args
args.each do |k,v|
instance_variable_set("##{k}", v) unless v.nil?
end
end
end
#=> nil
e1 = Example.new :name => 'foo', :age => 33
#=> #<Example:0x3f9a1c #name="foo", #age=33>
e2 = Example.new :name => 'bar'
#=> #<Example:0x3eb15c #name="bar">
e1.name
#=> "foo"
e1.age
#=> 33
e2.name
#=> "bar"
e2.age
#=> nil
BTW, you might take a look (if you haven't already) at the Struct class generator class, it's somewhat similar to what you are doing, but no hash-type initialization (but I guess it wouldn't be hard to make adequate generator class).
HasProperties
Trying to implement hurikhan's idea, this is what I came to:
module HasProperties
attr_accessor :props
def has_properties *args
#props = args
instance_eval { attr_reader *args }
end
def self.included base
base.extend self
end
def initialize(args)
args.each {|k,v|
instance_variable_set "##{k}", v if self.class.props.member?(k)
} if args.is_a? Hash
end
end
class Example
include HasProperties
has_properties :foo, :bar
# you'll have to call super if you want custom constructor
def initialize args
super
puts 'init example'
end
end
e = Example.new :foo => 'asd', :bar => 23
p e.foo
#=> "asd"
p e.bar
#=> 23
As I'm not that proficient with metaprogramming, I made the answer community wiki so anyone's free to change the implementation.
Struct.hash_initialized
Expanding on Marc-Andre's answer, here is a generic, Struct based method to create hash-initialized classes:
class Struct
def self.hash_initialized *params
klass = Class.new(self.new(*params))
klass.class_eval do
define_method(:initialize) do |h|
super(*h.values_at(*params))
end
end
klass
end
end
# create class and give it a list of properties
MyClass = Struct.hash_initialized :name, :age
# initialize an instance with a hash
m = MyClass.new :name => 'asd', :age => 32
p m
#=>#<struct MyClass name="asd", age=32>

Answer 2

The Struct clas can help you build such a class. The initializer takes the arguments one by one instead of as a hash, but it's easy to convert that:
class Example < Struct.new(:name, :age)
def initialize(h)
super(*h.values_at(:name, :age))
end
end
If you want to remain more generic, you can call values_at(*self.class.members) instead.

Answer 3

There are some useful things in Ruby for doing this kind of thing.
The OpenStruct class will make the values of a has passed to its initialize
method available as attributes on the class.
require 'ostruct'
class InheritanceExample < OpenStruct
end
example1 = InheritanceExample.new(:some => 'thing', :foo => 'bar')
puts example1.some # => thing
puts example1.foo # => bar
The docs are here:
http://www.ruby-doc.org/stdlib-1.9.3/libdoc/ostruct/rdoc/OpenStruct.html
What if you don't want to inherit from OpenStruct (or can't, because you're
already inheriting from something else)? You could delegate all method
calls to an OpenStruct instance with Forwardable.
require 'forwardable'
require 'ostruct'
class DelegationExample
extend Forwardable
def initialize(options = {})
#options = OpenStruct.new(options)
self.class.instance_eval do
def_delegators :#options, *options.keys
end
end
end
example2 = DelegationExample.new(:some => 'thing', :foo => 'bar')
puts example2.some # => thing
puts example2.foo # => bar
Docs for Forwardable are here:
http://www.ruby-doc.org/stdlib-1.9.3/libdoc/forwardable/rdoc/Forwardable.html

Answer 4

Given your hashes would include ActiveSupport::CoreExtensions::Hash::Slice, there is a very nice solution:
class Example
PROPERTIES = [:name, :age]
attr_reader *PROPERTIES #<-- use the star expansion operator here
def initialize(args)
args.slice(PROPERTIES).each {|k,v| #<-- slice comes from ActiveSupport
instance_variable_set "##{k}", v
} if args.is_a? Hash
end
end
I would abstract this to a generic module which you could include and which defines a "has_properties" method to set the properties and do the proper initialization (this is untested, take it as pseudo code):
module HasProperties
def self.has_properties *args
class_eval { attr_reader *args }
end
def self.included base
base.extend InstanceMethods
end
module InstanceMethods
def initialize(args)
args.slice(PROPERTIES).each {|k,v|
instance_variable_set "##{k}", v
} if args.is_a? Hash
end
end
end

Answer 5

My solution is similar to Marc-André Lafortune. The difference is that each value is deleted from the input hash as it is used to assign a member variable. Then the Struct-derived class can perform further processing on whatever may be left in the Hash. For instance, the JobRequest below retains any "extra" arguments from the Hash in an options field.
module Message
def init_from_params(params)
members.each {|m| self[m] ||= params.delete(m)}
end
end
class JobRequest < Struct.new(:url, :file, :id, :command, :created_at, :options)
include Message
# Initialize from a Hash of symbols to values.
def initialize(params)
init_from_params(params)
self.created_at ||= Time.now
self.options = params
end
end

Answer 6

Please take a look at my gem, Valuable:
class PhoneNumber < Valuable
has_value :description
has_value :number
end
class Person < Valuable
has_value :name
has_value :favorite_color, :default => 'red'
has_value :age, :klass => :integer
has_collection :phone_numbers, :klass => PhoneNumber
end
jackson = Person.new(name: 'Michael Jackson', age: '50', phone_numbers: [{description: 'home', number: '800-867-5309'}, {description: 'cell', number: '123-456-7890'})
> jackson.name
=> "Michael Jackson"
> jackson.age
=> 50
> jackson.favorite_color
=> "red"
>> jackson.phone_numbers.first
=> #<PhoneNumber:0x1d5a0 #attributes={:description=>"home", :number=>"800-867-5309"}>
I use it for everything from search classes (EmployeeSearch, TimeEntrySearch) to reporting ( EmployeesWhoDidNotClockOutReport, ExecutiveSummaryReport) to presenters to API endpoints. If you add some ActiveModel bits you can easily hook these classes up to forms for gathering criteria. I hope you find it useful.