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Describe database transactions

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Marcin K 2019-08-09 18:46:04 +02:00
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TUTORIAL.md
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@ -42,6 +42,22 @@ DROP TABLE IF EXISTS users;
``` ```
By adding `IF EXISTS/IF NOT EXISTS` we are making migrations idempotent - we can run the same sql code twice in a row with the same result. This makes our migrations more robust. On the other hand, it causes slightly less control over database schema - e.g. let's say you forgot to drop the table in down migration. You run down migration - the table is still there. When you run up migration again - `CREATE TABLE` would return an error, helping you find an issue in down migration, while `CREATE TABLE IF NOT EXISTS` would not. Use those conditions wisely. By adding `IF EXISTS/IF NOT EXISTS` we are making migrations idempotent - we can run the same sql code twice in a row with the same result. This makes our migrations more robust. On the other hand, it causes slightly less control over database schema - e.g. let's say you forgot to drop the table in down migration. You run down migration - the table is still there. When you run up migration again - `CREATE TABLE` would return an error, helping you find an issue in down migration, while `CREATE TABLE IF NOT EXISTS` would not. Use those conditions wisely.
In case you would like to run several commands/queries in one migration, you should wrap them in a transaction (if your database supports it).
In our postgres example it would be (migration up):
```
BEGIN;
CREATE TYPE enum_mood AS ENUM (
'happy',
'sad',
'neutral'
);
ALTER TABLE users ADD COLUMN mood enum_mood;
COMMIT;
```
This way if one of commands fails, our database will remain unchanged.
## Run migrations ## Run migrations
``` ```
migrate -database ${POSTGRESQL_URL} -path db/migrations up migrate -database ${POSTGRESQL_URL} -path db/migrations up