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The Elusive Attribute Chris Salzberg

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As we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns - the ones we don't know we don't know. - Donald Rumsfeld United States Secretary of Defense (2003)

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People who are really into bikes [...] start out from the frame. They represent that small percentage of people who get the drawing perfectly right. Most other people tend to start from what they are most sure of, so for nearly everyone it’s the wheels. [...] I saw many people drawing all the rest in the hope that positioning handlebar, saddle and pedals would help them figure out the frame. - Gianluca Gimini

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@attributes Name: Chris Salzberg Handle: shioyama Born: Montreal, CA Based: Tokyo, JP Company: Degica Blog: dejimata.com Tags: Module Builder, Mobility

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1. methods & columns

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class Post < ApplicationRecord end < 6.0

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class Post < ApplicationRecord end Post.ancestors => [Post, Post::GeneratedAssociationMethods, #, ApplicationRecord(abstract), ApplicationRecord::GeneratedAssociationMethods, #, AR::Base, ... ] < 6.0

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class Post < ApplicationRecord end Post.ancestors => [Post, Post::GeneratedAssociationMethods, #, ApplicationRecord(abstract), ApplicationRecord::GeneratedAssociationMethods, #, AR::Base, ... ] < 6.0

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class Post < ApplicationRecord end Post.ancestors => [Post, Post::GeneratedAssociationMethods, Post::GeneratedAttributeMethods, ApplicationRecord(abstract), ApplicationRecord::GeneratedAssociationMethods, ApplicationRecord::GeneratedAttributeMethods, ActiveRecord::Base, ... ] >= 6.0

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post = Post.new(title: "The Elusive Attribute")

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post = Post.new(title: "The Elusive Attribute") post.method(:title) => #

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post = Post.new(title: "The Elusive Attribute") post.method(:title).owner => Post::GeneratedAttributeMethods

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=> [ ... :title_before_type_cast, :title_came_from_user?, :title?, :title_changed?, :title_change, :title_will_change!, :title_was, :title_previously_changed?, ... ] post = Post.new(title: "The Elusive Attribute") post.method(:title).owner.instance_methods Attribute Methods

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post = Post.new(title: "The Elusive Attribute") mod = post.method(:title).owner

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post = Post.new(title: "The Elusive Attribute") mod = post.method(:title).owner mod.instance_methods.each do |meth| mod.send(:remove_method, meth) end

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post = Post.new(title: "The Elusive Attribute") mod = post.method(:title).owner mod.instance_methods.each do |meth| mod.send(:remove_method, meth) end mod.instance_methods #=> []

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post = Post.new(title: "The Elusive Attribute") mod = post.method(:title).owner mod.instance_methods.each do |meth| mod.send(:remove_method, meth) end mod.instance_methods post.method(:title) # raises NameError: undefined method `title'

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post = Post.new(title: "The Elusive Attribute") mod = post.method(:title).owner mod.instance_methods.each do |meth| mod.send(:remove_method, meth) end mod.instance_methods post.method(:title) post.title #=> ...

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post = Post.new(title: "The Elusive Attribute") mod = post.method(:title).owner mod.instance_methods.each do |meth| mod.send(:remove_method, meth) end mod.instance_methods post.method(:title) post.title #=> "The Elusive Attribute"

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cache = AR::Base.connection.schema_cache

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cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) #=> {}

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cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) columns_hash["posts"] = {}

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cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) columns_hash["posts"] = {} Post #=> Post()

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cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) columns_hash["posts"] = {} post = Post.new #=> #

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cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) columns_hash["posts"] = {} post = Post.new post.method(:title=) # raises NameError: undefined method `title='

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cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) columns_hash["posts"] = {} post = Post.new post.method(:title=) post.title = "The Elusive Attribute" # raises ActiveModel::UnknownAttributeError

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cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) columns_hash["posts"] = {} post = Post.new post.method(:title=) post.title = "The Elusive Attribute" post.attributes #=> {}

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cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) columns_hash["posts"] = {} post = Post.first # SELECT "posts".* FROM "posts" ... #=> #

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cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) columns_hash["posts"] = {} post = Post.first post.method(:title) # raises NameError: undefined method `title'

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cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) columns_hash["posts"] = {} post = Post.first post.method(:title) post.title #=> ?

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cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) columns_hash["posts"] = {} post = Post.first post.method(:title) post.title #=> "The Elusive Attribute"

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cache = AR::Base.connection.schema_cache columns_hash = cache.instance_variable_get(:@columns_hash) columns_hash["posts"] = {} post = Post.first post.method(:title) post.title post.attributes #=> {"id"=>1, "title"=>"The Elusive Attribute", # "created_at"=>2019-03-13 05:09:10 UTC, # "updated_at"=>2019-03-29 07:57:54 UTC}

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result = AR::Base.connection.exec_query( 'SELECT posts.* FROM posts') #=> #

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result = AR::Base.connection.exec_query( 'SELECT posts.* FROM posts') #=> # result.columns #=> ["id", "title", "created_at", "updated_at"] result.rows #=> [[1, "The Elusive Attribute", "2019-03-15...

