title: How do I keep all gems in Gemfile compatible after an update
body: <p>My question has already been asked <a href="https://stackoverflow.com/questions/7860778/ruby-on-rails-rspec-error">here</a>, but I am trying to understand the reasons behind it as opposed to how to work around it.</p>
  
  <p>The error I got was;</p>
  
  <pre><code>You have already activated rspec-core 2.7.1, but your Gemfile requires rspec-core 2.6.4. Using bundle exec may solve this. (Gem::LoadError)
  </code></pre>
  
  <p>Now I have been given various solutions like using "mpapis-bundler", or to create a shorthand for "bundle exec", but I was under the impression that that was what</p>
  
  <pre><code>$bundle install --binstubs
  </code></pre>
  
  <p>was for.</p>
  
  <p>More specifically, since I have no version numbers stated in my gemfile for rspec-rails, why do I have this incompatibility? My error also occurred when I tried</p>
  
  <pre><code>$rake db:migrate
  </code></pre>
  
  <p>telling me that</p>
  
  <pre><code>You have already activated rake 0.9.2.2, but your Gemfile requires rake 0.9.2. Consider using bundle exec.
  </code></pre>
  
  <p>Any explanations would be appreciated.</p>
  
  <p><strong>EDIT:</strong><br/>
  All my gems for my app are in a gemset, and I have updated my gems again. Should an update not make sure that related gems are compatible?</p>