class ActiveRecord::Migration::CommandRecorder

Parent:: Object

ActiveRecord::Migration::CommandRecorder records commands done during a migration and knows how to reverse those commands. The CommandRecorder knows how to invert the following commands:

add_column
add_foreign_key
add_index
add_reference
add_timestamps
change_column
change_column_default (must supply a :from and :to option)
change_column_null
change_column_comment (must supply a :from and :to option)
change_table_comment (must supply a :from and :to option)
create_join_table
create_table
disable_extension
drop_join_table
drop_table (must supply a block)
enable_extension
remove_column (must supply a type)
remove_columns (must specify at least one column name or more)
remove_foreign_key (must supply a second table)
remove_index
remove_reference
remove_timestamps
rename_column
rename_index
rename_table

Constants

ReversibleAndIrreversibleMethods

Attributes

commands[RW]

delegate[RW]

reverting[RW]

Public Class Methods

new(delegate = nil) Show source

# File activerecord/lib/active_record/migration/command_recorder.rb, line 47
def initialize(delegate = nil)
  @commands = []
  @delegate = delegate
  @reverting = false
end

Public Instance Methods

inverse_of(command, args, &block) Show source

# File activerecord/lib/active_record/migration/command_recorder.rb, line 89
      def inverse_of(command, args, &block)
        method = :"invert_#{command}"
        raise IrreversibleMigration, <<~MSG unless respond_to?(method, true)
          This migration uses #{command}, which is not automatically reversible.
          To make the migration reversible you can either:
          1. Define #up and #down methods in place of the #change method.
          2. Use the #reversible method to define reversible behavior.
        MSG
        send(method, args, &block)
      end

Returns the inverse of the given command. For example:

recorder.inverse_of(:rename_table, [:old, :new])
# => [:rename_table, [:new, :old]]

This method will raise an IrreversibleMigration exception if it cannot invert the command.

record(*command, &block) Show source

# File activerecord/lib/active_record/migration/command_recorder.rb, line 74
def record(*command, &block)
  if @reverting
    @commands << inverse_of(*command, &block)
  else
    @commands << (command << block)
  end
end

Record command. command should be a method name and arguments. For example:

recorder.record(:method_name, [:arg1, :arg2])

replay(migration) Show source

# File activerecord/lib/active_record/migration/command_recorder.rb, line 114
def replay(migration)
  commands.each do |cmd, args, block|
    migration.send(cmd, *args, &block)
  end
end

revert() { || ... } Show source

# File activerecord/lib/active_record/migration/command_recorder.rb, line 60
def revert
  @reverting = !@reverting
  previous = @commands
  @commands = []
  yield
ensure
  @commands = previous.concat(@commands.reverse)
  @reverting = !@reverting
end

While executing the given block, the recorded will be in reverting mode. All commands recorded will end up being recorded reverted and in reverse order. For example:

recorder.revert{ recorder.record(:rename_table, [:old, :new]) }
# same effect as recorder.record(:rename_table, [:new, :old])