I am just getting my hands on Concerns in Rails and try to implement a simple logging for ActiveRecord classes. In there I want to define the field that should go into the log and have the log written automatically after save.
What I have is this:
#logable.rb (the concern)
module Logable
extend ActiveSupport::Concern
#field = nil
module ClassMethods
def set_log_field(field)
#feild = field
end
end
def print_log
p "LOGGING: #{self[#index.to_s]}"
end
end
#houses.rb (the model using the concern)
class House < ActiveRecord::Base
include Logable
after_save :print_log
set_log_field :id
end
Unfortunately the call to set_log_field does not have an effect - or rather the given value does not make it to print_log.
What am I doing wrong?
Thanks for your help!
You probably mean this (btw, why not Loggable?):
# logable.rb
module Logable
extend ActiveSupport::Concern
# Here we define class-level methods.
# Note, that #field, defined here cannot be referenced as #field from
# instance (it's class level!).
# Note also, in Ruby there is no need to declare #field in the body of a class/module.
class_methods do
def set_log_field(field)
#field = field
end
def log_field
#field
end
end
# Here we define instance methods.
# In order to access class level method (log_field), we use self.class.
included do
def print_log
p "LOGGING: #{self.class.log_field}"
end
end
end
Update You also asked about what's the difference between methods in included block and those within method body.
To make a short resume there is seemingly no difference. In very good approximation you can consider them the same. The only minor difference is in dependency management. Great illustration of it is given in the end of ActiveSupport::Concern documentation. It worth reading, take a look!
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 want to define 3 classes:
a MotherClass (abstract, can not be inferred)
a SubClassA (inherits from MotherClass)
a SubClassB (inherits from MotherClass)
What is the best solution to declare it in Rails ?
1. Put everything in app/models/
MotherClass < AR::Base in app/models/mother_class.rb
SubClassA < MotherClass in app_models/sub_class_a.rb
SubClassB < MotherClass in app/models/sub_class_b.rb
Advantage: not very complicated to implement
Inconvenient: a big mess in models folder
2. Create a module for the two subclasses
MotherClass < AR::Base in app/models/mother_class.rb
MotherModule::SubClassA < MotherClass in app/models/mother_module/sub_class_a.rb
MotherModule::SubClassB < MotherClass in app/models/mother_module/sub_class_b.rb
Advantage: same than Solution 1
Inconvenient: naming MotherModule and MotherClass with different names, but they mean almost the same thing
3. Create a module for the 3 classes
MotherModule::Base < AR::Base in app/models/mother_module/base.rb
MotherModule::SubClassA < MotherModule::Base in app/models/mother_module/sub_class_a.rb
MotherModule::SubClassB < MotherModule::Base in app/models/mother_module/sub_class_b.rb
Advantage: very clean
Inconvenient: need some functions in Base to override (table_name for example)
So my question is: What is the best practice in Rails and
- how to name my classes?
- what are their directories?
First of all, I think you must already realize that ruby does not have true abstract classes. But we can approximate the behavior. And while doing so, it sounds like you have a preference toward organizational structure which I will attempt to address.
I must start by saying, however, that I'm surprised that you're coming at the problem so strongly from the organizational angle. First on my mind would be whether I really wanted to implement single table inheritance or not and then let that drive the organizational problem. Usually the answer here is that Single Table Inheritance is not what you actually want. But... let's dive in!
