How to use class_eval for multi attributes value - ruby

I tried to to extend the code from this question for keeping records of an attribute value. However, my code fails in the case of more than one attributes. Here is the code:
class Class
def attr_accessor_with_history(attr_name)
attr_name = attr_name.to_s
attr_reader attr_name
ah=attr_name+"_history"
attr_reader ah
class_eval %Q{
def #{attr_name}= (attr_name)
#attr_name=attr_name
if #ah == nil
#ah=[nil]
end
#ah.push(attr_name)
end
def #{ah}
#ah
end
def #{attr_name}
#attr_name
end
}
end
end
Here a dummy class for testing
class Foo
attr_accessor_with_history :bar
attr_accessor_with_history :bar1
end
f = Foo.new
f.bar = 1
f.bar = 2
f.bar1 = 5
p f.bar_history
p f.bar1_history
For some reason, f.bar and f.bar1 both return 5 and f.bar_history = f.bar1_history = [nil, 1, 2, 5]. Any idea why that is?

You were using #ah and #attr_name instead of ##{ah} and ##{attr_name} when getting/setting in the methods. This meant that they were always setting and returning the same instance variable, instead of different, dynamically named ones.
class Class
def attr_accessor_with_history(attr_name)
class_eval %{
attr_reader :#{attr_name}, :#{attr_name}_history
def #{attr_name}=(value)
##{attr_name} = value
##{attr_name}_history ||= [nil]
##{attr_name}_history << value
end
}
end
end
I've also generally cleaned up your code a little to make it (I think) clearer and more concise.

Related

Creating Ruby builder object with re-usable code

I'm working to create a few Ruby builder objects, and thinking on how I could reuse some of Ruby's magic to reduce the logic of the builder to a single class/module. It's been ~10 years since my last dance with the language, so a bit rusty.
For example, I have this builder:
class Person
PROPERTIES = [:name, :age]
attr_accessor(*PROPERTIES)
def initialize(**kwargs)
kwargs.each do |k, v|
self.send("#{k}=", v) if self.respond_to?(k)
end
end
def build
output = {}
PROPERTIES.each do |prop|
if self.respond_to?(prop) and !self.send(prop).nil?
value = self.send(prop)
# if value itself is a builder, evalute it
output[prop] = value.respond_to?(:build) ? value.build : value
end
end
output
end
def method_missing(m, *args, &block)
if m.to_s.start_with?("set_")
mm = m.to_s.gsub("set_", "")
if PROPERTIES.include?(mm.to_sym)
self.send("#{mm}=", *args)
return self
end
end
end
end
Which can be used like so:
Person.new(name: "Joe").set_age(30).build
# => {name: "Joe", age: 30}
I would like to be able to refactor everything to a class and/or module so that I could create multiple such builders that'll only need to define attributes and inherit or include the rest (and possibly extend each other).
class BuilderBase
# define all/most relevant methods here for initialization,
# builder attributes and object construction
end
module BuilderHelper
# possibly throw some of the methods here for better scope access
end
class Person < BuilderBase
include BuilderHelper
PROPERTIES = [:name, :age, :email, :address]
attr_accessor(*PROPERTIES)
end
# Person.new(name: "Joe").set_age(30).set_email("joe#mail.com").set_address("NYC").build
class Server < BuilderBase
include BuilderHelper
PROPERTIES = [:cpu, :memory, :disk_space]
attr_accessor(*PROPERTIES)
end
# Server.new.set_cpu("i9").set_memory("32GB").set_disk_space("1TB").build
I've been able to get this far:
class BuilderBase
def initialize(**kwargs)
kwargs.each do |k, v|
self.send("#{k}=", v) if self.respond_to?(k)
end
end
end
class Person < BuilderBase
PROPERTIES = [:name, :age]
attr_accessor(*PROPERTIES)
def build
...
end
def method_missing(m, *args, &block)
...
end
end
Trying to extract method_missing and build into the base class or a module keeps throwing an error at me saying something like:
NameError: uninitialized constant BuilderHelper::PROPERTIES
OR
NameError: uninitialized constant BuilderBase::PROPERTIES
Essentially the neither the parent class nor the mixin are able to access the child class' attributes. For the parent this makes sense, but not sure why the mixin can't read the values inside the class it was included into. This being Ruby I'm sure there's some magical way to do this that I have missed.
Help appreciated - thanks!
I reduced your sample to the required parts and came up with:
module Mixin
def say_mixin
puts "Mixin: Value defined in #{self.class::VALUE}"
end
end
class Parent
def say_parent
puts "Parent: Value defined in #{self.class::VALUE}"
end
end
class Child < Parent
include Mixin
VALUE = "CHILD"
end
child = Child.new
child.say_mixin
child.say_parent
This is how you could access a CONSTANT that lives in the child/including class from the parent/included class.
But I don't see why you want to have this whole Builder thing in the first place. Would an OpenStruct not work for your case?
Interesting question. As mentioned by #Pascal, an OpenStruct might already do what you're looking for.
Still, it might be more concise to explicitly define the setter methods. It might also be clearer to replace the PROPERTIES constants by methods calls. And since I'd expect a build method to return a complete object and not just a Hash, I renamed it to to_h:
class BuilderBase
def self.properties(*ps)
ps.each do |property|
attr_reader property
define_method :"set_#{property}" do |value|
instance_variable_set(:"##{property}", value)
#hash[property] = value
self
end
end
end
def initialize(**kwargs)
#hash = {}
kwargs.each do |k, v|
self.send("set_#{k}", v) if self.respond_to?(k)
end
end
def to_h
#hash
end
end
class Person < BuilderBase
properties :name, :age, :email, :address
end
p Person.new(name: "Joe").set_age(30).set_email("joe#mail.com").set_address("NYC").to_h
# {:name=>"Joe", :age=>30, :email=>"joe#mail.com", :address=>"NYC"}
class Server < BuilderBase
properties :cpu, :memory, :disk_space
end
p Server.new.set_cpu("i9").set_memory("32GB").set_disk_space("1TB").to_h
# {:cpu=>"i9", :memory=>"32GB", :disk_space=>"1TB"}
I think no need to declare PROPERTIES, we can create a general builder like this:
class Builder
attr_reader :build
def initialize(clazz)
#build = clazz.new
end
def self.build(clazz, &block)
builder = Builder.new(clazz)
builder.instance_eval(&block)
builder.build
end
def set(attr, val)
#build.send("#{attr}=", val)
self
end
def method_missing(m, *args, &block)
if #build.respond_to?("#{m}=")
set(m, *args)
else
#build.send("#{m}", *args, &block)
end
self
end
def respond_to_missing?(method_name, include_private = false)
#build.respond_to?(method_name) || super
end
end
Using
class Test
attr_accessor :x, :y, :z
attr_reader :w, :u, :v
def set_w(val)
#w = val&.even? ? val : 0
end
def add_u(val)
#u = val if val&.odd?
end
end
test1 = Builder.build(Test) {
x 1
y 2
z 3
} # <Test:0x000055b6b0fb2888 #x=1, #y=2, #z=3>
test2 = Builder.new(Test).set(:x, 1988).set_w(6).add_u(2).build
# <Test:0x000055b6b0fb23b0 #x=1988, #w=6>

