migrate:check

Command /migrate:check

Check migration for common problems before applying.

What we check

1. Enum types — is there drop in downgrade
2. Nullable changes — is there data migration
3. Index/Constraint names — are names unique
4. Foreign keys — correct order of operations
5. Data loss — potential data loss

Instructions

Step 1: Find migration file

# Latest migration
alembic heads

# Specific revision
alembic show {{ revision }}

Step 2: Read migration

# Get file path
alembic show {{ revision }} | grep "Path"

Step 3: Check for problems

Enum without drop in downgrade

# Look for enum in upgrade
sa.Enum("value1", "value2", name="enum_name")

# Check downgrade
# Should be:
# sa.Enum(name="enum_name").drop(op.get_bind(), checkfirst=False)

Nullable False without data migration

# Dangerous:
op.alter_column("users", "name", nullable=False)

# Safe:
op.execute("UPDATE users SET name = 'default' WHERE name IS NULL")
op.alter_column("users", "name", nullable=False)

Drop column without backup

# Dangerous — data loss:
op.drop_column("users", "important_data")

# Recommendation: first deprecate, then delete

Foreign key order

# Correct — first FK, then table:
def downgrade():
    op.drop_constraint("fk_orders_user_id", "orders")
    op.drop_table("users")

# Incorrect — FK constraint error:
def downgrade():
    op.drop_table("users")  # FK still exists!

Step 4: Test upgrade/downgrade

# Full cycle
alembic upgrade head
alembic downgrade -1
alembic upgrade head

Checklist

• All enum types are deleted in downgrade
• Nullable changes have data migration
• No hardcoded constraint names (use naming convention)
• FK are deleted before tables in downgrade
• Drop column — conscious decision
• Test upgrade/downgrade passes

Response format

## Migration check: {{ revision }}

### Status: OK / Needs attention / Problems

### Found problems

#### Enum without drop in downgrade
- `status_type` is created, but not deleted
- **Fix:** add to downgrade:
  ```python
  sa.Enum(name="status_type").drop(op.get_bind(), checkfirst=False)

Nullable change without data migration

• users.name changes to NOT NULL
• Recommendation: add:

  op.execute("UPDATE users SET name = 'Unknown' WHERE name IS NULL")
  

Recommendations

1. ...
2. ...

Test

alembic upgrade head && alembic downgrade -1 && alembic upgrade head