Using Single Table Inheritance
Here's the standard way to utilize and organize models using Single Table Inheritance:
# app/models/mother_class.rb
class MotherClass < ActiveRecord::Base
# An "abstract" method
def method1
raise NotImplementedError, "Subclasses must define `method1`."
end
def method2
puts method1 # raises NotImplementedError if `method1` is not redefined by a subclass
end
end
# app/models/sub_class_a.rb
class SubClassA < MotherClass
def method1
# do something
end
end
# app/models/sub_class_b.rb
class SubClassB < MotherClass
def method1
# do something
end
end
Given the above, we would get an exception when calling MotherClass.new.method2 but not when calling SubClassA.new.method2 or SubClassB.new.method2. So we've satisfied the "abstract" requirements. Organizationally, you called this a big mess in the models folder... which I can understand if you've got tons of these subclasses or something. But, remember that in single table inheritance even then parent class is a model and is / should be usable as such! So, that said, if you'd really like to organize your models file system better then you are free to do so. For example, you could do:
app/models/<some_organizational_name>/mother_class.rb
app/models/<some_organizational_name>/sub_class_a.rb
app/models/<some_organizational_name>/sub_class_b.rb
In this, we are keeping all other things (i.e. the Code for each of these models) the same. We're not namespacing these models in any way, we're just organizing them. To make this work it's just a matter of helping Rails to find the models now that we've placed them in a subfolder of the models folder without any other clues (i.e. without namespacing them). Please refer to this other Stack Overflow post for this. But, in short, you simply need to add the following to your config/application.rb file:
config.autoload_paths += Dir[Rails.root.join('app', 'models', '{**/}')]
Using Mixins
If you decide that Single Table Inheritance is not what you want (and they often aren't, really) then mixins can give you the same quasi-abstract functionality. And you can, again, be flexible on file organization. The common, organizational pattern for mixins is this:
# app/models/concerns/mother_module.rb
module MotherModule
extend ActiveSupport::Concern
# An "abstract" method
def method1
raise NotImplementedError, "Subclasses must define `method1`."
end
def method2
puts method1 # raises NotImplementedError if `method1` is not redefined
end
end
# app/models/sub_class_a.rb
class SubClassA
include MotherModule
def method1
# do something
end
end
# app/models/sub_class_b.rb
class SubClassB
include MotherModule
def method1
# do something
end
end
With this approach, we continue to not get an exception when calling SubClassA.new.method2 or SubClassB.new.method2 because we've overridden these methods in the "subclasses". And since we can't really call MotherModule#method1 directly it is certainly an abstract method.
In the above organization, we've tucked MotherModule away into the models/concerns folder. This is the common location for mixins in Rails these days. You didn't mention what rails version you're on, so if you don't already have a models/concerns folder you'll want to make one and then make rails autoload models from there. This would, again, be done in config/application.rb with the following line:
config.autoload_paths += Dir[Rails.root.join('app', 'concerns', '{**/}')]
The organization with the mixins approach is, in my opinion, simple and clear in that SubclassA and SubClassB are (obviously) models and, since they include the MotherModule concern they get the behaviors of MotherModule. If you wanted to group the subclass models, organizationally, into a folder then you could still do this of course. Just use the same approach outlined at the end of the Single Table Inheritance section, above. But I'd probably keep MotherModule located in the models/concerns folder still.
Even though ruby doesn't really have abstract classes, it's powerful enough to let you implement it yourself by implementing self.included on a mixin module. Hopefully this generic example gives you enough to go on for your particular implementation.
module MotherInterface
def self.included base
required_class_methods = [:method1, :method2]
required_instance_methods = [:fizzle, :fazzle]
required_associations = [:traits, :whatevers]
required_class_methods.each do |cm|
raise "MotherInterface: please define .#{cm} class method on host class #{base.name}" unless base.respond_to?(cm)
end
required_instance_methods.each do |im|
raise "MotherInterface: please define ##{im} instance method on host class #{base.name}" unless base.instance_methods.include?(im)
end
required_associations.each do |ass|
raise "MotherInterface: please be sure #{base.name} has a :#{ass} association" unless base.reflections.key?(ass)
end
base.send :include, InstanceMethods
base.extend ClassMethods
end
# inherited instance methods
module InstanceMethods
def foo
end
def bar
end
end
# inherited class methods
module ClassMethods
def baz
end
def bat
end
end
end
class SubClassA < ActiveRecord::Base
include MotherInterface
# ... define required methods here ...
end
class SubClassB < ActiveRecord::Base
include MotherInterface
end
Some advantages to this approach are:
Yes, you can still technically instantiate the mixin, but it's not actually tied to active record, so it tastes more like an abstract class.
