How to write conditional migrations in rails?

Question

I am looking for ways to write migrations in rails that can be executed against the database many times without failing.

For instance let say I have this migration:

class AddUrlToProfile < ActiveRecord::Migration   def self.up     add_column :profile, :url, :string   end    def self.down     remove_column :profile, :url   end end 

If the url column already exists in the Profile table (if the schema.rb has been modified unexpectedly for instance), my migration will fail saying that it's a duplicate!

So how to execute this migration only if it has to?

Thanks

Answer 1

You can do something like this:

class AddUrlToProfile < ActiveRecord::Migration   def self.up     Profile.reset_column_information     add_column(:profile, :url, :string) unless Profile.column_names.include?('url')    end    def self.down     Profile.reset_column_information     remove_column(:profile, :url) if Profile.column_names.include?('url')   end end 

This will reset the column information before it begins - making sure that the Profile model has the up-to-date column information from the actual table. It will then only add the column if it doesn't exist. The same thing happens for the down function, but it only removes the column if it exists.

If you have multiple use cases for this you could factor the code out into a function and re-use that in your migrations.