Ensure Migrations Use The Latest Schema

Real-world migrations in Rails apps can sometimes involve both schema changes and data changes. For instance, if you are moving a column from one table to another, you'll need to add a new column, move some data, and then delete the old column.

# Assume the following are defined:
# GenericAuthor for table 'authors'
# GenericBook for table 'books'

def up
  add_column :books, :genre, :string

  GenericAuthor.find_each do |author|
    book = GenericBook.find_by(author_id: author.id)
    book.update!(genre: author.genre)
  end

  remove_column :authors, :genre
end

This migration looks straightforward, but you may find that no data actually gets transferred to the genre column on books. This is because as a performance optimization, Rails has cached the scema. Thus an ActiveRecord modification like book.update! will be working off a version of the schema that doesn't include genre as a column.

We can ensure ActiveRecord is using the latest column informtion for the books table by calling reset_column_information.

def up
  add_column :books, :genre, :string

  GenericBook.reset_column_information

  GenericAuthor.find_each do |author|
    book = GenericBook.find_by(author_id: author.id)
    book.update!(genre: author.genre)
  end

  remove_column :authors, :genre
end

Now the update will work and genre will be set on books.