Method build_graph

django/db/migrations/loader.py:274–342

Build a migration dependency graph using both the disk and database. You'll need to rebuild the graph if you apply migrations. This isn't usually a problem as generally migration stuff runs in a one-shot process.

(self)

        self.replacements_progress[migration_key] = True

    def build_graph(self):
        """
        Build a migration dependency graph using both the disk and database.
        You'll need to rebuild the graph if you apply migrations. This isn't
        usually a problem as generally migration stuff runs in a one-shot
        process.
        """
        # Load disk data
        self.load_disk()
        # Load database data
        if self.connection is None:
            self.applied_migrations = {}
        else:
            recorder = MigrationRecorder(self.connection)
            self.applied_migrations = recorder.applied_migrations()
        # To start, populate the migration graph with nodes for ALL migrations
        # and their dependencies. Also make note of replacing migrations at
        # this step.
        self.graph = MigrationGraph()
        self.replacements = {}
        for key, migration in self.disk_migrations.items():
            self.graph.add_node(key, migration)
            # Replacing migrations.
            if migration.replaces:
                self.replacements[key] = migration
        for key, migration in self.disk_migrations.items():
            # Internal (same app) dependencies.
            self.add_internal_dependencies(key, migration)
        # Add external dependencies now that the internal ones have been
        # resolved.
        for key, migration in self.disk_migrations.items():
            self.add_external_dependencies(key, migration)
        # Carry out replacements where possible and if enabled.
        if self.replace_migrations:
            self.replacements_progress = {}
            for migration_key in self.replacements.keys():
                self.replace_migration(migration_key)
        # Ensure the graph is consistent.
        try:
            self.graph.validate_consistency()
        except NodeNotFoundError as exc:
            # Check if the missing node could have been replaced by any squash
            # migration but wasn't because the squash migration was partially
            # applied before. In that case raise a more understandable
            # exception (#23556).
            # Get reverse replacements.
            reverse_replacements = {}
            for key, migration in self.replacements.items():
                for replaced in migration.replaces:
                    reverse_replacements.setdefault(replaced, set()).add(key)
            # Try to reraise exception with more detail.
            if exc.node in reverse_replacements:
                candidates = reverse_replacements.get(exc.node, set())
                is_replaced = any(
                    candidate in self.graph.nodes for candidate in candidates
                )
                if not is_replaced:
                    tries = ", ".join("%s.%s" % c for c in candidates)