Load Rake, railties tasks and invoke the registered hooks. Check Rails::Railtie.rake_tasks for more info.Source: hide | on GitHub# File railties/lib/rails/engine.rb, line 451
def load_tasks(app=self)require "rake"run_tasks_blocks(app)self
end

rake_tasks(&block)
If you try to define a set of rake tasks on the instance, these will get passed up to the rake tasks defined on the application's class.

Source: hide | on GitHub# File railties/lib/rails/application.rb, line 270
def rake_tasks(&block)self.class.rake_tasks(&block)
end

# 根据下面的博客做了一点点修改
# http://eewang.github.io/blog/2013/07/29/how-to-use-rake-tasks-to-generate-migration-sql/
namespace :db do               [ :migrate, :rollback ].each do |n|namespace n do |migration_task| # original_task = migration_task.instance_variable_get("@scope").join ":"
[:with_sql, :to_sql ].each do |t|desc "Run migration, and generated SQL" if t == :with_sqldesc "Generate migration SQL" if t == :to_sqltask t => :environment do |sql_task| original_task = sql_task.name.sub(":#{t}", '')case original_task   when 'db:migrate'    filename = 'upgrade.sql'        when 'db:rollback'   filename = 'rollback.sql'       else                 raise "unkown migration type #{original_task}"end                  ActiveRecord::Base.connection.class.class_eval do# alias the adapter's execute for later usealias :old_execute :execute

  SQL_FILENAME = filename         RUN_SQL = sql_task.name.ends_with?("with_sql")# define our own execute        def execute(sql, name = nil)    # check for some DDL and DML statementsif /^(create|alter|drop|insert|delete|update)/i.match sqlFile.open(SQL_FILENAME, 'a') { |f| f.puts "#{sql};\n" } old_execute sql, name if RUN_SQLelse# pass everything else to the aliased execute
                old_execute sql, nameendendend# create or delete content of migration.sqlFile.open(SQL_FILENAME, 'w') { |f| f.puts "-- Script created @ #{Time.now}" }# invoke the normal migration procedure now
          Rake::Task[original_task].invokeputs "Ran #{original_task} and wrote sql to #{filename}"endendendend
end

rake db:create              # Creates the database from DATABASE_URL or config/database.yml for the current RAILS_ENV (use db:create:all t...
rake db:drop                # Drops the database from DATABASE_URL or config/database.yml for the current RAILS_ENV (use db:drop:all to dr...
rake db:fixtures:load       # Load fixtures into the current environment's database
rake db:migrate             # Migrate the database (options: VERSION=x, VERBOSE=false, SCOPE=blog)
rake db:migrate:status      # Display status of migrations
rake db:migrate:to_sql      # Generate migration SQL
rake db:migrate:with_sql    # Run migration, and generated SQL
rake db:rollback            # Rolls the schema back to the previous version (specify steps w/ STEP=n)
rake db:rollback:to_sql     # Generate migration SQL
rake db:rollback:with_sql   # Run migration, and generated SQL