Exception
that can be raised to stop migrations from being rolled back. For example the following migration is not reversible. Rolling back this migration will raise an ActiveRecord::IrreversibleMigration
error.
class IrreversibleMigrationExample < ActiveRecord::Migration[7.0]
def change
create_table :distributors do |t|
t.string :zipcode
end
execute <<~SQL
ALTER TABLE distributors
ADD CONSTRAINT zipchk
CHECK (char_length(zipcode) = 5) NO INHERIT;
SQL
end
end
There are two ways to mitigate this problem.
-
Define
#up
and#down
methods instead of#change
:
class ReversibleMigrationExample < ActiveRecord::Migration[7.0]
def up
create_table :distributors do |t|
t.string :zipcode
end
execute <<~SQL
ALTER TABLE distributors
ADD CONSTRAINT zipchk
CHECK (char_length(zipcode) = 5) NO INHERIT;
SQL
end
def down
execute <<~SQL
ALTER TABLE distributors
DROP CONSTRAINT zipchk
SQL
drop_table :distributors
end
end
-
Use the reversible method in
#change
method:
class ReversibleMigrationExample < ActiveRecord::Migration[7.0]
def change
create_table :distributors do |t|
t.string :zipcode
end
reversible do |dir|
dir.up do
execute <<~SQL
ALTER TABLE distributors
ADD CONSTRAINT zipchk
CHECK (char_length(zipcode) = 5) NO INHERIT;
SQL
end
dir.down do
execute <<~SQL
ALTER TABLE distributors
DROP CONSTRAINT zipchk
SQL
end
end
end
end