How do I find the source file for a rake task?

Despite what others have said, you can programmatically get the source location of rake tasks in a rails application. To do this, just run something like the following in your code or from a console:

# load all the tasks associated with the rails app
Rails.application.load_tasks

# get the source locations of actions called by a task
task_name = 'db:schema:load' # fully scoped task name
Rake.application[task_name].actions.map(&:source_location)

This will return the source locations of any code that gets executed for this task. You can also use #prerequisites instead of #source_location to get a list of prerequisite task names (e.g. 'environment', etc).

You can also list all tasks loaded using:

Rake.application.tasks

UPDATE: See Magne's good answer below. For versions of rake >= 0.9.0 you can use rake -W to show the source location of your rake tasks.

There is no programmatic way to do this unfortunately. Rake tasks can be loaded either from rails itself, lib/tasks, or from any plugin with a tasks directory.

This should nab most everything not within Rails itself:

find . -name "*.rake" | xargs grep "whatever"

As for db:schema:dump, here's the source:

desc "Create a db/schema.rb file that can be portably used against any DB supported by AR"
task :dump => :environment do
  require 'active_record/schema_dumper'
  File.open(ENV['SCHEMA'] || "#{RAILS_ROOT}/db/schema.rb", "w") do |file|
    ActiveRecord::SchemaDumper.dump(ActiveRecord::Base.connection, file)
  end
end

It can be found on line 242 of lib/tasks/database.rake in the rails 2.2.2 gem. If you've got a different version of Rails, just search for "namespace :schema".

You probably actually want the source of the ActiveRecord::SchemaDumper, but I think you should have no trouble figuring out where that is. :-)

For most rake tasks in Rails, look in the Rails gem directory, in lib/tasks.

If you've vendored Rails into your app directory structure then look in vendor/rails/railties/lib/tasks instead

Either way, db:schema:dump is in databases.rake.