what does it do by "bundle exec rake db:migrate"?

from my research, bundle exec is responsible for executing a command in the context of a bundle.

anyway, I don't quite understand yet for what it does differently for just the command rake db:migrate comparing with bundle exec rake db:migrate.

for example in my case, I executed the first command, and what I got was the following errors:

$ rake db:migrate
(in c:/rails/rails_projects/soccerweb)
rake aborted!
uninitialized constant Rake::DSL
c:/Ruby192/lib/ruby/1.9.1/rake.rb:2482:in `const_missing'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/tasklib.rb:8:in `<clas
s:TaskLib>'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/tasklib.rb:6:in `<modu
le:Rake>'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/tasklib.rb:3:in `<top
(required)>'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/testtask.rb:4:in `requ
ire'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/testtask.rb:4:in `<top
 (required)>'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/test_unit/testing.r
ake:2:in `require'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/test_unit/testing.r
ake:2:in `<top (required)>'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/test_unit/railtie.r
b:12:in `load'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/test_unit/railtie.r
b:12:in `block in <class:TestUnitRailtie>'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/railtie.rb:183:in `
call'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/railtie.rb:183:in `
block in load_tasks'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/railtie.rb:183:in `
each'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/railtie.rb:183:in `
load_tasks'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/engine.rb:396:in `b
lock in load_tasks'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/application/railtie
s.rb:8:in `each'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/application/railtie
s.rb:8:in `all'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/engine.rb:396:in `l
oad_tasks'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/application.rb:103:
in `load_tasks'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/railties-3.1.1/lib/rails/railtie/configurabl
e.rb:30:in `method_missing'
c:/rails/rails_projects/soccerweb/Rakefile:7:in `<top (required)>'
c:/Ruby192/lib/ruby/1.9.1/rake.rb:2373:in `load'
c:/Ruby192/lib/ruby/1.9.1/rake.rb:2373:in `raw_load_rakefile'
c:/Ruby192/lib/ruby/1.9.1/rake.rb:2007:in `block in load_rakefile'
c:/Ruby192/lib/ruby/1.9.1/rake.rb:2058:in `standard_exception_handling'
c:/Ruby192/lib/ruby/1.9.1/rake.rb:2006:in `load_rakefile'
c:/Ruby192/lib/ruby/1.9.1/rake.rb:1991:in `run'
c:/Ruby192/bin/rake:31:in `<main>'

but when I used "bundle exec" preceding that "rake db:migrate" command, everything just worked for me gracefully!

anyone can shed some light on me?

Solution

bundle exec rake runs the version of rake which you've specified in your Gemfile. The default version of rake, the one which can be found in your $PATH, might be different than one executed by bundle exec rake.

The problem you've experienced has been covered in a separate question. Your bundle exec rake most probably runs rake 0.8.7 and rake from your $PATH is a newer version which is affected by this problem.

Quoting from man bundle exec:

bundle exec makes a number of changes to the shell environment, then executes the command you specify in full.

(...)

put the directory containing executables (like rails, rspec, rackup) for your bundle on $PATH