`db:migrate:data` task

migrate.rake

# Migrate data from two ActiveRecord databases.
# Works with ActiveRecord 3 and above.
#
# Written by Rémi Prévost
# Based on the work of Rama McIntosh & Mike Castleman
#
# This task is released under this BSD license:
#
# Copyright (c) 2008, Matson Systems, Inc.
# Copyright (c) 2010, Roasted Vermicelli LLC.
# Copyright (c) 2013, Mirego, Inc.
#
# All rights reserved.
#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions
# are met:
#
# * Redistributions of source code must retain the above copyright
#   notice, this list of conditions and the following disclaimer.
# * Redistributions in binary form must reproduce the above copyright
#   notice, this list of conditions and the following disclaimer in the
#   documentation and/or other materials provided with the distribution.
# * Neither the name of Matson Systems, Inc. nor the names of its
#   contributors may be used to endorse or promote products derived
#   from this software without specific prior written permission.
#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
# FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
# COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
# INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
# BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
# LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
# CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
# LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
# ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
# POSSIBILITY OF SUCH DAMAGE.namespace :db do

namespace :db do
  namespace :migrate do
    task data: :environment do
      puts '-------'
      puts 'Migrating all data from an old database to a new one. All data in the new one will be deleted.'
      puts "Old: #{ENV['OLD_DATABASE_URL']}"
      puts "New: #{ENV['NEW_DATABASE_URL']}"
      puts '-------'

      # Use separate models to avoid using application-specific code
      class OldModelClass < ActiveRecord::Base; end
      class NewModelClass < ActiveRecord::Base; end
      ActiveRecord::Base.establish_connection ENV['OLD_DATABASE_URL']

      tables = ActiveRecord::Base.connection.tables - %w(schema_info schema_migrations)
      tables.each do |table_name|
        # Configure the old table
        OldModelClass.table_name = table_name
        OldModelClass.reset_column_information

        # Configure the new table
        NewModelClass.table_name = table_name
        NewModelClass.establish_connection ENV['NEW_DATABASE_URL']
        NewModelClass.reset_column_information
        NewModelClass.record_timestamps = false

        # Remove all data in the new table
        NewModelClass.delete_all

        print "Migrating #{table_name}..."
        columns = nil

        # Loop through all records (in batches of 1000) and insert them into the new table
        OldModelClass.find_in_batches do |models|
          values = models.inject([]) do |memo, model|
            attrs = model.attributes
            columns ||= attrs.keys.map { |value| NewModelClass.connection.quote_column_name(value) }.join(',')
            memo << '(' + attrs.values.map { |value| NewModelClass.connection.quote(value) }.join(',') + ')'
          end

          # Insert those 1000 records
          NewModelClass.transaction do
            NewModelClass.connection.execute("INSERT INTO #{NewModelClass.connection.quote_table_name(table_name)} (#{columns}) VALUES #{values.join(',')}")
          end
        end
        # Reset the sequence of a table's primary key to the maximum value
        NewModelClass.connection.reset_pk_sequence!(table_name) if NewModelClass.connection.respond_to?(:reset_pk_sequence!)

        puts " #{OldModelClass.count} records migrated."
      end

      puts 'Done!'
    end
  end
end