How to get all instances variables in ruby class?

I have a ruby class, and in one of the methods, it calls an external function, and pass in all instance variables, and continue with the return value. Here is the code:
class MyClass
attr_accessor :name1
attr_accessor :name2
...
attr_accessor :namen
def inner_func():
all_vars = ???? # how to collect all my instance variables into a dict/Hash?
res = out_func(all_vars)
do_more_stuff(res)
end
end
The problem is the instance variables might vary in subclasses. I can't refer them as their names. So, is there a way to do this? Or Am I thinking in a wrong way?
You can use instance_variables to collect them in an Array. You will get all initialized instance variables.
class MyClass
attr_accessor :name1
attr_accessor :name2
...
attr_accessor :namen
def inner_func():
all_vars = instance_variables
res = out_func(all_vars)
do_more_stuff(res)
end
end
You could keep track of all accessors as you create them:
class Receiver
def work(arguments)
puts "Working with #{arguments.inspect}"
end
end
class MyClass
def self.attr_accessor(*arguments)
super
#__attribute_names__ ||= []
#__attribute_names__ += arguments
end
def self.attribute_names
#__attribute_names__
end
def self.inherited(base)
parent = self
base.class_eval do
#__attribute_names__ = parent.attribute_names
end
end
def attributes
self.class.attribute_names.each_with_object({}) do |attribute_name, result|
result[attribute_name] = public_send(attribute_name)
end
end
def work
Receiver.new.work(attributes)
end
attr_accessor :foo
attr_accessor :bar
end
class MySubclass < MyClass
attr_accessor :baz
end
Usage
my_class = MyClass.new
my_class.foo = 123
my_class.bar = 234
my_class.work
# Working with {:foo=>123, :bar=>234}
my_subclass = MySubclass.new
my_subclass.foo = 123
my_subclass.bar = 234
my_subclass.baz = 345
my_subclass.work
# Working with {:foo=>123, :bar=>234, :baz=>345}

