Rails migrations: Undo default setting for a column

I have the problem, that I have an migration in Rails that sets up a default setting for a column, like this example:

def self.up
  add_column :column_name, :bought_at, :datetime, :default => Time.now
end

Suppose, I like to drop that default settings in a later migration, how do I do that with using rails migrations?

My current workaround is the execution of a custom sql command in the rails migration, like this:

def self.up
  execute 'alter table column_name alter bought_at drop default'
end

But I don't like this approach, because I am now dependent on how the underlying database is interpreting this command. In case of a change of the database this query perhaps might not work anymore and the migration would be broken. So, is there a way to express the undo of a default setting for a column in rails?


change_column_default(table_name, column_name, nil) also works to drop the column's default value entirely. No need to use execute.


Sounds like you're doing the right thing with your 'execute', as the docs point out:

change_column_default(table_name, column_name, default)

Sets a new default value for a column. If you want to set the default value to NULL, you are out of luck. You need to DatabaseStatements#execute the appropriate SQL statement yourself. Examples

change_column_default(:suppliers, :qualification, 'new')
change_column_default(:accounts, :authorized, 1)

下面的代码片段用于使NULLNOT NULL ,但在模式级别跳过DEFAULT

def self.up
  change_column :table, :column, :string, :null => false, :default => ""
  change_column_default(:table, :column, nil)
end
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