gistfile1.rbx

# NOTE, all names, syntax, etc... are just off the top of my head to illustrate
# the general idea. Don't take anything too literally.

# I just started a blank project with DataMapper. DM Migrations knows
# that the current production database is empty because no schema has been "committed".
#
# I then created a new model:

class User
  include DataMapper::Resource
  
  property :id, Serial
end

# DataMapper.auto_migrate! can run and creates a new table users with the ID property.
# A new migration file is also created named work_in_progress.rb with the following
# content:

migrate(
    :from => table(:users).missing,
    :to   => table(:users).with(:id => Serial)
  ) do
  create_table(:users) do |u|
    u.column :id, Serial
  end
end

# I now decided that I want a username property and change the file:

class User
  include DataMapper::Resource
  
  property :id,       Serial
  property :username, String
end

# I run DataMapper.auto_migrate and the new column is created in development mode.
# DM still knows that I haven't committed so it goes ahead and modifies the current
# workign migration file to:

migrate(
    :from => table(:users).missing,
    :to   => table(:users).with(:id => Serial, :username => String)
  ) do
  create_table(:users) do |u|
    u.column :id, Serial
    u.column :username, String
  end
end

# So, I decide I want to deploy my awesome application and type a command rake dm:commit
# and datamapper "freezes" the current work_in_progress.rb migration file for deployment.
# So, I deploy my app and decide that it sucks and I need to improve it. I also get John
# to help me. So, now there are two developers working on this awesome app.

# Ok, I decided that I needed to add a password field:

class User
  include DataMapper::Resource
  
  property :id,       Serial
  property :username, String
  property :password, String
end

# I run auto_migrate. The old committed migration file is stashed somewhere and a new
# work_in_progress.rb migration is created with the following:

migrate(
  # As you can see, this migration only requires a users table (with anything in it)
  # as long as it can successfully add the password column
  :from => table(:users),
  :to => table(:users).with(:password => String)
) do
  add_column :users, :password, String
end

# Ok, now John decides that he wants to remove :username (wtf??):

class User
  include DataMapper::Resource
  
  property :id,       Serial
  property :password, String
end

# And he commits and gets the following migrate file:

migrate(
  # As you can see, this migration only requires a users table (with anything in it)
  # as long as it can successfully add the password column
  :from => table(:users).with(:username => String),
  :to => table(:users)
) do
  remove_column :users, :username
end  

# Now here's a cool bit. What if I decide to change :password to an Integer and John
# decides to change :password to a Boolean? My migrate file would look like:

migrate(
  :from => table(:users).with(:password => String),
  :to => table(:users).with(:password => Integer)
) do
  create_column :users, :new_password, Integer
  execute "update users set new_password = integer(password);"
  remove_column :users, :password
  rename_column :users, :new_password, :password
end

# And John's migration file would look like:

migrate(
  :from => table(:users).with(:password => String),
  :to => table(:users).with(:password => Boolean)
) do
  remove_column :users, :password
  create_column :users, :password, Boolean
end

# DM Migrator would know that there is a conflict here and something would happen.

# CONCLUSION:

# The whole point is to make development easy with auto_migrate and then being able
# to safely get changes to production with some kind of smart migration files. This
# is still a really rough and crazy idea.