Why Does The Sequelize Id Auto Increment Keep Going Up When Unseeded?

The Sequelize id auto-increment mechanism will typically continue to increase even when your database is "unseeded" or if you delete all rows from a table. This behavior is by design, and it stems from how databases handle auto-increment fields.

Here's a breakdown of why this happens:

1. Database Sequence or Counter:

- Most databases, including PostgreSQL, MySQL, and SQLite (which Sequelize often interfaces with), use an internal sequence or counter to manage auto-incrementing IDs. This sequence is not tied to the existing data in the table but is an independent tracking mechanism.

2. Independent of Table Data:

- When you insert a new record with an auto-incrementing ID, the database increments this internal sequence, assigns the next value, and then stores it in the id column. The database does not look back at previously stored ID's to decide what the next ID should be, it relies on its internal counter.

3. Deleting Data Doesn't Reset Sequence:

- Deleting rows from a table or even truncating the entire table does not usually reset this underlying sequence. The sequence's job is to ensure that, if a new entry is added, it receives a unique ID that is greater than any ID that has been used before, even if those have been deleted.

4. Why Keep Increasing?

- Continuing to increment the ID sequence ensures the uniqueness of each ID and prevents collisions. If the ID counter reset after every deletion or seeding, there would be a high probability of ID's being reused and conflicts would occur.

5. Resetting the Auto Increment Sequence:

- If you need to reset the sequence to start from the beginning, database-specific SQL commands can be used. For example, in PostgreSQL, you might use:

ALTER SEQUENCE tablename_id_seq RESTART WITH 1;

- In MySQL, you might use:

ALTER TABLE tablename AUTO_INCREMENT = 1;

- Be aware that resetting the auto-increment sequence after data has been created can cause conflicts and should be done with caution, especially in live environments. It is important to test that no data is lost, or that no other database errors are produced.

6. Sequelize's Role:

- Sequelize (and similar ORMs) typically operate by utilizing these database-native auto-incrementing features. They do not intervene with this behavior unless you explicitly specify otherwise during table or model creation. When using Sequelize with the autoIncrement: true setting on the id field it tells the database to manage this ID creation.

In summary, the auto-incrementing ID in Sequelize and the underlying database is designed to ensure uniqueness and it keeps incrementing regardless of the existing data in the table. This behavior is not specific to Sequelize but is a standard database functionality to manage unique identifiers.

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