Changing the type of a column in mysql

rubyonrails-talk
1

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?

2

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

Looks like problem with migrations.

3

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