migration.py 文件源码

def get_current_heads(self):
        """Return a tuple of the current 'head versions' that are represented
        in the target database.

        For a migration stream without branches, this will be a single
        value, synonymous with that of
        :meth:`.MigrationContext.get_current_revision`.   However when multiple
        unmerged branches exist within the target database, the returned tuple
        will contain a value for each head.

        If this :class:`.MigrationContext` was configured in "offline"
        mode, that is with ``as_sql=True``, the ``starting_rev``
        parameter is returned in a one-length tuple.

        If no version table is present, or if there are no revisions
        present, an empty tuple is returned.

        .. versionadded:: 0.7.0

        """
        if self.as_sql:
            start_from_rev = self._start_from_rev
            if start_from_rev is not None and self.script:
                start_from_rev = \
                    self.script.get_revision(start_from_rev).revision

            return util.to_tuple(start_from_rev, default=())
        else:
            if self._start_from_rev:
                raise util.CommandError(
                    "Can't specify current_rev to context "
                    "when using a database connection")
            if not self._has_version_table():
                return ()
        return tuple(
            row[0] for row in self.connection.execute(self._version.select())
        )