Ruby Tutorials

Usually I use migrations to just add and remove columns. But today I needed to modify the database schema of a live app and should thus preserve the data already present. Although not hard implementing it revealed some pitfalls.

The task at hand was to augment the users in the system with roles. We had a column in the Members table which made a member a manager or not(boolean). This information is now extracted to a roles table (we are using RoleRequirement). It is now possible to assign many different roles to each member.

The migration I wrote at first looked something like this:

def self.up
r = Role.new(:name => "manager"
r.save
remove_column :members, :manager
managers = Member.find_all_by_manager(true)
managers.each do |manager|
manager.roles << r
manager.save
end
end

However this migration failed on the manager.save, complaining that the column manager on the table members no longer existed. And it didn’t of course. The column had just been removed! I figured that the attributes in the manager objects still contained the :manager => true key-value pair. So I deleted that but the error persisted. Which makes sense when I thought about it. The migration loaded the Rails framework with all inferred classes. One of these classes was the Member class which contained at the time of loading still a manager field. Trying to save the member without one of the attributes causes an error. Reloading framework seemed a bit overkill as well.

In the end the solution was, as most of the time, deceptively simple. Remove the column at the end of the migration!

def self.up
r = Role.new(:name => "manager")
r.save
managers = Member.find_all_by_manager(true)
managers.each do |manager|
manager.roles << r
manager.save
end
remove_column :members, :manager
end

But how do you test your migration? I took a stab at this in the form of an RSpec test:

describe Member, "should migrate properly" do
it "should migrate correctly from a column manager to a role manager" do

`rake db:drop`
`rake db:create`
`rake db:migrate VERSION=59`
m1 = Member.new(:manager => true)
m1.save_with_validation(false)
m2 = Member.new(:manager => true)
m2.save_with_validation(false)
m3 = Member.new(:manager => true)
m3.save_with_validation(false)
`rake db:migrate VERSION=60`
Member.find(m1.id).has_role ?("manager").should be_true
Member.find(m2.id).has_role?("manager").should be_true
Member.find(m3.id).has_role?("manager").should be_true
end
end

But this does not work yielding a MySQL timeout. This has to do with the fact that all tests are run in transaction mode preventing other database changes during the tests. This particular test waits for the lock to be released which never happens and thus times out.

I have yet to figure out how to do this, but I think this is really important! I would love to be able to test my migrations as these migrations manipulate live data which can not be lost. Any suggestions are welcome.