The sub classes get to define their own connection information. You have two databases? Differing columns? Cool, no problem. Just implement your instance methods and stuff appropriately.
The dividing line between parent and child is very obvious.
But, there are disadvantages too:
All the meta programming is a bit more complex. You'll have to think a little abstractly (HA!) about how to organize your code.
There are probably other advantages and disadvantages I haven't considered, kind of in a hurry here.
Now, as far as file locations, I would suggest that the mixin itself, presumably mother_interface.rb, go someplace other than your models folder.
In config/application.rb, throw in a line like this:
config.autoload_paths << File.join(Rails.root, 'app', 'lib')
...and then you can create (rails)/app/lib/mother_interface.rb. Really, you should do it however makes sense to you. I dislike the word "concerns" for this, and other people dislike the word "lib." So, use whatever word you like, or make up your own.
Using Single Table Inheritance with little bit meta programming
# app/models/mother_class.rb
class MotherClass < ActiveRecord::Base
def self.inherited(subclass)
subclass.include(OnlyChildMethods)
end
module OnlyChildMethods
extend ActiveSupport::Concern
included do
def child_method_one
puts 'hi one'
end
def child_method_two
puts 'hi two'
end
end
end
end
# app/models/sub_class_a.rb
class SubClassA < MotherClass
def some_specific_method
#some code
end
end
# app/models/sub_class_b.rb
class SubClassB < MotherClass
def some_specific_method
#some code
end
end
mother_class_instance.child_method_one
=> NoMethodError: undefined method 'child_method_one'
sub_class_a_instance.child_method_one
hi one
=> nil
I have a very big function in my model and I want to store it somewhere else in order to keep my model dry. I read that storing methods in ApplicationHelper and then calling them from a model is a bad idea. What is a good idea then?
I want to have a separate file with my big methods and call them from a model.
You can create a "plain old ruby object (PORO)" to do your work for you. let's say you had a method that calculates the amount overdue for a user.
So, you can create app/services/calculates_overages.rb
class CalculatesOverages
def initialize(user)
#user = user
end
def calculate
# your method goes here
end
end
Then, you can:
class User < ActiveRecord::Base
def overage_amount
CaluclatesOverage.new(self).calculate
end
end
Or, in a controller you could:
def show
#amount = CaluclatesOverage.new(current_user).calculate
end
The app/services directory could also be app/models, or the lib directory. There's no set convention for this (yet).
Use a Concern. https://gist.github.com/1014971
It's simple. In app/models/concerns create a file your_functionality.rb as follows:
module YourFunctionality
extend ActiveSupport::Concern
def your_fat_method
# insert...
end
end
And in your model simply:
include YourFunctionality
If a few of my models have a privacy column, is there a way I can write one method shared by all the models, lets call it is_public?
so, I'd like to be able to do object_var.is_public?
One possible way is to put shared methods in a module like this (RAILS_ROOT/lib/shared_methods.rb)
module SharedMethods
def is_public?
# your code
end
end
Then you need to include this module in every model that should have these methods (i.e. app/models/your_model.rb)
class YourModel < ActiveRecord::Base
include SharedMethods
end
UPDATE:
In Rails 4 there is a new way to do this. You should place shared Code like this in app/models/concerns instead of lib
Also you can add class methods and execute code on inclusion like this
module SharedMethods
extend ActiveSupport::Concern
included do
scope :public, -> { where(…) }
end
def is_public?
# your code
end
module ClassMethods
def find_all_public
where #some condition
end
end
end
You can also do this by inheriting the models from a common ancestor which includes the shared methods.
class BaseModel < ActiveRecord::Base
def is_public?
# blah blah
end
end
class ChildModel < BaseModel
end
In practice, jigfox's approach often works out better, so don't feel obligated to use inheritance merely out of love for OOP theory :)