Deploy this initializer before migrations involving dropping/changing names of columns

remove_columns.rb

model = MyModel
column_names = %w{ first_column second_column }

columns = model.columns.select { |c| column_names.include?(c.name) }
columns.each do |c|
  model.columns.delete(c)
end

If you just run migration with drop column statement ActiveRecord in your app instance will continue to issue queries with non-existing items in column list causing exceptions.

To avoid downtime deploy this initialzier before dropping columns.