I am relatively new to meta programming and I am trying to define getters and setters in children models under a single table inheritance, like so:
# app/models/player.rb
class Player < ActiveRecord::Base
serialize :equipments
protected
# Define getters for setters for individual sports
def attr_equipments(*attributes)
attributes.each do |attribute|
define_singleton_method(attribute) do
self.equipments[attribute.to_sym]
end
define_singleton_method("#{attribute}=") do |value|
self.equipments[attribute.to_sym] = value
end
end
end
end
# app/models/baseball_player.rb
class BaseballPlayer < Player
after_initialize do
attr_equipments :bat, :glove
end
end
On a higher layer I would like to do something like this:
# app/controllers/baseball_players_controller.rb
class BaseballPlayersController < ApplicationController
def new
#baseball_player = BaseballPlayer
end
end
# app/views/baseball_players/new.html.haml
=form_for(#baseball_player) do |f|
=f.text_field :bat
=f.text_field :glove
=f.submit
But with the example above I cannot get or set the attributes even from the console.
> p = BaseballPlayer.new
> p.bat
> # returns NoMethodError: undefined method `[]' for nil:NilClass
> p = BaseballPlayer.new
> p.bat = "Awesome bat"
> # returns ArgumentError: wrong number of arguments (0 for 2..3)
Could someone help me find the right direction? Many thanks in advance.
I just solved the issue myself..
For future references, this part needed to be store as serialize does not automatically define any accessors.
# app/models/player.rb
class Player < ActiveRecord::Base
# serialize :equipments -> does not create a setter or getter needed
store :equipments
I also encountered an error in create method in the controller:
def create
#alarm = BaseballPlayer.new(alarm_params)
# Error: undefined attribute 'bat'
which was solved by:
def create
#alarm = BaseballPlayer.new
#alarm.assign_attributes(alarm_params)
Related
In my model, I'm trying to dynamically expose objects that are inside an array as a top level attribute. Here's the code snippet:
class Widget < ActiveRecord::Base
# attr_accessor :name
end
class MyModel < ActiveRecord::Base
has_many :widgets
#attr_accessor :widgets
after_initialize :init_widgets
def init_widgets
widgets
widgets.each_with_index do |widget, index|
define_method(widget.name) do
widgets[index]
end
end
end
end
Is there any way for me to define the value of index into the new method I am creating so that it will be associated with the proper index?
I might create an accessor / assignment methods that overload the [] operator:
class BracketOperator
def initialize
#values = (1..100).to_a
end
def [](index)
#values[index]
end
def []=(index, value)
#values[index] = value
end
end
bo = BracketOperator.new
bo[3] # => 4
bo[3] = 17
bo[3] # => 17
So for reference, here's the answer I came up with. Apparently I don't understand scoping for ruby too well. The variable n somehow remains referenced to the n within the each loop. Hence for my original question, I can just use the index variable within the method and it will be mapped to what I am expecting it to be mapped to.
class Test
def setup
names = [ "foo","bar" ]
names.each do |n|
self.class.send :define_method, n do
puts "method is called #{n}!"
end
end
end
end
I am trying to DRY my code by implementing modules. However, I have constants stored in models (not the module) that I am trying to access with self.class.
Here are (I hope) the relevant snippets:
module Conversion
def constant(name_str)
self.class.const_get(name_str.upcase)
end
end
module DarkElixir
def dark_elixir(th_level)
structure.map { |name_str| structure_dark_elixir(name_str, th_level) if constant(name_str)[0][:dark_elixir_cost] }.compact.reduce(:+)
end
end
class Army < ActiveRecord::Base
include Conversion, DarkElixir
TH_LEVEL = [...]
end
def structure_dark_elixir(name_str, th_level)
name_sym = name_str.to_sym
Array(0..send(name_sym)).map { |level| constant(name_str)[level][:dark_elixir_cost] }.reduce(:+) * TH_LEVEL[th_level][sym_qty(name)]
end
When I place the structure_dark_elixir method inside the DarkElixir module, I get an error, "uninitialized constant DarkElixir::TH_LEVEL"
While if I place it inside the Army class, it finds the appropriate constant.
I believe it is because I am not scoping the self.constant_get correctly. I would like to keep the method in question in the module as other models need to run the method referencing their own TH_LEVEL constants.
How might I accomplish this?
