Dynamic method chain? - ruby

How can I call a nested hash of methods names on an object?
For example, given the following hash:
hash = {:a => {:b => {:c => :d}}}
I would like to create a method that, given the above hash, does the equivalent of the following:
object.send(:a).send(:b).send(:c).send(:d)
The idea is that I need to get a specific attribute from an unknown association (unknown to this method, but known to the programmer).
I would like to be able to specify a method chain to retrieve that attribute in the form of a nested hash. For example:
hash = {:manufacturer => {:addresses => {:first => :postal_code}}}
car.execute_method_hash(hash)
=> 90210

I'd use an array instead of a hash, because a hash allows inconsistencies (what if there is more than one key in a (sub)hash?).
object = Thing.new
object.call_methods [:a, :b, :c, :d]
Using an array, the following works:
# This is just a dummy class to allow introspection into what's happening
# Every method call returns self and puts the methods name.
class Thing
def method_missing(m, *args, &block)
puts m
self
end
end
# extend Object to introduce the call_methods method
class Object
def call_methods(methods)
methods.inject(self) do |obj, method|
obj.send method
end
end
end
Within call_methods we use inject in the array of symbols, so that we send every symbol to the result of the method execution that was returned by the previous method send. The result of the last send is automatically returned by inject.

There's a much simpler way.
class Object
def your_method
attributes = %w(thingy another.sub_thingy such.attribute.many.method.wow)
object = Object.find(...)
all_the_things << attributes.map{ |attr| object.send_chain(attr.split('.')) }
end
def send_chain(methods)
methods.inject(self, :try)
end
end

There is no predefined method, but you can define your own method for that:
class Object
def send_chain(chain)
k = chain.keys.first
v = chain.fetch(k)
r = send(k)
if v.kind_of?(Hash)
r.send_chain(v)
else
r.send(v)
end
end
end
class A
def a
B.new
end
end
class B
def b
C.new
end
end
class C
def c
D.new
end
end
class D
def d
12345
end
end
chain = { a: { b: { c: :d } } }
a = A.new
puts a.send_chain(chain) # 12345
Tested with http://ideone.com/mQpQmp

Related

Ruby equivalent of lisp-like "apply"?

I would like to create an instance method that takes another instance method of its own class as a parameter, and then applies the passed method on the instance it's working on (known as self):
class MyClass
attr_reader :called_methods
def initialize
#called_methods = []
end
def my_first_method!
#called_methods << :my_first_method
self
end
def my_second_method!
#called_methods << :my_second_method
self
end
def my_strange_method!(secondary)
# Want to apply method whose name is given by secondary, to self
end
end
p MyClass.new.my_second_method!.my_strange_method!(:my_first_method!).called_methods
I suspect the unary & may be key, but all the web pages I can find on that operator involve calling methods on multiple objects, as when iterating over an Enumerable with #each or #map.
Use Object#public_send (or Object#send to apply protected/private method).
def my_strange_method!(secondary)
public_send(secondary)
self
end
p MyClass.new.
my_second_method!.
my_strange_method!(:my_first_method!).
called_methods
#⇒ [:my_second_method, :my_first_method]
There could be more defensive way to apply if and only the method is known:
def my_strange_method!(secondary)
raise ArgumentError.new("Unknown method #{secondary}") \
unless methods.include? secondary.to_s.to_sym
public_send(secondary)
end
p MyClass.new.
my_second_method!.
my_strange_method!(:my_first_method!).
called_methods
#⇒ [:my_second_method, :my_first_method]
p MyClass.new.
my_second_method!.
my_strange_method!(:inexisting_method!).
called_methods
#⇒ ArgumentError: Unknown method inexisting_method!
This is tagged with functional-programming so I'm going to offer a persistent (immutable) design -
class MyClass
attr_reader :called_methods
def initialize(m = [])
#called_methods = m
end
def my_first_method!
MyClass.new(called_methods + [ :first ])
end
def my_second_method!
MyClass.new(called_methods + [ :second ])
end
def my_strange_method!(secondary)
public_send secondary
end
end
MyClass.new.my_second_method!.my_strange_method!(:my_first_method!).called_methods
# [:second, :first]

