Ruby on Rails – How to Rename Column and Change Type in Migration

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In my general_exams table, I have a column named semester, type is string. Now I want to change its name to semester_id, type is integer. I have read about migration and it has available transformations:

rename_column(table_name, column_name, new_column_name): Renames a column but keeps the type and content.
change_column(table_name, column_name, type, options): Changes the column to a different type using the same parameters as add_column.

So, I create my migration file like this:

class RenameSemesterFromGeneralExams < ActiveRecord::Migration

  def change
    rename_column :general_exams, :semester, :semester_id
    change_column :general_exams, :semester_id, :integer
  end
end

But, when I run rake db:migrate, it has error:

==  RenameSemesterFromGeneralExams: migrating =================================
-- rename_column(:general_exams, :semester, :semester_id)
   -> 0.0572s
-- change_column(:general_exams, :semester_id, :integer)
rake aborted!
An error has occurred, this and all later migrations canceled:

PG::Error: ERROR:  column "semester_id" cannot be cast to type integer
: ALTER TABLE "general_exams" ALTER COLUMN "semester_id" TYPE integer

In my table GeneralExam, I destroyed all data.
So, anyone can tell me how can I do that? Or I must create two migration files?

Best Answer

This works as of Rails 4

def change
  rename_column :general_exams, :semester, :semester_id
  change_column :general_exams, :semester_id, :integer
end

Related Solutions

Ruby on Rails – How to Rename a Database Column in a Migration

rename_column :table, :old_column, :new_column

You'll probably want to create a separate migration to do this. (Rename FixColumnName as you will.):

bin/rails generate migration FixColumnName
# creates  db/migrate/xxxxxxxxxx_fix_column_name.rb

Then edit the migration to do your will:

# db/migrate/xxxxxxxxxx_fix_column_name.rb
class FixColumnName < ActiveRecord::Migration
  def self.up
    rename_column :table_name, :old_column, :new_column
  end

  def self.down
    # rename back if you need or do something else or do nothing
  end
end

For Rails 3.1 use:

While, the up and down methods still apply, Rails 3.1 receives a change method that "knows how to migrate your database and reverse it when the migration is rolled back without the need to write a separate down method".

See "Active Record Migrations" for more information.

rails g migration FixColumnName

class FixColumnName < ActiveRecord::Migration
  def change
    rename_column :table_name, :old_column, :new_column
  end
end

If you happen to have a whole bunch of columns to rename, or something that would have required repeating the table name over and over again:

rename_column :table_name, :old_column1, :new_column1
rename_column :table_name, :old_column2, :new_column2
...

You could use change_table to keep things a little neater:

class FixColumnNames < ActiveRecord::Migration
  def change
    change_table :table_name do |t|
      t.rename :old_column1, :new_column1
      t.rename :old_column2, :new_column2
      ...
    end
  end
end

Then just db:migrate as usual or however you go about your business.

For Rails 4:

While creating a Migration for renaming a column, Rails 4 generates a change method instead of up and down as mentioned in the above section. The generated change method is:

$ > rails g migration ChangeColumnName

which will create a migration file similar to:

class ChangeColumnName < ActiveRecord::Migration
  def change
    rename_column :table_name, :old_column, :new_column
  end
end

Rails Migration to Change Column – How to Guide

Use #change_column.

change_column(:table_name, :column_name, :date)

# a few more examples:
change_column(:suppliers, :name, :string, limit: 80)
change_column(:accounts, :description, :text)

NOTE: the same result can be achieved even outside of db migrations, this might be handy for testing/debugging but this method needs to be used very cautiously:

ActiveRecord::Base.connection.change_column(:table_name, :column_name, :date)

Best Answer

Related Solutions

Ruby on Rails – How to Rename a Database Column in a Migration

Rails Migration to Change Column – How to Guide

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