Rails Migrations: Check Existence and Keep Going?

Question

I was doing this kind of thing in my migrations:

add_column :statuses, :hold_reason, :string rescue puts "column already added"

but it turns out that, while this works for SQLite, it does not work for PostgreSQL. It seems like if the add_column blows up, even if the Exception is caught, the transaction is dead and so the Migration can't do any additional work.

Is there any non-DB sepecific ways to check if a column or table already exist? Failing that, is there any way to get my rescue block to really work?

Answer 1

You could check the column ActiveRecord style, assuming you've created a model for the table at this point:

Status.column_names.include?("hold_reason")

If you'll be doing it a lot you could make it a method:

def safe_add_column table_name, column_name, type, options
  unless table_name.to_s.singularize.camelcase.constantize.column_names.include?(column_name.to_s)
    add_column table_name, column_name, type, options
  else
    announce "Column #{column_name} already exists in table #{table_name}"
  end
end

However I'm not quite sure if this holds up if multiple migrations are run at once. I guess you could trigger a reload if necessary to update the Model metadata.

Also I haven't run this code for bugs, but the concept is sound.