Add NOT NULL field without default value into a populated DB - ruby-on-rails

Question

I have a table, let's call it MyTable. It is part of a Postgresql database.
In MyTable are a lot of entries, let's say over a million. I would like to add a field to this table, let's call it MyNewField. It is to be added by an ActiveRecord Migration.
This field is to be without default values and not nullable. The result, in it's migration class would be something like so:
class AddMyFieldToMyTable < ActiveRecord::Migration
def change
add_column :my_table, :my_field, :text, null: false
end
end
However, it will trigger an error (PG::NotNullViolation), because the table already contains rows, all which will have MyField set to NULL.
What I would like to do is: Add the row without default value and nullable set to false (without triggering a PG::NotNullViolation). Then, insert a value from another table into each records.
This would probably be achievable by adding the field with nullable set to true, then adding the values, then changing back to nullable set to false. However, I am interested to know if it is possible to do so in a single shot.

Answer 1

You have to make sure that the other table has the my_field value for each entry in my_table.
class AddMyFieldToMyTable < ActiveRecord::Migration
def up
add_column :my_table, :my_field, :text
execute("insert into my_table(my_field) values (select my_field from different_table where my_table.id = different_table.different_id)")
change_column :my_table, :my_field, :text, null: false
end
def down
remove_column :my_table, :my_field
end
end

Answer 2

All indications seems to show that this will not be possible to do in one shot; you'll have to add the column without the null constraint, populate the data, then add the null constraint afterwards.
class AddMyFieldToMyTable < ActiveRecord::Migration
def change
add_column :my_table, :my_field, :text
reversible do |dir|
dir.up do
# populate my_field col
change_column :my_table, :my_field, :text, null: false
end
end
end
end
Resources:
How to solve "Cannot add a NOT NULL column with default value NULL" in SQLite3?
http://strd6.com/2009/04/adding-a-non-null-column-with-no-default-value-in-a-rails-migration/
If you really only want to set the column once, perhaps you could generate it with a temporary default value then immediately update it with the real data.
class AddMyFieldToMyTable < ActiveRecord::Migration
def change
add_column :my_table, :my_field, :text, default: 'tmp', null: false
reversible do |dir|
dir.up do
# populate my_field col
end
end
end
end

Answer 3

In my case I need to deviate from #usha's answer in two ways: 1) I needed to UPDATE rather than INSERT INTO since I'm updating the NULL values of existing rows rather than creating new rows. 2) I needed to use change_column_null instead of change_column ... null: false. A stylized example is below.
class AddMyFieldToMyTable < ActiveRecord::Migration
def up
add_column :my_table, :my_field, :text
execute <<-SQL
WITH values AS (
SELECT
my_table.id AS my_table_id,
different_table.my_value
FROM my_table
JOIN different_table ON
my_table.id = different_table.different_id
)
UPDATE my_table
SET my_field = values.my_value
FROM values
WHERE my_table.id = values.my_table_id
SQL
change_column_null :my_table, :my_field, false
end
def down
remove_column :my_table, :my_field
end
end