Iterate and Set Ruby Object Instance Variables

Question

I'm looking for a way to iterate through a Ruby object's instance variables, and set them individually (from a provided hash) using a generic setter. I am assuming I can iterate through them directly in a method and simply set each individually.

Here is my object instance, u:

u = #<Waffle:0x00007fc6b1145530 
    @id=nil,
    @alpha="",
    @bravo="",
    @charlie="",
    @delta=nil,
    @echo=nil,
    @status=new>

I would like to populate it with a given hash, h:

h = {
    "id"=>"141",
     "alpha"=>"Muccahiya""
     "bravo"=>"$2a$10$xR2g",
     "charlie"=>"2018-02-21 10:41:56-05",
     "delta"=>"2018-02-05 18:17:16.752606-05",
     "echo"=>"wobbly",
     "status"=>"active"
}

This is the method I have and it is throwing this error:

def to_obj(h)
    self.instance_variables.each do |i|
        self.i = h[i.sub('@','')]
    end
end

Error:

Traceback (most recent call last):
3: from /users/rich/app.rb:31:in `<main>'
2: from /Library/WebServer/Documents/dingbat/models/waffle.rb:240:in `to_obj'
1: from /Library/WebServer/Documents/dingbat/models/waffle.rb:240:in `each'
/Library/WebServer/Documents/dingbat/models/waffle.rb:241:in `block in to_obj': no implicit conversion of String into Integer (TypeError)

self.instance_variables is indeed an array of symbols.

I am thinking that an object can set its instance_variables given a hash. This can't be difficult.

You want instance_variable_set method

Sergio Tulentsev
– Sergio Tulentsev

2018-02-08 09:01:25 +00:00
Commented Feb 8, 2018 at 9:01 — Sergio Tulentsev
– Sergio Tulentsev, Commented Feb 8, 2018 at 9:01

sawa · Accepted Answer · 2018-02-08 11:03:12Z

4

def to_obj(h)
  h.each{|k, v| instance_variable_set("@{k}", v)}
end

If you want to set a value only if such instance variable is defined, then do:

def to_obj(h)
  h.each do
    |k, v|
    next unless instance_variable_defined?("@{k}")
    instance_variable_set("@{k}", v)
  end
end

edited Feb 8, 2018 at 11:03

answered Feb 8, 2018 at 9:04

sawa

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11 Comments

Rich_F Over a year ago

For security reasons, I'm iterating the object's ivars and setting accordingly from the hash, instead of the other way around. Cheers

Rich_F Over a year ago

@SergioTulentsev The provided hash could be larger than the array of instance_variables I want to actually modify. So if I qualify the instance_variables, I can only set those, instead of having the hash take the lead.

Sergio Tulentsev Over a year ago

@Stefan: yes, but if something were to set an "unknown" instance variable, it wouldn't be used by anything (except maybe reflection-based serialization or something). Plus, this mass-assign is "unpredicable" (depends on what instance variables in the object are "alive" at the moment)

Sergio Tulentsev Over a year ago

@Rich_F: if you whitelist "settable" instance variables (of which we have no evidence in this thread) - then yes. but in this case you might just as well filter hash keys. It's the whitelisting that adds security, not the inversion.

Stefan Over a year ago

@SergioTulentsev instance_variables seems to be that whitelist.

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