Ruby convert Object to Hash

Let's say I have a Gift object with #name = "book" & #price = 15.95. What's the best way to convert that to the Hash {name: "book", price: 15.95} in Ruby, not Rails (although feel free to give the Rails answer too)?
Just say (current object) .attributes
.attributes returns a hash of any object. And it's much cleaner too.
class Gift
def initialize
#name = "book"
#price = 15.95
end
end
gift = Gift.new
hash = {}
gift.instance_variables.each {|var| hash[var.to_s.delete("#")] = gift.instance_variable_get(var) }
p hash # => {"name"=>"book", "price"=>15.95}
Alternatively with each_with_object:
gift = Gift.new
hash = gift.instance_variables.each_with_object({}) { |var, hash| hash[var.to_s.delete("#")] = gift.instance_variable_get(var) }
p hash # => {"name"=>"book", "price"=>15.95}
Implement #to_hash?
class Gift
def to_hash
hash = {}
instance_variables.each { |var| hash[var.to_s.delete('#')] = instance_variable_get(var) }
hash
end
end
h = Gift.new("Book", 19).to_hash
Gift.new.instance_values # => {"name"=>"book", "price"=>15.95}
You can use as_json method. It'll convert your object into hash.
But, that hash will come as a value to the name of that object as a key. In your case,
{'gift' => {'name' => 'book', 'price' => 15.95 }}
If you need a hash that's stored in the object use as_json(root: false). I think by default root will be false. For more info refer official ruby guide
http://api.rubyonrails.org/classes/ActiveModel/Serializers/JSON.html#method-i-as_json
For Active Record Objects
module ActiveRecordExtension
def to_hash
hash = {}; self.attributes.each { |k,v| hash[k] = v }
return hash
end
end
class Gift < ActiveRecord::Base
include ActiveRecordExtension
....
end
class Purchase < ActiveRecord::Base
include ActiveRecordExtension
....
end
and then just call
gift.to_hash()
purch.to_hash()
class Gift
def to_hash
instance_variables.map do |var|
[var[1..-1].to_sym, instance_variable_get(var)]
end.to_h
end
end
If you are not in an Rails environment (ie. don't have ActiveRecord available), this may be helpful:
JSON.parse( object.to_json )
You can write a very elegant solution using a functional style.
class Object
def hashify
Hash[instance_variables.map { |v| [v.to_s[1..-1].to_sym, instance_variable_get v] }]
end
end
Recursively convert your objects to hash using 'hashable' gem (https://rubygems.org/gems/hashable)
Example
class A
include Hashable
attr_accessor :blist
def initialize
#blist = [ B.new(1), { 'b' => B.new(2) } ]
end
end
class B
include Hashable
attr_accessor :id
def initialize(id); #id = id; end
end
a = A.new
a.to_dh # or a.to_deep_hash
# {:blist=>[{:id=>1}, {"b"=>{:id=>2}}]}
You should override the inspect method of your object to return the desired hash, or just implement a similar method without overriding the default object behaviour.
If you want to get fancier, you can iterate over an object's instance variables with object.instance_variables
Might want to try instance_values. That worked for me.
To plagiarize #Mr. L in a comment above, try #gift.attributes.to_options.
You can use symbolize_keys and in-case you have nested attributes we can use deep_symbolize_keys:
gift.as_json.symbolize_keys => {name: "book", price: 15.95}
Produces a shallow copy as a hash object of just the model attributes
my_hash_gift = gift.attributes.dup
Check the type of the resulting object
my_hash_gift.class
=> Hash
If you need nested objects to be converted as well.
# #fn to_hash obj {{{
# #brief Convert object to hash
#
# #return [Hash] Hash representing converted object
#
def to_hash obj
Hash[obj.instance_variables.map { |key|
variable = obj.instance_variable_get key
[key.to_s[1..-1].to_sym,
if variable.respond_to? <:some_method> then
hashify variable
else
variable
end
]
}]
end # }}}
Gift.new.attributes.symbolize_keys
To do this without Rails, a clean way is to store attributes on a constant.
class Gift
ATTRIBUTES = [:name, :price]
attr_accessor(*ATTRIBUTES)
end
And then, to convert an instance of Gift to a Hash, you can:
class Gift
...
def to_h
ATTRIBUTES.each_with_object({}) do |attribute_name, memo|
memo[attribute_name] = send(attribute_name)
end
end
end
This is a good way to do this because it will only include what you define on attr_accessor, and not every instance variable.
class Gift
ATTRIBUTES = [:name, :price]
attr_accessor(*ATTRIBUTES)
def create_random_instance_variable
#xyz = 123
end
def to_h
ATTRIBUTES.each_with_object({}) do |attribute_name, memo|
memo[attribute_name] = send(attribute_name)
end
end
end
g = Gift.new
g.name = "Foo"
g.price = 5.25
g.to_h
#=> {:name=>"Foo", :price=>5.25}
g.create_random_instance_variable
g.to_h
#=> {:name=>"Foo", :price=>5.25}
I started using structs to make easy to hash conversions.
Instead of using a bare struct I create my own class deriving from a hash this allows you to create your own functions and it documents the properties of a class.
require 'ostruct'
BaseGift = Struct.new(:name, :price)
class Gift < BaseGift
def initialize(name, price)
super(name, price)
end
# ... more user defined methods here.
end
g = Gift.new('pearls', 20)
g.to_h # returns: {:name=>"pearls", :price=>20}
Following Nate's answer which I haven't been able to compile:
Option 1
class Object
def to_hash
instance_variables.map{ |v| Hash[v.to_s.delete("#").to_sym, instance_variable_get(v)] }.inject(:merge)
end
end
And then you call it like that:
my_object.to_hash[:my_variable_name]
Option 2
class Object
def to_hash
instance_variables.map{ |v| Hash[v.to_s.delete("#"), instance_variable_get(v)] }.inject(:merge)
end
end
And then you call it like that:
my_object.to_hash["my_variable_name"]

