# Database, migrations and schema

### When to use

* Migration fails on deploy.
* App starts, but gives error of non-existent table/column.
* Database connection errors.

### Quick checklist (2 min)

1. Confirm in which environment the error occurs.
2. Check the most recently applied migration.
3. See if there is a mismatch between expected schema and actual schema.

### Diagnosis

#### 1) Connection

* Correct credentials/URL?
* Network/VPC open (if applicable)?

#### 2) Order and idempotence of migrations

* Does a migration depend on another that didn't run?
* Does a migration run twice and break?

#### 3) Existing data

* Altering a column with old data often breaks.
* Constraints (NOT NULL / FK) require backfill.

### How to resolve (patterns)

* **Column/table error**: review the last migration and the schema.
* **Constraint failed**: perform backfill before applying constraint.
* **Connection**: validate secrets and network permissions.

### When to escalate

* Migration repeatedly breaks in the pipeline.
* Corruption/inconsistent data.
* Error only in Prod and not reproducible in Staging.