Migration errors

The migrations raise their own HareError subclasses, from hare.migrations.exceptions:

HareError
└── HareMigrationError
    ├── UnknownMigrationError(HareMigrationError, LookupError)
    ├── MigrationLoadError
    ├── CircularDependencyError
    ├── IncompatibleStateError
    ├── IrreversibleMigrationError
    ├── PartiallyAppliedMigrationError
    ├── TenantMigrationError
    ├── FieldNarrowingDataLossError
    ├── ForeignKeyTargetChangeError
    ├── InconsistentMigrationStateError(HareMigrationError, RuntimeError)
    └── MigrationAutodetectorError(HareMigrationError, RuntimeError)

When each is raised

Exception	When
UnknownMigrationError	An app label, a migration name or a migrate target names nothing that exists — or a migration name matches several migrations. Also a LookupError.
MigrationLoadError	A migration file can't be loaded: it doesn't import, has no Migration class, a migration is defined twice, a dependency names a migration or app that doesn't exist, or a squashed migration replaces migrations only some of which are applied while the files of the others are gone.
CircularDependencyError	The dependencies of the migrations form a cycle.
IncompatibleStateError	An operation can't run on the state the migrations before it leave.
IrreversibleMigrationError	A migration is unapplied while one of its operations can't be reversed — raised before the first migration of the run is unapplied, so the later ones stay applied.
PartiallyAppliedMigrationError	A non-atomic migration failed partway through — some of its operations already ran. One refused before its first statement ran (no RunPython in it) raises its own error instead: nothing of it landed.
TenantMigrationError	With tenant_schema_template, the migrations failed in the schemas of some tenants. The other tenants' schemas are migrated; the error names each failed schema with its error, and the next migrate runs in each schema what its own journal lacks.
FieldNarrowingDataLossError	An AlterField narrowing a column would cut stored values.
ForeignKeyTargetChangeError	An AlterField points a relation at another target column (to_field) while rows store key values of the old one.
InconsistentMigrationStateError	Replaying the migration files leaves a relation pointing at a model that no longer exists.
MigrationAutodetectorError	makemigrations checks the migrations it made and finds them wrong — applied, they don't reach the models, their files don't read back as the same operations, or they depend on each other in a cycle. An error of hare's, raised before any file is written. Also a RuntimeError.

Other errors of migrations

A migration operation or a value that can't be declared or written into a migration file (a lambda default, RemoveIndex() without a name or fields) raises ConfigurationError; migrate targets that contradict each other raise QueryError.