Changing the type of a column in mysql

I need to change the type of a column (wasserstand) from string to
integer, which already contains text.

My idea is to copy the string into a new scratch column, modify the
column type to integer and then manually set the appropriate ints.

After this, I want to remove the scratch string column in the next
migration.

The first migration should be done like this:

  def self.up
    tz = Teilzaehlung.all
    ws = []
    tz.each do |b|
      ws << [b.id, b.wasserstand]
    end

    change_column(:teilzaehlungs, :wasserstand, :integer)
    add_column(:teilzaehlungs, :wst, :string)

    ws.each do |b|
      beob = Teilzaehlung.find(b[0])
      beob.update_attribute(:wasserstand, 0)
      beob.update_attribute(:wst, b[1])
    end
  end

Interestingly, after running the migration, the new interger typed
column contains 0, but the scratch column wst is always NULL - even if
b[1] contains a string.

Why?

Solution:
Split the migration: The first creats the new column, the second
processes the data.

Looks like problem with migrations.

Solution:
Split the migration: The first creats the new column, the second
processes the data.

Looks like problem with migrations.

Classes cache column information, but you can call
MyClass.reset_column_information to clear that cache.

Fred