Why not just use class methods?
module DarkElixir
def dark_elixir(th_level)
# simplified example
th_level * self.class.my_th_level
end
end
class Army < ActiveRecord::Base
include DarkElixir
def self.my_th_level
5
end
end
Ugh. Method in question uses two constants. It was the second constant that was tripping up, not the first. Added "self.class::" prior to the second constant--back in business.
def structure_dark_elixir(name_str, th_lvl)
name_sym = name_str.to_sym
Array(0..send(name_sym)).map { |level| constant(name_str)[level][:dark_elixir_cost] }.reduce(:+) * self.class::TH_LEVEL[th_lvl][sym_qty(name_str)]
end
I am facing a design decision I cannot solve. In the application a user will have the ability to create a campaign from a set of different campaign types available to them.
Originally, I implemented this by creating a Campaign and CampaignType model where a campaign has a campaign_type_id attribute to know which type of campaign it was.
I seeded the database with the possible CampaignType models. This allows me to fetch all CampaignType's and display them as options to users when creating a Campaign.
I was looking to refactor because in this solution I am stuck using switch or if/else blocks to check what type a campaign is before performing logic (no subclasses).
The alternative is to get rid of CampaignType table and use a simple type attribute on the Campaign model. This allows me to create Subclasses of Campaign and get rid of the switch and if/else blocks.
The problem with this approach is I still need to be able to list all available campaign types to my users. This means I need to iterate Campaign.subclasses to get the classes. This works except it also means I need to add a bunch of attributes to each subclass as methods for displaying in UI.
Original
CampaignType.create! :fa_icon => "fa-line-chart", :avatar=> "spend.png", :name => "Spend Based", :short_description => "Spend X Get Y"
In STI
class SpendBasedCampaign < Campaign
def name
"Spend Based"
end
def fa_icon
"fa-line-chart"
end
def avatar
"spend.png"
end
end
Neither way feels right to me. What is the best approach to this problem?
A not very performant solution using phantom methods. This technique only works with Ruby >= 2.0, because since 2.0, unbound methods from modules can be bound to any object, while in earlier versions, any unbound method can only be bound to the objects kind_of? the class defining that method.
# app/models/campaign.rb
class Campaign < ActiveRecord::Base
enum :campaign_type => [:spend_based, ...]
def method_missing(name, *args, &block)
campaign_type_module.instance_method(name).bind(self).call
rescue NameError
super
end
def respond_to_missing?(name, include_private=false)
super || campaign_type_module.instance_methods(include_private).include?(name)
end
private
def campaign_type_module
Campaigns.const_get(campaign_type.camelize)
end
end
# app/models/campaigns/spend_based.rb
module Campaigns
module SpendBased
def name
"Spend Based"
end
def fa_icon
"fa-line-chart"
end
def avatar
"spend.png"
end
end
end
Update
Use class macros to improve performance, and keep your models as clean as possible by hiding nasty things to concerns and builder.
This is your model class:
# app/models/campaign.rb
class Campaign < ActiveRecord::Base
include CampaignAttributes
enum :campaign_type => [:spend_based, ...]
campaign_attr :name, :fa_icon, :avatar, ...
end
And this is your campaign type definition:
# app/models/campaigns/spend_based.rb
Campaigns.build 'SpendBased' do
name 'Spend Based'
fa_icon 'fa-line-chart'
avatar 'spend.png'
end
A concern providing campaign_attr to your model class:
# app/models/concerns/campaign_attributes.rb
module CampaignAttributes
extend ActiveSupport::Concern
module ClassMethods
private
def campaign_attr(*names)
names.each do |name|
class_eval <<-EOS, __FILE__, __LINE__ + 1
def #{name}
Campaigns.const_get(campaign_type.camelize).instance_method(:#{name}).bind(self).call
end
EOS
end
end
end
end
And finally, the module builder:
# app/models/campaigns/builder.rb
module Campaigns
class Builder < BasicObject
def initialize
#mod = ::Module.new
end
def method_missing(name, *args)
value = args.shift
#mod.send(:define_method, name) { value }
end
def build(&block)
instance_eval &block
#mod
end
end
def self.build(module_name, &block)
const_set module_name, Builder.new.build(&block)
end
end
How do you define a method for an attribute of an instance in Ruby?
Let's say we've got a class called HtmlSnippet, which extends ActiveRecord::Base of Rails and has got an attribute content. And, I want to define a method replace_url_to_anchor_tag! for it and get it called in the following way;
html_snippet = HtmlSnippet.find(1)
html_snippet.content = "Link to http://stackoverflow.com"
html_snippet.content.replace_url_to_anchor_tag!
# => "Link to <a href='http://stackoverflow.com'>http://stackoverflow.com</a>"
# app/models/html_snippet.rb
class HtmlSnippet < ActiveRecord::Base
# I expected this bit to do what I want but not
class << #content
def replace_url_to_anchor_tag!
matching = self.match(/(https?:\/\/[\S]+)/)
"<a href='#{matching[0]}'/>#{matching[0]}</a>"
end
end
end
As content is an instance of String class, redefine String class is one option. But I don't feel like to going for it because it overwrites behaviour of all instances of String;
class HtmlSnippet < ActiveRecord::Base
class String
def replace_url_to_anchor_tag!