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result = AR::Base.connection.exec_query( 'SELECT posts.* FROM posts') #=> # result.columns #=> ["id", "title", "created_at", "updated_at"] result.rows #=> [[1, "The Elusive Attribute", "2019-03-15... Post.column_names #=> []

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Post.select('title as foo')

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post = Post.select('title as foo').first post.foo #=> "The Elusive Attribute"

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2. Matchers & Dispatchers

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Constraints

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Constraints 1. Methods are fast, method missing is slow

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Constraints 1. Methods are fast, method missing is slow 2. schema is static, attributes are dynamic

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Constraints 1. Methods are fast, method missing is slow 2. schema is static, attributes are dynamic “front door” “back door”

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prefix: "" suffix: "_was" Matcher (Attribute Method)

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prefix: "" suffix: "_was" "title" attr_name

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prefix: "" suffix: "_was" "title_was" "title" method_name

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prefix: "" suffix: "_was" "title_was" "attribute_was" "title" target

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prefix: "" suffix: "_was" def title_was(*args) attribute_was("title", *args) end "title" Dispatcher

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“Front Door” attribute_method_matchers.each do |matcher| method_name = matcher.method_name(attr_name) define_proxy_call true, generated_attribute_methods, method_name, matcher.target, attr_name.to_s end

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“Front Door” attribute_method_matchers.each do |matcher| method_name = matcher.method_name(attr_name) define_proxy_call true, generated_attribute_methods, method_name, matcher.target, attr_name.to_s end “title”

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“Front Door” attribute_method_matchers.each do |matcher| method_name = matcher.method_name(attr_name) define_proxy_call true, generated_attribute_methods, method_name, matcher.target, attr_name.to_s end “title_was” “attribute_was” “title”

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prefix: "" suffix: "_was"

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regex: /^(.*)_was$/

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"foo_was" regex: /^(.*)_was$/ method_name

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"foo" regex: /^(.*)_was$/ match("foo_was") attr_name

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"foo", "attribute_was" regex: /^(.*)_was$/ match("foo_was") target

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“Back Door” def attribute_method_matchers_matching(method_name) matchers = attribute_method_matchers. partition(&:plain?).reverse.flatten(1) matchers.map { |matcher| matcher.match(method_name) }.compact end

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“Back Door” def attribute_method_matchers_matching(method_name) matchers = attribute_method_matchers. partition(&:plain?).reverse.flatten(1) matchers.map { |matcher| matcher.match(method_name) }.compact end (See PR #36005)

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“Back Door” def attribute_method_matchers_matching(method_name) matchers = attribute_method_matchers. partition(&:plain?).reverse.flatten(1) matchers.map { |matcher| matcher.match(method_name) }.compact end “foo_was”

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module ActiveModel::Dirty extend ActiveSupport::Concern include ActiveModel::AttributeMethods included do attribute_method_suffix "_changed?", "_change", "_will_change!", "_was" # Dispatch target for *_was attribute methods. def attribute_was(attr) attribute_changed?(attr) ? changed_attributes[attr] : _read_attribute(attr) end

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module ActiveModel::Dirty extend ActiveSupport::Concern include ActiveModel::AttributeMethods included do attribute_method_suffix "_changed?", "_change", "_will_change!", "_was" # Dispatch target for *_was attribute methods. def attribute_was(attr) attribute_changed?(attr) ? changed_attributes[attr] : _read_attribute(attr) end

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module ActiveModel::AttributeMethods included do class_attribute :attribute_method_matchers, instance_writer: false, default: [ ClassMethods::AttributeMethodMatcher.new ] # ... end end

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module ActiveModel::AttributeMethods included do class_attribute :attribute_method_matchers, instance_writer: false, default: [ ClassMethods::AttributeMethodMatcher.new ] # ... end end

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module ActiveModel::AttributeMethods included do class_attribute :attribute_method_matchers, instance_writer: false, default: [ ClassMethods::AttributeMethodMatcher.new ] # ... end end def initialize(options = {}) @prefix = options.fetch(:prefix, "") @suffix = options.fetch(:suffix, "")

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module ActiveRecord module AttributeMethods module Read # ... def _read_attribute(attr_name, &b) @attributes.fetch_value(attr_name.to_s, &b) end alias :attribute :_read_attribute private :attribute end end end

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3. A Better Frame

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post = Post.select('title as foo').first post.foo #=> "The Elusive Attribute"

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post = Post.select('title as foo').first post.foo #=> "The Elusive Attribute" post.foo = "This cannot be saved"

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post = Post.select('title as foo').first post.foo #=> "The Elusive Attribute" post.foo = "This cannot be saved" post.foo_was

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post = Post.select('title as foo').first post.foo #=> "The Elusive Attribute" post.foo = "This cannot be saved" post.foo_was post.foo_change

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post = Post.select('1 as one').first post.one #=> 1 post.one = 2 post.one_was post.one_change

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post = Post.select('1 as one').first post.one #=> 1 post.one = 2 # post.one #=> 2 post.one_was #=> 1 post.one_change #=> [1, 2]

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payments = Payment.select( 'amount, amount*0.08 as tax, amount*0.02 as tax_change')

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payments = Payment.select( 'amount, amount*0.08 as tax, amount*0.02 as tax_change') payment = payments.first payment.amount #=> 100 payment.tax #=> 8 payment.tax_change #=> ?

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payments = Payment.select( 'amount, amount*0.08 as tax, amount*0.02 as tax_change') payment = payments.first payment.amount #=> 100 payment.tax #=> 8 payment.tax_change #=> nil

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"tax", "attribute_change" regex: /^(.*)_change$/ match("tax_change")

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DON’T JUST UNPACK IT,

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DON’T JUST UNPACK IT, FIX IT

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Chris Salzberg @shioyama /dejimata.com