When I do rake db:migrate, I get the following error:
rake aborted!
Gem::LoadError: You have already activated rake 10.2.2, but your Gemfile requires rake 10.1.0. Using bundle exec may solve this.
How can solve this?
There might be other gems in the Gemfile which are dependent on rake 10.2.2, while you are trying to install rake 10.1.0 via your gemfile or explicitly mentioned it. A look into your Gemfile will help.
In case you have specific environment, you may want to run
bundle exec rake db:migrate
to make sure you are running it appropriately.
As per another answer given on this topic, you can also try deleting the Gemfile.lock file and re-running bundle install to regenerate the Gem dependencies.