title: Rake db:migrate error "don't know how to build task"
body: <p>I've got a table where I used integer on a field which needs decimal places, so I'm trying to create a migration which changes the field type from integer to float/real. My database is sqllite3 and I'm using rails3.</p>
  
  <p>I ran </p>
  
  <pre>
  rails generate migration ChangeMeasureColumnOnIngredients</pre>
  
  <p>to create the initial migration files, then updated the  class to </p>
  
  <pre>
  class ChangeMeasureColumnOnIngredients &lt; ActiveRecord::Migration
    def self.up
      change_column :ingredients, :measure, :real
    end
  </pre>
  
  <p>I ran rake db:migrate and it returned fine. </p>
  
  <p>When I inserted a value via my rails app, it didn't return the decimal place. I started to think that many rails doesn't know what 'real' is as a datatype, so I changed the migration to <pre>change_column :ingredients, :measure, :float</pre></p>
  
  <p>I then ran <pre>rake db:migrate change_measure_column_on_ingredients</pre>
  and now I get the following error<pre>c:\Ruby192\rails3rc>rake db:migrate change_measure_column_on_ingredients
  (in c:/Ruby192/rails3rc)
  rake aborted!
  Don't know how to build task 'change_measure_column_on_ingredients'
  C:/Ruby192/lib/ruby/1.9.1/rake.rb:1720:in <code>[]'
  C:/Ruby192/lib/ruby/1.9.1/rake.rb:2040:in</code>invoke_task'
  C:/Ruby192/lib/ruby/1.9.1/rake.rb:2019:in <code>block (2 levels) in top_level'
  C:/Ruby192/lib/ruby/1.9.1/rake.rb:2019:in</code>each'
  C:/Ruby192/lib/ruby/1.9.1/rake.rb:2019:in <code>block in top_level'
  C:/Ruby192/lib/ruby/1.9.1/rake.rb:2058:in</code>standard_exception_handling'
  C:/Ruby192/lib/ruby/1.9.1/rake.rb:2013:in <code>top_level'
  C:/Ruby192/lib/ruby/1.9.1/rake.rb:1992:in</code>run'
  C:/Ruby192/bin/rake:31:in `'</pre></p>
  
  <p>I tried changing the :float back to :real, but I still get that error. </p>
  
  <p>can somebody tell me what I'm doing wrong? 
  I'm new to rails and still learning. </p>