I define class methods dynamically in Rails as follows:
class << self
%w[school1 school2].each do |school|
define_method("self.find_by_#{school}_id") do |id|
MyClass.find_by(school: school, id: id)
end
end
end
How can I use method missing to call find_by_SOME_SCHOOL_id without having to predefine these schools in %w[school1 school2]?
It is not completely clear to me what you want to achieve. If you want to call a method, you naturally have to define it first (as long as ActiveRecord does not handle this). You could do something like this:
class MyClass
class << self
def method_missing(m, *args, &block)
match = m.to_s.match(/find_by_school([0-9]+)_id/)
if match
match.captures.first
else
nil
end
end
end
end
puts MyClass.find_by_school1_id
puts MyClass.find_by_school2_id
puts MyClass.find_by_school22_id
puts MyClass.find_by_school_id
This will output:
1
2
22
nil
You could then do something with the ID contained in the method name. If you are sure that the method is defined, you can also use send(m, args) to call that method on an object/class. Beware though, if you do that on the same class that receives the method missing call and the method is not defined, you will get a stack overflow.
I recommend return the super unless you have a match
class MyClass
class << self
def method_missing(m, *args, &block)
match = m.to_s.match(/find_by_school([0-9]+)_id/)
match ? match.captures.first : super
end
end
end
Related
I am taking ruby-kickstart (Josh Cheek) challenges and even though I managed to pass all the test there is one thing I cannot understand.
In the exercise you are being asked to override the << method for an instance variable. Specifically here is what you have to do:
In Ruby on Rails, when a person goes to a URL on your site, your
application looks at the url, and maps it to a controller method to
handle the request
My boss wanted to be able to specify what CSS class the body of the
HTML output should have, based on which controller method was being
accessed. It fell to me to provide a method, which, when invoked,
would return a String that could handle the request There are a few
nuances, though. The String you return must be retained during the
object's entire life The method must be able to be called multiple
times The String you return should know how to add new classes: each
class is separated by a space The only method you need to worry about
being invoked is the << method.
(plus a few other irrelevant things)
EXAMPLE:
controller = ApplicationController.new
controller.body_class
#=> ""
controller.body_class << 'admin'
controller.body_class
#=> "admin"
controller.body_class << 'category'
controller.body_class
#=> "admin category"
controller.body_class << 'page' << 'order'
controller.body_class
#=> "admin category page order"
My working solution:
class ApplicationController
def initialize
#body_class = ""
end
def body_class
def #body_class.<<(str)
puts "self is:"+self
return self if self=~/\b#{Regexp.escape(str)}\b/
if self==""
self.concat(str)
else
self.concat(" ")
self.concat(str)
end
end
return #body_class
end
end
Everything works perfectly fine.
But an earlier solution I gave (and it didn't work) was the following
class ApplicationController
attr_accessor :body_class
def initialize
#body_class = ""
end
def #body_class.<<(str)
puts "self is:"+self
return self if self=~/\b#{Regexp.escape(str)}\b/
if self==""
self.concat(str)
else
self.concat(" ")
self.concat(str)
end
end
def body_class #Even this for the way I work it out on my mind is unnecessary
return #body_class
end
end
When someone runs on the second not-working sollution the following
obj = ApplicationController.new
obj.body_class << "Hi"
The << method is not being overridden by the object's singleton.
I do not understand why I have to wrap the singleton methods inside the body_class method. (Mind that in the second solution there is an attr_accessor.
Could anyone enlighten me please!
Thanks!
I do not understand why I have to wrap the singleton methods inside the body_class method.
To access the correct instance variable. When you attempt to override it outside of method, you're in the class context. This code runs at class load time. No instances have been created yet. (And even if instances did exist at this point, #body_class instance variable belongs to class ApplicationController, which is itself an instance of class Class).
You need instance context.
Also I am pretty sure that this problem can be solved without any method patching voodoo. Just provide a different object (conforming to the same API. This is called "duck typing").
class ApplicationController
def body_class
#body_class ||= CompositeClass.new
end
class CompositeClass
def initialize
#classes = []
end
def <<(new_class)
#classes << new_class
end
# I imagine, this is what is ultimately called on ApplicationController#body_class,
# when it's time to render the class in the html.
def to_s
#classes.join(' ')
end
end
end
Didn't test this code, naturally.
BTW, the proper way to do it is to explicitly extend the instance variable:
class A
attr_reader :body_class
def initialize
#body_class = "".extend(Module.new do
def <<(other)
return self if self[/\b#{Regexp.escape(other)}\b/]
concat(' ') unless empty?
concat(other)
end
end)
end
end
I have a several classes, each of which define various statistics.
class MonthlyStat
attr_accessor :cost, :size_in_meters
end
class DailyStat
attr_accessor :cost, :weight
end
I want to create a decorator/presenter for a collection of these objects, that lets me easily access aggregate information about each collection, for example:
class YearDecorator
attr_accessor :objs
def self.[]= *objs
new objs
end
def initialize objs
self.objs = objs
define_helpers
end
def define_helpers
if o=objs.first # assume all objects are the same
o.instance_methods.each do |method_name|
# sums :cost, :size_in_meters, :weight etc
define_method "yearly_#{method_name}_sum" do
objs.inject(0){|o,sum| sum += o.send(method_name)}
end
end
end
end
end
YearDecorator[mstat1, mstat2].yearly_cost_sum
Unfortunately define method isn't available from within an instance method.