Ruby: add custom properties to built-in classes

Question:
Using Ruby it is simple to add custom methods to existing classes, but how do you add custom properties? Here is an example of what I am trying to do:
myarray = Array.new();
myarray.concat([1,2,3]);
myarray._meta_ = Hash.new(); # obviously, this wont work
myarray._meta_['createdby'] = 'dreftymac';
myarray._meta_['lastupdate'] = '1993-12-12';
## desired result
puts myarray._meta_['createdby']; #=> 'dreftymac'
puts myarray.inspect() #=> [1,2,3]
The goal is to construct the class definition in such a way that the stuff that does not work in the example above will work as expected.
Update: (clarify question) One aspect that was left out of the original question: it is also a goal to add "default values" that would ordinarily be set-up in the initialize method of the class.
Update: (why do this) Normally, it is very simple to just create a custom class that inherits from Array (or whatever built-in class you want to emulate). This question derives from some "testing-only" code and is not an attempt to ignore this generally acceptable approach.
Isn't a property just a getter and a setter? If so, couldn't you just do:
class Array
# Define the setter
def _meta_=(value)
#_meta_ = value
end
# Define the getter
def _meta_
#_meta_
end
end
Then, you can do:
x = Array.new
x._meta_
# => nil
x._meta_ = {:name => 'Bob'}
x._meta_
# => {:name => 'Bob'}
Does that help?
Recall that in Ruby, you do not have access to attributes (instance variables) outside of that instance. You only have access to an instance's public methods.
You can use attr_accessor to create a method for a class that acts as a property as you describe:
irb(main):001:0> class Array
irb(main):002:1> attr_accessor :_meta_
irb(main):003:1> end
=> nil
irb(main):004:0>
irb(main):005:0* x = [1,2,3]
=> [1, 2, 3]
irb(main):006:0> x._meta_ = Hash.new
=> {}
irb(main):007:0> x._meta_[:key] = 'value'
=> "value"
irb(main):008:0>
For a simple way to do a default initialization for an accessor, we'll need to basically reimplement attr_accessor ourselves:
class Class
def attr_accessor_with_default accessor, default_value
define_method(accessor) do
name = "##{accessor}"
instance_variable_set(name, default_value) unless instance_variable_defined?(name)
instance_variable_get(name)
end
define_method("#{accessor}=") do |val|
instance_variable_set("##{accessor}", val)
end
end
end
class Array
attr_accessor_with_default :_meta_, {}
end
x = [1,2,3]
x._meta_[:key] = 'value'
p x._meta_
y = [4,5,6]
y._meta_[:foo] = 'bar'
p y._meta_
But wait! The output is incorrect:
{:key=>"value"}
{:foo=>"bar", :key=>"value"}
We've created a closure around the default value of a literal hash.
A better way might be to simply use a block:
class Class
def attr_accessor_with_default accessor, &default_value_block
define_method(accessor) do
name = "##{accessor}"
instance_variable_set(name, default_value_block.call) unless instance_variable_defined?(name)
instance_variable_get(name)
end
define_method("#{accessor}=") do |val|
instance_variable_set("##{accessor}", val)
end
end
end
class Array
attr_accessor_with_default :_meta_ do Hash.new end
end
x = [1,2,3]
x._meta_[:key] = 'value'
p x._meta_
y = [4,5,6]
y._meta_[:foo] = 'bar'
p y._meta_
Now the output is correct because Hash.new is called every time the default value is retrieved, as opposed to reusing the same literal hash every time.
{:key=>"value"}
{:foo=>"bar"}