using a virtual attribute as a hash

I want to be able to have a virtual attribute on a non-database model that is a hash. I just can't figure out what the syntax is for adding and removing items from this hash:
If I define:
attr_accessor :foo, :bar
then in a method in the model, I can use:
self.foo = "x"
But I can't say:
self.bar["item"] = "value"
Try
self.bar = Hash.new
self.bar["item"] = "value"
class YourModel
def bar
#bar ||= Hash.new
end
def foo
bar["item"] = "value"
end
end
but classic approach would be:
class YourModel
def initialize
#bar = Hash.new
end
def foo
#bar["item"] = "value"
end
end
Just use OpenStruct, Hash with Indifferent Access or Active Model.
When you are calling:
attr_accessor :foo, :bar
on your class, Ruby does something like the following behind the curtains:
def foo
return #foo
end
def foo=(val)
#foo = val
end
def bar
return #bar
end
def bar=(val)
#bar = val
end
The methods #foo and #bar are just returning the instance variables and #foo= and #bar= just set them. So if you want one of them to contain a Hash, you have to assign this Hash somewhere.
My favorite solution would be the following:
class YourModel
# generate the default accessor methods
attr_accessor :foo, :bar
# overwrite #bar so that it always returns a hash
def bar
#bar ||= {}
end
end

Ruby.Metaprogramming. class_eval

There seem to be a mistake in my code. However I just can't find it out.
class Class
def attr_accessor_with_history(attr_name)
attr_name = attr_name.to_s
attr_reader attr_name
attr_writer attr_name
attr_reader attr_name + "_history"
class_eval %Q{
##{attr_name}_history=[1,2,3]
}
end
end
class Foo
attr_accessor_with_history :bar
end
f = Foo.new
f.bar = 1
f.bar = 2
puts f.bar_history.to_s
I would expect it to return an array [1,2,3]. However, it doesn't return anything.
You shouldn't be opening Class to add new methods. That's what modules are for.
module History
def attr_accessor_with_history(attr_name)
attr_name = attr_name.to_s
attr_accessor attr_name
class_eval %Q{
def #{attr_name}_history
[1, 2, 3]
end
}
end
end
class Foo
extend History
attr_accessor_with_history :bar
end
f = Foo.new
f.bar = 1
f.bar = 2
puts f.bar_history.inspect
# [1, 2, 3]
And here's the code you probably meant to write (judging from the names).
module History
def attr_accessor_with_history(attr_name)
attr_name = attr_name.to_s
class_eval %Q{
def #{attr_name}
##{attr_name}
end
def #{attr_name}= val
##{attr_name}_history ||= []
##{attr_name}_history << #{attr_name}
##{attr_name} = val
end
def #{attr_name}_history
##{attr_name}_history
end
}
end
end
class Foo
extend History
attr_accessor_with_history :bar
end
f = Foo.new
f.bar = 1
f.bar = 2
puts f.bar_history.inspect
# [nil, 1]
Solution:
class Class
def attr_accessor_with_history(attr_name)
ivar = "##{attr_name}"
history_meth = "#{attr_name}_history"
history_ivar = "##{history_meth}"
define_method(attr_name) { instance_variable_get ivar }
define_method "#{attr_name}=" do |value|
instance_variable_set ivar, value
instance_variable_set history_ivar, send(history_meth) << value
end
define_method history_meth do
value = instance_variable_get(history_ivar) || []
value.dup
end
end
end
Tests:
describe 'Class#attr_accessor_with_history' do
let(:klass) { Class.new { attr_accessor_with_history :bar } }
let(:instance) { instance = klass.new }
it 'acs as attr_accessor' do
instance.bar.should be_nil
instance.bar = 1
instance.bar.should == 1
instance.bar = 2
instance.bar.should == 2
end
it 'remembers history of setting' do
instance.bar_history.should == []
instance.bar = 1
instance.bar_history.should == [1]
instance.bar = 2
instance.bar_history.should == [1, 2]
end
it 'is not affected by mutating the history array' do
instance.bar_history << 1
instance.bar_history.should == []
instance.bar = 1
instance.bar_history << 2
instance.bar_history.should == [1]
end
end
You will find a solution for your problem in Sergios answer. Here an explanation, what's going wrong in your code.
With
class_eval %Q{
##{attr_name}_history=[1,2,3]
}
you execute
#bar_history = [1,2,3]
You execute this on class level, not in object level.
The variable #bar_history is not available in a Foo-object, but in the Foo-class.
With
puts f.bar_history.to_s
you access the -never on object level defined- attribute #bar_history.
When you define a reader on class level, you have access to your variable:
class << Foo
attr_reader :bar_history
end
p Foo.bar_history #-> [1, 2, 3]
#Sergio Tulentsev's answer works, but it promotes a problematic practice of using string eval which is in general fraught with security risks and other surprises when the inputs aren't what you expect. For example, what happens to Sergio's version if one calls (no don't try it):
attr_accessor_with_history %q{foo; end; system "rm -rf /"; def foo}
It is often possible to do ruby meta-programming more carefully without string eval. In this case, using simple interpolation and define_method of closures with instance_variable_[get|set], and send:
module History
def attr_accessor_with_history(attr_name)
getter_sym = :"#{attr_name}"
setter_sym = :"#{attr_name}="
history_sym = :"#{attr_name}_history"
iv_sym = :"##{attr_name}"
iv_hist = :"##{attr_name}_history"
define_method getter_sym do
instance_variable_get(iv_sym)
end
define_method setter_sym do |val|
instance_variable_set( iv_hist, [] ) unless send(history_sym)
send(history_sym).send( :'<<', send(getter_sym) )
instance_variable_set( iv_sym, val #)
end
define_method history_sym do
instance_variable_get(iv_hist)
end
end
end
Here is what should be done. The attr_writer need be defined withing class_eval instead in Class.
class Class
def attr_accessor_with_history(attr_name)
attr_name = attr_name.to_s
attr_reader attr_name
#attr_writer attr_name ## moved into class_eval
attr_reader attr_name + "_history"
class_eval %Q{
def #{attr_name}=(value)
##{attr_name}_history=[1,2,3]
end
}
end
end

