To add a default value to a column in a Rails migration, you can use the change_column_default
method. This method takes three arguments: the name of the table, the name of the column, and the default value you want to set. For example, to add a default value of false
to a published
column in a posts
table, you could use the following migration:
👋 Check out our easy to use desktop GUI for SQL
Beekeeper Studio is a truly cross-platform SQL GUI with a clean, uncluttered interface. Write SQL, edit data, alter tables, and more!
Available for MacOS, Linux, and Windows.
class AddDefaultToPosts < ActiveRecord::Migration[6.0]
def change
change_column_default :posts, :published, from: nil, to: false
end
end
The change_column_default
method has a number of options that you can use to customize its behavior. For example, you can use the from:
option to specify the current default value of the column, if any. This is useful if you are changing the default value of a column that already has a default value set. The to:
option specifies the new default value that you want to set.
In addition to the change_column_default
method, you can also use the add_column
method to add a new column to a table and set a default value at the same time. The add_column
method takes three arguments: the name of the table, the name of the column, and a hash of options for the column. To specify a default value, you can use the :default
option. For example, to add a new published
column to a posts
table with a default value of false
, you could use the following migration:
class AddPublishedToPosts < ActiveRecord::Migration[6.0]
def change
add_column :posts, :published, :boolean, default: false
end
end
It’s important to note that default values are not applied retroactively to existing rows in the table. If you add a default value to a column that already has data in it, the default value will only be applied to new rows that are inserted into the table after the migration is run. If you want to update the existing rows in the table with the new default value, you can use the update_all
method in a separate migration. For example:
class UpdatePublishedToPosts < ActiveRecord::Migration[6.0]
def change
Post.where(published: nil).update_all(published: false)
end
end
In this example, we are using the update_all
method to set the published
column to false
for all rows where the published
column is currently nil
.
Overall, Rails migrations are a powerful and convenient way to make changes to the structure of a database. By using the change_column_default
or add_column
methods, you can easily add default values to columns in your database tables.