import os from django.utils.importlib import import_module from django.utils.functional import cached_property from django.db.models.loading import cache from django.db.migrations.recorder import MigrationRecorder from django.db.migrations.graph import MigrationGraph class MigrationLoader(object): """ Loads migration files from disk, and their status from the database. Migration files are expected to live in the "migrations" directory of an app. Their names are entirely unimportant from a code perspective, but will probably follow the 1234_name.py convention. On initialisation, this class will scan those directories, and open and read the python files, looking for a class called Migration, which should inherit from django.db.migrations.Migration. See django.db.migrations.migration for what that looks like. Some migrations will be marked as "replacing" another set of migrations. These are loaded into a separate set of migrations away from the main ones. If all the migrations they replace are either unapplied or missing from disk, then they are injected into the main set, replacing the named migrations. Any dependency pointers to the replaced migrations are re-pointed to the new migration. This does mean that this class MUST also talk to the database as well as to disk, but this is probably fine. We're already not just operating in memory. """ def __init__(self, connection): self.connection = connection self.disk_migrations = None self.applied_migrations = None def load_disk(self): """ Loads the migrations from all INSTALLED_APPS from disk. """ self.disk_migrations = {} for app in cache.get_apps(): # Get the migrations module directory module_name = ".".join(app.__name__.split(".")[:-1] + ["migrations"]) app_label = module_name.split(".")[-2] try: module = import_module(module_name) except ImportError as e: # I hate doing this, but I don't want to squash other import errors. # Might be better to try a directory check directly. if "No module named migrations" in str(e): continue directory = os.path.dirname(module.__file__) # Scan for .py[c|o] files migration_names = set() for name in os.listdir(directory): if name.endswith(".py") or name.endswith(".pyc") or name.endswith(".pyo"): import_name = name.rsplit(".", 1)[0] if import_name[0] not in "_.~": migration_names.add(import_name) # Load them for migration_name in migration_names: migration_module = import_module("%s.%s" % (module_name, migration_name)) if not hasattr(migration_module, "Migration"): raise BadMigrationError("Migration %s in app %s has no Migration class" % (migration_name, app_label)) self.disk_migrations[app_label, migration_name] = migration_module.Migration(migration_name, app_label) @cached_property def graph(self): """ Builds a migration dependency graph using both the disk and database. """ # Make sure we have the disk data if self.disk_migrations is None: self.load_disk() # And the database data if self.applied_migrations is None: recorder = MigrationRecorder(self.connection) self.applied_migrations = recorder.applied_migrations() # Do a first pass to separate out replacing and non-replacing migrations normal = {} replacing = {} for key, migration in self.disk_migrations.items(): if migration.replaces: replacing[key] = migration else: normal[key] = migration # Calculate reverse dependencies - i.e., for each migration, what depends on it? # This is just for dependency re-pointing when applying replacements, # so we ignore run_before here. reverse_dependencies = {} for key, migration in normal.items(): for parent in migration.dependencies: reverse_dependencies.setdefault(parent, set()).add(key) # Carry out replacements if we can - that is, if all replaced migrations # are either unapplied or missing. for key, migration in replacing.items(): # Do the check can_replace = True for target in migration.replaces: if target in self.applied_migrations: can_replace = False break if not can_replace: continue # Alright, time to replace. Step through the replaced migrations # and remove, repointing dependencies if needs be. for replaced in migration.replaces: if replaced in normal: del normal[replaced] for child_key in reverse_dependencies.get(replaced, set()): normal[child_key].dependencies.remove(replaced) normal[child_key].dependencies.append(key) normal[key] = migration # Finally, make a graph and load everything into it graph = MigrationGraph() for key, migration in normal.items(): graph.add_node(key, migration) for key, migration in normal.items(): for parent in migration.dependencies: graph.add_dependency(key, parent) return graph class BadMigrationError(Exception): """ Raised when there's a bad migration (unreadable/bad format/etc.) """ pass