...
end
end
end
Any suggestions please?
The reason why your code is not working is simple - you are working with #content which is nil in the context of execution (the self is the class, not the instance). So you are basically modifying eigenclass of nil.
So you need to extend the instance of #content when it's set. There are few ways, there is one:
class HtmlSnippet < ActiveRecord::Base
# getter is overrided to extend behaviour of freshly loaded values
def content
value = read_attribute(:content)
decorate_it(value) unless value.respond_to?(:replace_url_to_anchor_tag)
value
end
def content=(value)
dup_value = value.dup
decorate_it(dup_value)
write_attribute(:content, dup_value)
end
private
def decorate_it(value)
class << value
def replace_url_to_anchor_tag
# ...
end
end
end
end
For the sake of simplicity I've ommited the "nil scenario" - you should handle nil values differently. But that's quite simple.
Another thing is that you might ask is why I use dup in the setter. If there is no dup in the code, the behaviour of the following code might be wrong (obviously it depends on your requirements):
x = "something"
s = HtmlSnippet.find(1)
s.content = x
s.content.replace_url_to_anchor_tag # that's ok
x.content.replace_url_to_anchor_tag # that's not ok
Wihtout dup you are extending not only x.content but also original string that you've assigned.
I am using Ruby on Rails 3.2.2 and I would like to know if the following is a "proper"/"correct"/"sure" way to override a setter method for a my class attribute.
attr_accessible :attribute_name
def attribute_name=(value)
... # Some custom operation.
self[:attribute_name] = value
end
The above code seems to work as expected. However, I would like to know if, by using the above code, in future I will have problems or, at least, what problems "should I expect"/"could happen" with Ruby on Rails. If that isn't the right way to override a setter method, what is the right way?
Note: If I use the code
attr_accessible :attribute_name
def attribute_name=(value)
... # Some custom operation.
self.attribute_name = value
end
I get the following error:
SystemStackError (stack level too deep):
actionpack (3.2.2) lib/action_dispatch/middleware/reloader.rb:70
===========================================================================
Update: July 19, 2017
Now the Rails documentation is also suggesting to use super like this:
class Model < ActiveRecord::Base
def attribute_name=(value)
# custom actions
###
super(value)
end
end
===========================================================================
Original Answer
If you want to override the setter methods for columns of a table while accessing through models, this is the way to do it.
class Model < ActiveRecord::Base
attr_accessible :attribute_name
def attribute_name=(value)
# custom actions
###
write_attribute(:attribute_name, value)
# this is same as self[:attribute_name] = value
end
end
See Overriding default accessors in the Rails documentation.
So, your first method is the correct way to override column setters in Models of Ruby on Rails. These accessors are already provided by Rails to access the columns of the table as attributes of the model. This is what we call ActiveRecord ORM mapping.
Also keep in mind that the attr_accessible at the top of the model has nothing to do with accessors. It has a completely different functionlity (see this question)
But in pure Ruby, if you have defined accessors for a class and want to override the setter, you have to make use of instance variable like this:
class Person
attr_accessor :name
end
class NewPerson < Person
def name=(value)
# do something
#name = value
end
end
This will be easier to understand once you know what attr_accessor does. The code attr_accessor :name is equivalent to these two methods (getter and setter)
def name # getter
#name
end
def name=(value) # setter
#name = value
end
Also your second method fails because it will cause an infinite loop as you are calling the same method attribute_name= inside that method.
Use the super keyword:
def attribute_name=(value)
super(value.some_custom_encode)
end
Conversely, to override the reader:
def attribute_name
super.some_custom_decode
end
In rails 4
let say you have age attribute in your table
def age=(dob)
now = Time.now.utc.to_date
age = now.year - dob.year - ((now.month > dob.month || (now.month == dob.month && now.day >= dob.day)) ? 0 : 1)
super(age) #must add this otherwise you need to add this thing and place the value which you want to save.
end
Note:
For new comers in rails 4 you don't need to specify attr_accessible in model. Instead you have to white-list your attributes at controller level using permit method.
I have found that (at least for ActiveRecord relationship collections) the following pattern works:
has_many :specialties
def specialty_ids=(values)
super values.uniq.first(3)
end
(This grabs the first 3 non-duplicate entries in the array passed.)
Using attr_writer to overwrite setter
attr_writer :attribute_name
def attribute_name=(value)
# manipulate value
# then send result to the default setter
super(result)
end