54 lines
1.7 KiB
Markdown
54 lines
1.7 KiB
Markdown
|
# Swapping Tables
|
||
|
|
|
||
|
|
Sometimes you need to replace one table with another. For example, when
|
||
|
|
migrating data in a very large table it's often better to create a copy of the
|
||
|
|
table and insert & migrate the data into this new table in the background.
|
||
|
|
|
||
|
|
Let's say you want to swap the table "events" with "events_for_migration". In
|
||
|
|
this case you need to follow 3 steps:
|
||
|
|
|
||
|
|
1. Rename "events" to "events_temporary"
|
||
|
|
2. Rename "events_for_migration" to "events"
|
||
|
|
3. Rename "events_temporary" to "events_for_migration"
|
||
|
|
|
||
|
|
Rails allows you to do this using the `rename_table` method:
|
||
|
|
|
||
|
|
```ruby
|
||
|
|
rename_table :events, :events_temporary
|
||
|
|
rename_table :events_for_migration, :events
|
||
|
|
rename_table :events_temporary, :events_for_migration
|
||
|
|
```
|
||
|
|
|
||
|
|
This does not require any downtime as long as the 3 `rename_table` calls are
|
||
|
|
executed in the _same_ database transaction. Rails by default uses database
|
||
|
|
transactions for migrations, but if it doesn't you'll need to start one
|
||
|
|
manually:
|
||
|
|
|
||
|
|
```ruby
|
||
|
|
Event.transaction do
|
||
|
|
rename_table :events, :events_temporary
|
||
|
|
rename_table :events_for_migration, :events
|
||
|
|
rename_table :events_temporary, :events_for_migration
|
||
|
|
end
|
||
|
|
```
|
||
|
|
|
||
|
|
Once swapped you _have to_ reset the primary key of the new table. For
|
||
|
|
PostgreSQL you can use the `reset_pk_sequence!` method like so:
|
||
|
|
|
||
|
|
```ruby
|
||
|
|
reset_pk_sequence!('events')
|
||
|
|
```
|
||
|
|
|
||
|
|
For MySQL however you need to do run the following:
|
||
|
|
|
||
|
|
```ruby
|
||
|
|
amount = Event.pluck('COALESCE(MAX(id), 1)').first
|
||
|
|
|
||
|
|
execute "ALTER TABLE events AUTO_INCREMENT = #{amount}"
|
||
|
|
```
|
||
|
|
|
||
|
|
Failure to reset the primary keys will result in newly created rows starting
|
||
|
|
with an ID value of 1. Depending on the existing data this can then lead to
|
||
|
|
duplicate key constraints from popping up, preventing users from creating new
|
||
|
|
data.
|