How can one set property values when initializing an object in Ruby?

Given the following class:
class Test
attr_accessor :name
end
When I create the object, I want to do the following:
t = Test.new {name = 'Some Test Object'}
At the moment, it results in the name attribute still being nil.
Is that possible without adding an initializer?
ok,
I came up with a solution. It uses the initialize method but on the other hand do exactly what you want.
class Test
attr_accessor :name
def initialize(init)
init.each_pair do |key, val|
instance_variable_set('#' + key.to_s, val)
end
end
def display
puts #name
end
end
t = Test.new :name => 'hello'
t.display
happy ? :)
Alternative solution using inheritance. Note, with this solution, you don't need to explicitly declare the attr_accessor!
class CSharpStyle
def initialize(init)
init.each_pair do |key, val|
instance_variable_set('#' + key.to_s, val)
instance_eval "class << self; attr_accessor :#{key.to_s}; end"
end
end
end
class Test < CSharpStyle
def initialize(arg1, arg2, *init)
super(init.last)
end
end
t = Test.new 'a val 1', 'a val 2', {:left => 'gauche', :right => 'droite'}
puts "#{t.left} <=> #{t.right}"
As mentioned by others, the easiest way to do this would be to define an initialize method. If you don't want to do that, you could make your class inherit from Struct.
class Test < Struct.new(:name)
end
So now:
>> t = Test.new("Some Test Object")
=> #<struct Test name="Some Test Object">
>> t.name
=> "Some Test Object"
There is a general way of doing complex object initialization by
passing a block with necessary actions. This block is evaluated in the
context of the object to be initialized, so you have an easy access to
all instance variables and methods.
Continuing your example, we can define this generic initializer:
class Test
attr_accessor :name
def initialize(&block)
instance_eval(&block)
end
end
and then pass it the appropriate code block:
t = Test.new { #name = 'name' }
or
t = Test.new do
self.name = 'name'
# Any other initialization code, if needed.
end
Note that this approach does not require adding much complexity
to the initialize method, per se.
As previously mentioned, the sensible way to do this is either with a Struct or by defining an Test#initialize method. This is exactly what structs and constructors are for. Using an options hash corresponding to attributes is the closest equivalent of your C# example, and it's a normal-looking Ruby convention:
t = Test.new({:name => "something"})
t = Test.new(name: "something") # json-style or kwargs
But in your example you are doing something that looks more like variable assignment using = so let's try using a block instead of a hash. (You're also using Name which would be a constant in Ruby, we'll change that.)
t = Test.new { #name = "something" }
Cool, now let's make that actually work:
class BlockInit
def self.new(&block)
super.tap { |obj| obj.instance_eval &block }
end
end
class Test < BlockInit
attr_accessor :name
end
t = Test.new { #name = "something" }
# => #<Test:0x007f90d38bacc0 #name="something">
t.name
# => "something"
We've created a class with a constructor that accepts a block argument, which is executed within the newly-instantiated object.
Because you said you wanted to avoid using initialize, I'm instead overriding new and calling super to get the default behavior from Object#new. Normally we would define initialize instead, this approach isn't recommended except in meeting the specific request in your question.
When we pass a block into a subclass of BlockInit we can do more than just set variable... we're essentially just injecting code into the initialize method (which we're avoiding writing). If you also wanted an initialize method that does other stuff (as you mentioned in comments) you could add it to Test and not even have to call super (since our changes aren't in BlockInit#initialize, rather BlockInit.new)
Hope that's a creative solution to a very specific and intriguing request.
The code you're indicating is passing parameters into the initialize function. You will most definitely have to either use initialize, or use a more boring syntax:
test = Test.new
test.name = 'Some test object'
Would need to subclass Test (here shown with own method and initializer) e.g.:
class Test
attr_accessor :name, :some_var
def initialize some_var
#some_var = some_var
end
def some_function
"#{some_var} calculation by #{name}"
end
end
class SubClassedTest < Test
def initialize some_var, attrbs
attrbs.each_pair do |k,v|
instance_variable_set('#' + k.to_s, v)
end
super(some_var)
end
end
tester = SubClassedTest.new "some", name: "james"
puts tester.some_function
outputs: some calculation by james
You could do this.
class Test
def not_called_initialize(but_act_like_one)
but_act_like_one.each_pair do |variable,value|
instance_variable_set('#' + variable.to_s, value)
class << self
self
end.class_eval do
attr_accessor variable
end
end
end
end
(t = Test.new).not_called_initialize :name => "Ashish", :age => 33
puts t.name #=> Ashish
puts t.age #=> 33
One advantage is that you don't even have to define your instance variables upfront using attr_accessor. You could pass all the instance variables you need through not_called_initialize method and let it create them besides defining the getters and setters.
If you don't want to override initialize then you'll have to move up the chain and override new. Here's an example:
class Foo
attr_accessor :bar, :baz
def self.new(*args, &block)
allocate.tap do |instance|
if args.last.is_a?(Hash)
args.last.each_pair do |k,v|
instance.send "#{k}=", v
end
else
instance.send :initialize, *args
end
end
end
def initialize(*args)
puts "initialize called with #{args}"
end
end
If the last thing you pass in is a Hash it will bypass initialize and call the setters immediately. If you pass anything else in it will call initialize with those arguments.

