Default value not populating in migration with Rails and Postgresql

Question

I am currently trying to run this migration:

class AddDroppedProjectsCountToUser < ActiveRecord::Migration
  def self.up
    add_column :users, :dropped_projects, :integer, {:default=>0, :required=>true}
  end

  def self.down
    remove_column :users, :dropped_projects
  end
end

The column is added correctly, but none of the old records are populated with 0. They are nil. I have tried using default=>'0' as well, to no avail. Any idea why this might be happening? (Rails 3.0.3)

Edited to add: When I create a new user it works fine, and everything looks correct. It's just the old users that still have nil for that value in the table.

mu is too short · Accepted Answer · 2018-11-26 18:37:06Z

13

What happens if you say:

def self.up
  add_column :users, :dropped_projects, :integer, :null => false, :default => 0
end

instead? Without the :null=>false you're still allowing NULL in dropped_projects so there's no reason for PostgreSQL to make them 0. Also, I don't think :required is a valid option for add_column; since the options are just a simple Hash and add_column only looks for options it knows about, your stray :required option is silently ignored.

edited Nov 26, 2018 at 18:37

answered Mar 4, 2011 at 19:37

mu is too short

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4 Comments

Dave G Over a year ago

Great Success! Thanks Mu. So weird that it takes all that just to fill some 0's.

mu is too short Over a year ago

@Dave G: Also see my little update about :required. The problem with NULL is that sometimes it means "unknown" and sometimes it means "not there", the database has no way of differentiated these two cases; if you don't explicitly say "no NULLs" then the DB can't know if the NULLs are default values or explicit NULLs.

B Seven Over a year ago

It worked for me. What if you don't want to force no nulls? Is there another way to do it? What if you don't want to force a default, but just change the values for rows that are already in the database and subject to the migration?

mu is too short Over a year ago

@B Seven: Just leave the :null=>false out but that probably won't do anything to existing rows, you could always execute "UPDATE table ..." and do the update half with raw SQL; there's no good reason to be avoid SQL when it is the most direct and efficient way to get the job done.

random-forest-cat · Accepted Answer · 2015-01-01 20:20:46Z

3

you could do this:

(taken from http://apidock.com/rails/ActiveRecord/Migration)

Using a model after changing its table

Sometimes you’ll want to add a column in a migration and populate it immediately after. In that case, you’ll need to make a call to Base#reset_column_information in order to ensure that the model has the latest column data from after the new column was added. Example:

class AddPeopleSalary < ActiveRecord::Migration
  def up
    add_column :people, :salary, :integer
    Person.reset_column_information
    Person.all.each do |p|
      p.update_column :salary, SalaryCalculator.compute(p)
    end
  end
end

edited Jan 1, 2015 at 20:20

random-forest-cat

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answered Dec 9, 2012 at 20:26

studiou

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1 Comment

Bogdan Popa Over a year ago

What happens if you have a huge collection of existing people on production or so?

techdreams · Accepted Answer · 2017-07-11 06:17:12Z

1

I believe this is due to the fact that you are changing the old migration, instead of creating a new. In this case, the solution is to check the schema file (schema.rb). It does not change automatically, and add

t.integer "dropped_projects", default: 0, null: false

edited Jul 11, 2017 at 6:17

techdreams

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answered Jul 11, 2017 at 4:51

Alex Strizhak

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