The Art Of Self-Defense - How To Protect Yourself From Nils

Presentation given on Łódź Ruby User Group.

Transcript

1. The art of self-defense - how
   to protect yourself from nils
   Karol Galanciak - full-stack developer at
   Twitter: @Azdaroth
   

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2. NoMethodError: undefined
   method 'name' for nil:NilClass
   

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3. 1. Use right set of methods
   

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4. 1. Use right set of methods
   1. Prefer Hash#fetch over Hash#[]
   name = params[:name]
   name = params.fetch(:name)
   

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5. 1. Use right set of methods
   2. Use bang finder methods for required objects
   user = User.find_by(name: name)
   user = User.find_by!(name: name)
   

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6. 1. Use right set of methods
   3. Use Hash#dig for hashes from input you don’t own
   payload = {
   data: {
   type: "users",
   attributes: {
   email: "email@example.com"
   },
   relationships: {
   source: {
   data: {
   type: "sources",
   id: 20
   }
   }
   }
   }
   }
   source_id = payload[:data][:relationships][:source][:data][:id]
   source_id = payload.dig(:data, :relationships, :source, :data, :id)
   

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7. 2. Null Object Pattern
   

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8. 2. Null Object Pattern
   # controller
   def current_user
   @current_user ||= User.find_by(id: user_id) || GuestUser.new
   end
   # model
   class GuestUser
   def name
   "Anonymous user"
   end
   end
   # view
   <%= current_user.name %>
   

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9. 2. Null Object Pattern
   class GuestUser
   def name
   "Anonymous user"
   end
   def comments
   Comment.none
   end
   end
   

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10. 2. Null Object Pattern
    class Comment::Persistence
    attr_reader :logger
    private :logger
    def initialize(logger: nil)
    @logger = logger
    end
    def save!(comment)
    comment.save!
    logger.log("comment: #{comment.attributes} created!") if logger
    end
    end
    

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11. 2. Null Object Pattern
    class Comment::Persistence
    attr_reader :logger
    private :logger
    def initialize(logger: NullLoger)
    @logger = logger
    end
    def save!(comment)
    comment.save!
    logger.log("comment: #{comment.attributes} created!")
    end
    module NullLogger
    def self.log(*)
    end
    end
    end
    

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12. 3. Do Not Make Demeter Sad
    

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13. 3. Do Not Make Demeter Sad
    comment.author.name
    

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14. 3. Do Not Make Demeter Sad
    class Comment < ActiveRecord::Base
    belongs_to :author
    delegate :name, to: :author, prefix: true, allow_nil: true
    end
    class Comment < ActiveRecord::Base
    belongs_to :author
    def author_name
    author && author.name || "Anonymous"
    end
    end
    comment.author_name
    

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15. 4. Apply Explicit Conversions
    class Tax < ActiveRecord::Base
    # percentage
    def calculate(value)
    value * percentage
    end
    end
    

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16. 4. Apply Explicit Conversions
    class Tax < ActiveRecord::Base
    # percentage
    def calculate(value)
    value.to_d * percentage
    end
    end
    

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17. 4. Apply Explicit Conversions
    class Tax < ActiveRecord::Base
    # percentage
    def calculate(value)
    value.to_s.to_d * percentage
    end
    end
    

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18. 5. Use proper database constraints
    class CreateComments < ActiveRecord::Migration
    def change
    create_table :comments do |t|
    t.integer :author_id, null: false
    t.text :content, null: false
    t.timestamps
    end
    add_foreign_key :comments, :authors
    end
    end
    

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19. Bonus: Never, ever, use Object#try
    and safe navigation operator &.
    

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20. Bonus: Never, ever, use Object#try
    and safe navigation operator &.
    comment.try(:author).try(:address).try(:primary)
    .try(:city).try(:region).try(:country)
    comment&.author&.address&.primary&.city&.region
    &.country
    http://www.reactionface.info/face/troll-face
    

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21. http://www.jasondemakis.com/wp-content/uploads/2015/09/yoda-there-is-no-try1.png
    

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