ar_sample_impl.rb

# To execute:
#    ruby ar_sample_impl.rb

require 'pp'

class ActiveRecord
  class Migration
    # See original implementation of `create_table` in ActiveRecord:
    # https://github.com/rails/rails/blob/main/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb#L296
    def self.create_table(table_name)
      migration = CreateTableDefinition.new(table_name)

      # `yield` will execute the block that is passed to the `create_table` method. The block
      # can take a parameter, in our case it is `|t|`. The parameter for yield, here `migration`, will
      # take the place of `t`.
      yield(migration)

      migration.execute
    end
  end
end

class ActiveRecord
  class CreateTableDefinition
    attr_reader :table_name, :entries

    def initialize(table_name)
      @table_name = table_name
      @entries = []
    end

    # See original implementation of the `column` method in ActiveRecord:
    # https://github.com/rails/rails/blob/main/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb#L398
    def column(column_name, column_type)
      @entries.push({ type: :column, column_name: column_name, column_type: column_type })
    end

    def execute
      # Here is where we actually execute the migration. ActiveRecord will create the appropriate SQL instructions
      # and run them on the server, but we'll only print out the collected migration information.
      puts "create_table migration: #{table_name}"
      entries.each do |entry|
        pp entry
      end
    end
  end
end

ActiveRecord::Migration.create_table(:todos) do |t|
  t.column :todo_text, :text
  t.column :due_date, :date
  t.column :completed, :bool
end

# Running this will print the following to console:
#
# create_table migration: todos
# {:type=>:column, :column_name=>:todo_text, :column_type=>:text}
# {:type=>:column, :column_name=>:due_date, :column_type=>:date}
# {:type=>:column, :column_name=>:completed, :column_type=>:bool}