Django/venv/lib/python3.7/site-packages/django/db/migrations/migration.py

from django.db.transaction import atomic

from .exceptions import IrreversibleError


class Migration:
    """
    The base class for all migrations.

    Migration files will import this from django.db.migrations.Migration
    and subclass it as a class called Migration. It will have one or more
    of the following attributes:

     - operations: A list of Operation instances, probably from django.db.migrations.operations
     - dependencies: A list of tuples of (app_path, migration_name)
     - run_before: A list of tuples of (app_path, migration_name)
     - replaces: A list of migration_names

    Note that all migrations come out of migrations and into the Loader or
    Graph as instances, having been initialized with their app label and name.
    """

    # Operations to apply during this migration, in order.
    operations = []

    # Other migrations that should be run before this migration.
    # Should be a list of (app, migration_name).
    dependencies = []

    # Other migrations that should be run after this one (i.e. have
    # this migration added to their dependencies). Useful to make third-party
    # apps' migrations run after your AUTH_USER replacement, for example.
    run_before = []

    # Migration names in this app that this migration replaces. If this is
    # non-empty, this migration will only be applied if all these migrations
    # are not applied.
    replaces = []

    # Is this an initial migration? Initial migrations are skipped on
    # --fake-initial if the table or fields already exist. If None, check if
    # the migration has any dependencies to determine if there are dependencies
    # to tell if db introspection needs to be done. If True, always perform
    # introspection. If False, never perform introspection.
    initial = None

    # Whether to wrap the whole migration in a transaction. Only has an effect
    # on database backends which support transactional DDL.
    atomic = True

    def __init__(self, name, app_label):
        self.name = name
        self.app_label = app_label
        # Copy dependencies & other attrs as we might mutate them at runtime
        self.operations = list(self.__class__.operations)
        self.dependencies = list(self.__class__.dependencies)
        self.run_before = list(self.__class__.run_before)
        self.replaces = list(self.__class__.replaces)

    def __eq__(self, other):
        return (
            isinstance(other, Migration) and
            self.name == other.name and
            self.app_label == other.app_label
        )

    def __repr__(self):
        return "<Migration %s.%s>" % (self.app_label, self.name)

    def __str__(self):
        return "%s.%s" % (self.app_label, self.name)

    def __hash__(self):
        return hash("%s.%s" % (self.app_label, self.name))

    def mutate_state(self, project_state, preserve=True):
        """
        Take a ProjectState and return a new one with the migration's
        operations applied to it. Preserve the original object state by
        default and return a mutated state from a copy.
        """
        new_state = project_state
        if preserve:
            new_state = project_state.clone()

        for operation in self.operations:
            operation.state_forwards(self.app_label, new_state)
        return new_state

    def apply(self, project_state, schema_editor, collect_sql=False):
        """
        Take a project_state representing all migrations prior to this one
        and a schema_editor for a live database and apply the migration
        in a forwards order.

        Return the resulting project state for efficient reuse by following
        Migrations.
        """
        for operation in self.operations:
            # If this operation cannot be represented as SQL, place a comment
            # there instead
            if collect_sql:
                schema_editor.collected_sql.append("--")
                if not operation.reduces_to_sql:
                    schema_editor.collected_sql.append(
                        "-- MIGRATION NOW PERFORMS OPERATION THAT CANNOT BE WRITTEN AS SQL:"
                    )
                schema_editor.collected_sql.append("-- %s" % operation.describe())
                schema_editor.collected_sql.append("--")
                if not operation.reduces_to_sql:
                    continue
            # Save the state before the operation has run
            old_state = project_state.clone()
            operation.state_forwards(self.app_label, project_state)
            # Run the operation
            atomic_operation = operation.atomic or (self.atomic and operation.atomic is not False)
            if not schema_editor.atomic_migration and atomic_operation:
                # Force a transaction on a non-transactional-DDL backend or an
                # atomic operation inside a non-atomic migration.
                with atomic(schema_editor.connection.alias):
                    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
            else:
                # Normal behaviour
                operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
        return project_state

    def unapply(self, project_state, schema_editor, collect_sql=False):
        """
        Take a project_state representing all migrations prior to this one
        and a schema_editor for a live database and apply the migration
        in a reverse order.

        The backwards migration process consists of two phases:

        1. The intermediate states from right before the first until right
           after the last operation inside this migration are preserved.
        2. The operations are applied in reverse order using the states
           recorded in step 1.
        """
        # Construct all the intermediate states we need for a reverse migration
        to_run = []
        new_state = project_state
        # Phase 1
        for operation in self.operations:
            # If it's irreversible, error out
            if not operation.reversible:
                raise IrreversibleError("Operation %s in %s is not reversible" % (operation, self))
            # Preserve new state from previous run to not tamper the same state
            # over all operations
            new_state = new_state.clone()
            old_state = new_state.clone()
            operation.state_forwards(self.app_label, new_state)
            to_run.insert(0, (operation, old_state, new_state))

        # Phase 2
        for operation, to_state, from_state in to_run:
            if collect_sql:
                schema_editor.collected_sql.append("--")
                if not operation.reduces_to_sql:
                    schema_editor.collected_sql.append(
                        "-- MIGRATION NOW PERFORMS OPERATION THAT CANNOT BE WRITTEN AS SQL:"
                    )
                schema_editor.collected_sql.append("-- %s" % operation.describe())
                schema_editor.collected_sql.append("--")
                if not operation.reduces_to_sql:
                    continue
            atomic_operation = operation.atomic or (self.atomic and operation.atomic is not False)
            if not schema_editor.atomic_migration and atomic_operation:
                # Force a transaction on a non-transactional-DDL backend or an
                # atomic operation inside a non-atomic migration.
                with atomic(schema_editor.connection.alias):
                    operation.database_backwards(self.app_label, schema_editor, from_state, to_state)
            else:
                # Normal behaviour
                operation.database_backwards(self.app_label, schema_editor, from_state, to_state)
        return project_state


class SwappableTuple(tuple):
    """
    Subclass of tuple so Django can tell this was originally a swappable
    dependency when it reads the migration file.
    """

    def __new__(cls, value, setting):
        self = tuple.__new__(cls, value)
        self.setting = setting
        return self


def swappable_dependency(value):
    """Turn a setting value into a dependency."""
    return SwappableTuple((value.split(".", 1)[0], "__first__"), value)