Customising attr_reader to do lazy instantiation of attributes

(Big edit, I got part of the way there…)
I've been hacking away and I've come up with this as a way to specify things that need to be done before attributes are read:
class Class
def attr_reader(*params)
if block_given?
params.each do |sym|
define_method(sym) do
yield
self.instance_variable_get("##{sym}")
end
end
else
params.each do |sym|
attr sym
end
end
end
end
class Test
attr_reader :normal
attr_reader(:jp,:nope) { changethings if #nope.nil? }
def initialize
#normal = "Normal"
#jp = "JP"
#done = false
end
def changethings
p "doing"
#jp = "Haha!"
#nope = "poop"
end
end
j = Test.new
p j.normal
p j.jp
But changethings isn't being recognised as a method — anyone got any ideas?
You need to evaluate the block in the context of the instance. yield by default will evaluate it in its native context.
class Class
def attr_reader(*params, &blk)
if block_given?
params.each do |sym|
define_method(sym) do
self.instance_eval(&blk)
self.instance_variable_get("##{sym}")
end
end
else
params.each do |sym|
attr sym
end
end
end
end
Here's another alternative approach you can look at. It's not as elegant as what you're trying to do using define_method but it's maybe worth looking at.
Add a new method lazy_attr_reader to Class
class Class
def lazy_attr_reader(*vars)
options = vars.last.is_a?(::Hash) ? vars.pop : {}
# get the name of the method that will populate the attribute from options
# default to 'get_things'
init_method = options[:via] || 'get_things'
vars.each do |var|
class_eval("def #{var}; #{init_method} if !defined? ##{var}; ##{var}; end")
end
end
end
Then use it like this:
class Test
lazy_attr_reader :name, :via => "name_loader"
def name_loader
#name = "Bob"
end
end
In action:
irb(main):145:0> t = Test.new
=> #<Test:0x2d6291c>
irb(main):146:0> t.name
=> "Bob"
IMHO changing the context of the block is pretty counter-intuitive, from a perspective of someone who would use such attr_reader on steroids.
Perhaps you should consider plain ol' "specify method name using optional arguments" approach:
def lazy_attr_reader(*args, params)
args.each do |e|
define_method(e) do
send(params[:init]) if params[:init] && !instance_variable_get("##{e}")
instance_variable_get("##{e}")
end
end
end
class Foo
lazy_attr_reader :foo, :bar, :init => :load
def load
#foo = 'foo'
#bar = 'bar'
end
end
f = Foo.new
puts f.bar
#=> bar

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