Merge pull request #21412 from yui-knk/feature/irreversible_migration_error_msg

Add detailed error message to `IrreversibleMigration`

2 files changed, 77 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/migration.rb b/activerecord/lib/active_record/migration.rb
index b4dd8eff5a..112d52024f 100644
--- a/activerecord/lib/active_record/migration.rb
+++ b/activerecord/lib/active_record/migration.rb
@@ -9,7 +9,77 @@ module ActiveRecord
     end
   end
 
-  # Exception that can be raised to stop migrations from going backwards.
+  # Exception that can be raised to stop migrations from being rolled back.
+  # For example the following migration is not reversible.
+  # Rolling back this migration will raise an ActiveRecord::IrreversibleMigration error.
+  #
+  #   class IrreversibleMigrationExample < ActiveRecord::Migration
+  #     def change
+  #       create_table :distributors do |t|
+  #         t.string :zipcode
+  #       end
+  #
+  #       execute <<-SQL
+  #         ALTER TABLE distributors
+  #           ADD CONSTRAINT zipchk
+  #             CHECK (char_length(zipcode) = 5) NO INHERIT;
+  #       SQL
+  #     end
+  #   end
+  #
+  # There are two ways to mitigate this problem.
+  #
+  # 1. Define <tt>#up</tt> and <tt>#down</tt> methods instead of <tt>#change</tt>:
+  #
+  #  class ReversibleMigrationExample < ActiveRecord::Migration
+  #    def up
+  #      create_table :distributors do |t|
+  #        t.string :zipcode
+  #      end
+  #
+  #      execute <<-SQL
+  #        ALTER TABLE distributors
+  #          ADD CONSTRAINT zipchk
+  #            CHECK (char_length(zipcode) = 5) NO INHERIT;
+  #      SQL
+  #    end
+  #
+  #    def down
+  #      execute <<-SQL
+  #        ALTER TABLE distributors
+  #          DROP CONSTRAINT zipchk
+  #      SQL
+  #
+  #      drop_table :distributors
+  #    end
+  #  end
+  #
+  # 2. Use the #reversible method in <tt>#change</tt> method:
+  #
+  #   class ReversibleMigrationExample < ActiveRecord::Migration
+  #     def change
+  #       create_table :distributors do |t|
+  #         t.string :zipcode
+  #       end
+  #
+  #       reversible do |dir|
+  #         dir.up do
+  #           execute <<-SQL
+  #             ALTER TABLE distributors
+  #               ADD CONSTRAINT zipchk
+  #                 CHECK (char_length(zipcode) = 5) NO INHERIT;
+  #           SQL
+  #         end
+  #
+  #         dir.down do
+  #           execute <<-SQL
+  #             ALTER TABLE distributors
+  #               DROP CONSTRAINT zipchk
+  #           SQL
+  #         end
+  #       end
+  #     end
+  #   end
   class IrreversibleMigration < MigrationError
   end
 
diff --git a/activerecord/lib/active_record/migration/command_recorder.rb b/activerecord/lib/active_record/migration/command_recorder.rb
index 3ab0f28c9b..4c4afb4dbd 100644
--- a/activerecord/lib/active_record/migration/command_recorder.rb
+++ b/activerecord/lib/active_record/migration/command_recorder.rb
@@ -81,7 +81,12 @@ module ActiveRecord
       # invert the +command+.
       def inverse_of(command, args, &block)
         method = :"invert_#{command}"
-        raise IrreversibleMigration unless respond_to?(method, true)
+        raise IrreversibleMigration, <<-MSG.strip_heredoc 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