Replacing this with:
class << self
define_method "yearly_#{method_name}_sum" do
objs.inject(0){|o,sum| sum += o.send(method_name)}
end
end
...also fails because the variables method_name and objs which are defined in the instance are no longer available. Is there an idomatic was to accomplish this in ruby?
(EDITED: I get what you're trying to do now.)
Well, I tried the same approaches that you probably did, but ended up having to use eval
class Foo
METHOD_NAMES = [:foo]
def def_foo
METHOD_NAMES.each { |method_name|
eval <<-EOF
def self.#{method_name}
\"#{method_name}\".capitalize
end
EOF
}
end
end
foo=Foo.new
foo.def_foo
p foo.foo # => "Foo"
f2 = Foo.new
p f2.foo # => "undefined method 'foo'..."
I myself will admit it's not the most elegant solution (may not even be the most idiomatic) but I've run into similar situations in the past where the most blunt approach that worked was eval.
I'm curious what you're getting for o.instance_methods. This is a class-level method and isn't generally available on instances of objects, which from what I can tell, is what you're dealing with here.
Anyway, you probably are looking for method_missing, which will define the method dynamically the first time you call it, and will let you send :define_method to the object's class. You don't need to redefine the same instance methods every time you instantiate a new object, so method_missing will allow you to alter the class at runtime only if the called method hasn't already been defined.
Since you're expecting the name of a method from your other classes surrounded by some pattern (i.e., yearly_base_sum would correspond to a base method), I'd recommend writing a method that returns a matching pattern if it finds one. Note: this would NOT involve making a list of methods on the other class - you should still rely on the built-in NoMethodError for cases when one of your objects doesn't know how to respond to message you send it. This keeps your API a bit more flexible, and would be useful in cases where your stats classes might also be modified at runtime.
def method_missing(name, *args, &block)
method_name = matching_method_name(name)
if method_name
self.class.send :define_method, name do |*args|
objs.inject(0) {|obj, sum| sum + obj.send(method_name)}
end
send name, *args, &block
else
super(name, *args, &block)
end
end
def matching_method_name(name)
# ... this part's up to you
end
I have created a class which I have some constant hashes. I'd like to type Myclass.myhash.hashkey and to show the value of the hash. Right Now I have created a similar behavior with method_missing but I have to initialize the object, so I am calling it like Myclass.new.myhash.hashkey and it works. Here is my code so far:
class Myclass
def initialize
#attributes = []
end
def method_missing(name, *args)
#attributes << name
if #attributes.length == 2
eval("#{#attributes.first.upcase}[:#{#attributes.last.downcase}]")
else
self
end
end
MYHASH = {
id: 1,
description: "A nice hash",
hashkey: "hash key"
}
end
How can I do it without initialize and without new so it won't create an object of MyClass everytime?
Update:
The first question was explained by toro2k but I don't know if using it I can have the behavior of my second question...
Question 2
I have many openstructs in my class, how can I define them as a class methods dynamically without every time adding something like:
def self.myhash
MYHASH
end
You could use an OpenStruct object instead of the Hash:
class MyClass
MYHASH = OpenStruct.new(id: 1,
description: 'A nice Ostruct',
hashkey: 'hash key')
def self.myhash
MYHASH
end
end
MyClass.myhash.id # => 1
MyClass.myhash.description # => "A nice Ostruct"
MyClass.myhash.foo # => nil
Update You could replace constants with class instance variables like this:
class MyClass
def self.myhash
#myhash ||= OpenStruct(id: ...)
end
end
MyClass.myhash.id
Or you could use class variables and cattr_reader:
class MyClass
cattr_reader :myhash
##myhash = OpenStruct(id: ...)
end
MyClass.myhash.id
Or you could get rid of the myhash method and access the constant directly:
class MyClass
MYHASH = OpenStruct(id: ...)
end
MyClass::MYHASH.id
I have finally found a solution for my second question also:
class << self
Myclass.constants.each do |constant|
define_method(constant.to_s.downcase) do
eval("#{constant}")
end
end
end
I just have to add it at the end of the class to work, after I have defined all the openstruct variables.
Hey guys.
How do I know the methods that a child class overrided in my super class?
I have this:
class Test
def self.inherited(child)
# child.overrided_methods???
end
def self.foo
end
def self.bar
end
end
def Child < Test
def self.bar
puts "bar"
end
end
The method self.inherited is called when a subclass of Test is loaded. So I get the reference to this subclass in child, but I don't know how to get the methods that were overrided by this subclass.
Any ideas?
