These are codes about instance_eval() in ruby: what is does, and how is it used in Rails ActiveRecord and ActiveSupport.

column_definition.rb

class ColumnDefinition < Struct.new(:base, :name, :type, :limit, :precision, :scale, :default, :null)

instance_eval.rb

# Object#instance_eval()
p Object.instance_methods.grep(/instance_eval/)

# evaluates a block in the context of an someoneect
# (the block is evaluated with the receiver as self, 
# so it can access the receiver's private methods and instance variables)
# it leaves all the other bindings alone

class Person
  def initialize(name)
    @name = name
  end
  
  private
  def credit_card
    "My credit card number is..."
  end
end
   
outter_val = "Tom"

someone = Person.new("Bob")
someone.instance_eval do
  # self became someone(an instance of Person)
  self  # => #<Person:0x007fa4e3843ca0 @name="Bob">
  self == someone # => true
  
  # access to the instance variable
  @name  # => "Bob"
  
  # call the private method
  credit_card # => "My credit card number is..."
  
  # access the outter variable
  @name = outter_val  # => "Tom"
  
end

instance_eval_in_rails_active_record.rb

def change_column(table_name, column_name, type, options = {}) #:nodoc:
  alter_table(table_name) do |definition|
    include_default = options_include_default?(options)
    definition[column_name].instance_eval do
      self.type    = type
      self.limit   = options[:limit] if options.include?(:limit)
      self.default = options[:default] if include_default
      self.null    = options[:null] if options.include?(:null)
    end
  end
end

instance_eval_in_rails_active_support.rb

class Time
  class << self
    alias_method :_load_without_utc_flag, :_load
    def _load(marshaled_time)
      time = _load_without_utc_flag(marshaled_time)
      time.instance_eval do
        if defined?(@marshal_with_utc_coercion)
          val = remove_instance_variable("@marshal_with_utc_coercion")
        end
        val ? utc : self
      end
    end
  end
end