Declaring instance variables iterating over a hash!

i want to do the following:
I want to declare the instance variables of a class iterating over a dictionary.
Let's assume that i have this hash
hash = {"key1" => "value1","key2" => "value2","key3" => "value3"}
and i want to have each key as instance variable of a class. I want to know if i could declare the variables iterating over that hash. Something like this:
class MyClass
def initialize()
hash = {"key1" => "value1","key2" => "value2","key3" => "value3"}
hash.each do |k,v|
#k = v
end
end
end
I know this doesn't work! I only put this piece of code to see if you could understand what i want more clearly.
Thanks!
class MyClass
def initialize()
hash = {"key1" => "value1","key2" => "value2","key3" => "value3"}
hash.each do |k,v|
instance_variable_set("##{k}",v)
# if you want accessors:
eigenclass = class<<self; self; end
eigenclass.class_eval do
attr_accessor k
end
end
end
end
The eigenclass is a special class belonging just to a single object, so methods defined there will be instance methods of that object but not belong to other instances of the object's normal class.
class MyClass
def initialize
# define a hash and then
hash.each do |k,v|
# attr_accessor k # optional
instance_variable_set(:"##{k}", v)
end
end
end
Chuck's answer is better than my last two attempts. The eigenclass is not self.class like I had thought; it took a better test than I had written to realize this.
Using my old code, I tested in the following manner and found that the class was indeed manipulated and not the instance:
a = MyClass.new :my_attr => 3
b = MyClass.new :my_other_attr => 4
puts "Common methods between a & b:"
c = (a.public_methods | b.public_methods).select { |v| a.respond_to?(v) && b.respond_to?(v) && !Object.respond_to?(v) }
c.each { |v| puts " #{v}" }
The output was:
Common methods between a & b:
my_other_attr=
my_attr
my_attr=
my_other_attr
This clearly disproves my presupposition. My apologies Chuck, you were right all along.
Older answer:
attr_accessor only works when evaluated in a class definition, not the initialization of an instance. Therefore, the only method to directly do what you want is to use instance_eval with a string:
class MyClass
def initialize(params)
#hash = {"key1" => "value1","key2" => "value2","key3" => "value3"}
params.each do |k,v|
instance_variable_set("##{k}", v)
instance_eval %{
def #{k}
instance_variable_get("##{k}")
end
def #{k}= (new_val)
instance_variable_set("##{k}", new_val)
end
}
end
end
end
To test this try:
c = MyClass.new :my_var => 1
puts c.my_var
http://facets.rubyforge.org/apidoc/api/more/classes/OpenStructable.html
OpensStructable is a mixin module
which can provide OpenStruct behavior
to any class or object. OpenStructable
allows extention of data objects with
arbitrary attributes.

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