active_record_mini.rb

require "sqlite3"

module ActiveRecord
  class Mini
    attr_reader :attributes

    def initialize(attributes = {})
      @attributes = attributes
    end

    def save
      if id.nil?
        sql = <<~SQL
          insert into
            #{self.class.table_name}
            (#{@attributes.keys.join(", ")})
          values
            (#{@attributes.size.times.map { |_| "?" }.join(", ")})
        SQL

        self.class.execute(sql, @attributes.values)
        @attributes["id"] = self.class.connection.last_insert_row_id

      else
        sql = <<~SQL
          update
            #{self.class.table_name}
          set #{@attributes.keys.map { |key| "#{key} = ?" }.join(", ")}
          where id = ?
        SQL

        self.class.execute(
          sql,
          @attributes.values,
          id
        )
      end
    end

    def reload
      @attributes = self.class.find(id).attributes
    end

    def delete
      self.class.execute("delete from #{self.class.table_name} where id = ?", id)
    end

    def self.table_name
      "#{name.downcase}s"
    end

    def self.column_names
      columns.map { |column| column["name"] }
    end

    def self.columns
      @columns ||= self.execute("pragma table_info(#{table_name});")
    end

    def self.connection
      @connection ||=
        begin
          connection = SQLite3::Database.new "#{__dir__}/data.db"
          connection.results_as_hash = true
          connection
        end
    end

    def self.find(id)
      data = self.execute("select * from #{table_name} where id = ?", id).first
      return nil if data.nil?
      self.new(data)
    end

    def self.execute(*args)
      puts "SQL: #{args}"
      if args.size > 1
        connection.prepare(args[0]).execute(args[1..])
      else
        connection.execute(args[0])
      end
    end

    def self.inherited(subclass)
      subclass.class_eval do
        column_names.each do |column_name|
          define_method(column_name) do
            @attributes[column_name]
          end

          define_method("#{column_name}=") do |value|
            @attributes[column_name] = value
          end
        end
      end
    end
  end
end

class User < ActiveRecord::Mini
end

p User.table_name # => "users"
p User.column_names # => ["id", "name"]

user = User.new(name: "John")
user.save
p user.id # => 1
user.reload
p user.name # => "John"
user.name = "Jane"
user.save
user.reload
p user.name # => "Jane"

user = User.find(1)
p user.name # => "Jane"
user.delete

p User.find(1) # => nil