--
Arsen suggested the use of self.method_added(name) instead of self.inherited(child), but this method catches only instance methods and I want to catch class methods. Does anyone know another methods that does the same thing but with class methods?
In the last case I'll consider using a singleton and convert all this class methods to instance methods then the problem is solved.
For instance methods there is an Object::method_added(name) method you can override, similar to 'inherited' you have used:
class test
def self.method_added(name)
puts "method_added(#{name.inspect})"
super
end
end
irb(main):002:0> class Child < Test; def foo; end; end
method_added(:foo)
=> nil
You can then compare a received name to a list of your methods:
Test.instance_methods.include?(name.to_s)
With class methods this approach does not work (even if you do things like class << self magic), but a helpful fellow knew the answer: http://www.ruby-forum.com/topic/120416 :
class Test
def self.singleton_method_added(name)
puts "Class method added #{name.inspect}"
end
end
This is only the first part of the problem, because you need to know which class defined the method (it will be self) and whether the method is a new one, or overridden one. Experiment with this code:
class Test
def self.singleton_method_added(name)
if self == Test
puts "My own class method added: #{self.name}.#{name.inspect}"
elsif Test.methods(false).include?(name.to_s)
puts "Class method overriden: #{self.name}.#{name.inspect}"
elsif Test.methods(true).include?(name.to_s)
puts "My parent's class method overriden: #{self.name}.#{name.inspect}"
else
puts "New class method added: #{self.name}.#{name.inspect}"
end
end
end
Maybe a first step to the solution:
By calling child.instance_method(:bar) (if child refers to the class) or child.method(:bar) (if it refers to an instance of Child) you can get an UnboundMethod or Method object representing your method:
a = Test.instance_method(:foo)
b = Child.instance_method(:foo)
Unfortunately, a == b evaluates to false, although both refer to the same method.
def overridden_methods
klass = self.class
klass.instance_methods.select {|m| klass.instance_method(m).owner == klass}
end
Change according to your needs.
For cruft-removal purposes I would like to log whenever a method from one of my AR models is called.
I can get get all those classes with something like this:
subclasses = [] ; ObjectSpace.each_object(Module) {|m| subclasses << m if m.ancestors.include? ActiveRecord::Base } ; subclasses.map(&:name)
But then I need a list of only the methods defined on those classes (instance and class methods), and a way to inject a logger statement in them.
The result would be the equivalent of inserting this into every method
def foo
logger.info "#{class.name} - #{__method__}"
# ...
end
def self.foo
logger.info "#{name} - #{__method__}"
# ...
end
How can I do that without actually adding it to every single method?
Some awesome meta perhaps?
If you want only the methods defined in the class you can do this:
>> Project.instance_methods
=> ["const_get", "validates_associated", "before_destroy_callback_chain", "reset_mocha", "parent_name", "inspect", "slug_normalizer_block", "set_sequence_name", "require_library_or_gem", "method_exists?", "valid_keys_for_has_and_belongs_to_many_association=", "table_name=", "validate_find_options_without_friendly", "quoted_table_name" (another 100 or so methods)]
Only the methods defined in your class
>> Project.instance_methods(false)
=> ["featured_asset", "category_list", "before_save_associated_records_for_slugs", "asset_ids", "primary_asset", "friendly_id_options", "description", "description_plain"]
You should be using Aspect Oriented Programming pattern for this. In Ruby Aquarium gem provides the AOP DSL.
Create a log_method_initializer.rb in config/initializers/ directory.
require 'aquarium'
Aspect.new(:around, :calls_to => :all_methods,
:in_types => [ActiveRecord::Base] ) do |join_point, object, *args|
log "Entering: #{join_point.target_type.name}##{join_point.method_name}"
result = join_point.proceed
log "Leaving: #{join_point.target_type.name}##{join_point.method_name}"
result
end
Every method calls of classes inherited from ActiveRecord::Base will be logged.
You have
AR::Base.instance_methods
and
AR::Base.class_eval "some string"
so you can probably use them to put a header on every existing method.
For instance method call you can use this proxy pattern:
class BlankSlate
instance_methods.each { |m| undef_method m unless m =~ /^__/ }
end
class MyProxy < BlankSlate
def initialize(obj, &proc)
#proc = proc
#obj = obj
end
def method_missing(sym, *args, &block)
#proc.call(#obj,sym, *args)
#obj.__send__(sym, *args, &block)
end
end
Example:
cust = Customer.first
cust = MyProxy.new(cust) do |obj, method_name, *args|
ActiveRecord::Base.logger.info "#{obj.class}##{method_name}"
end
cust.city
# This will log:
# Customer#city
This is inspired from: http://onestepback.org/index.cgi/Tech/Ruby/BlankSlate.rdoc
You will need to find a way to apply this pattern on ActiveRecord::Base object creation.
For Aquarium, seems like adding method_options => :exclude_ancestor_methods does the trick.
I had the stack too deep problem as well.
Source
http://andrzejonsoftware.blogspot.com/2011/08/tracing-aspect-for-rails-application.html