difference between rake db:autoupgrade and rake db:automigrate

Question

I am new to Ruby on Rails and Datamapper. Could anyone please tell me what is the difference between rake db:autoupgrade and rake db:automigrate in datamapper. Because in my application I have used model A and model B as follows:

 class A
    includes DataMapper::resource
    property :id, Serial
    belongs_to :B, :required=>true
  end

  class B
    includes DataMapper::resource
    property :id, Serial
    has n, :As
  end

Then after changing :required => false in the model A while doing rake db:autoupgrade no change happens to the A table in database but while doing rake db:automigrate the following error is coming:

 cannot delete or update a parent row: a foreign key constraint fails

Answer 1

if there a foreign key constant you can not delete the associated table. what else you can do is rake db:drop and create the table again.