"""
A custom manager for working with trees of objects.
"""
from django.db import connection, models, transaction
from django.utils.translation import ugettext as _

from mptt.exceptions import InvalidMove

__all__ = ('TreeManager',)

qn = connection.ops.quote_name

COUNT_SUBQUERY = """(
    SELECT COUNT(*)
    FROM %(rel_table)s
    WHERE %(mptt_fk)s = %(mptt_table)s.%(mptt_pk)s
)"""

CUMULATIVE_COUNT_SUBQUERY = """(
    SELECT COUNT(*)
    FROM %(rel_table)s
    WHERE %(mptt_fk)s IN
    (
        SELECT m2.%(mptt_pk)s
        FROM %(mptt_table)s m2
        WHERE m2.%(tree_id)s = %(mptt_table)s.%(tree_id)s
          AND m2.%(left)s BETWEEN %(mptt_table)s.%(left)s
                              AND %(mptt_table)s.%(right)s
    )
)"""

class TreeManager(models.Manager):
    """
    A manager for working with trees of objects.
    """
    def __init__(self, parent_attr, left_attr, right_attr, tree_id_attr,
                 level_attr):
        """
        Tree attributes for the model being managed are held as
        attributes of this manager for later use, since it will be using
        them a **lot**.
        """
        super(TreeManager, self).__init__()
        self.parent_attr = parent_attr
        self.left_attr = left_attr
        self.right_attr = right_attr
        self.tree_id_attr = tree_id_attr
        self.level_attr = level_attr

    def add_related_count(self, queryset, rel_model, rel_field, count_attr,
                          cumulative=False):
        """
        Adds a related item count to a given ``QuerySet`` using its
        ``extra`` method, for a ``Model`` class which has a relation to
        this ``Manager``'s ``Model`` class.

        Arguments:

        ``rel_model``
           A ``Model`` class which has a relation to this `Manager``'s
           ``Model`` class.

        ``rel_field``
           The name of the field in ``rel_model`` which holds the
           relation.

        ``count_attr``
           The name of an attribute which should be added to each item in
           this ``QuerySet``, containing a count of how many instances
           of ``rel_model`` are related to it through ``rel_field``.

        ``cumulative``
           If ``True``, the count will be for each item and all of its
           descendants, otherwise it will be for each item itself.
        """
        opts = self.model._meta
        if cumulative:
            subquery = CUMULATIVE_COUNT_SUBQUERY % {
                'rel_table': qn(rel_model._meta.db_table),
                'mptt_fk': qn(rel_model._meta.get_field(rel_field).column),
                'mptt_table': qn(opts.db_table),
                'mptt_pk': qn(opts.pk.column),
                'tree_id': qn(opts.get_field(self.tree_id_attr).column),
                'left': qn(opts.get_field(self.left_attr).column),
                'right': qn(opts.get_field(self.right_attr).column),
            }
        else:
            subquery = COUNT_SUBQUERY % {
                'rel_table': qn(rel_model._meta.db_table),
                'mptt_fk': qn(rel_model._meta.get_field(rel_field).column),
                'mptt_table': qn(opts.db_table),
                'mptt_pk': qn(opts.pk.column),
            }
        return queryset.extra(select={count_attr: subquery})

    def get_query_set(self):
        """
        Returns a ``QuerySet`` which contains all tree items, ordered in
        such a way that that root nodes appear in tree id order and
        their subtrees appear in depth-first order.
        """
        return super(TreeManager, self).get_query_set().order_by(
            self.tree_id_attr, self.left_attr)

    def insert_node(self, node, target, position='last-child',
                    commit=False):
        """
        Sets up the tree state for ``node`` (which has not yet been
        inserted into in the database) so it will be positioned relative
        to a given ``target`` node as specified by ``position`` (when
        appropriate) it is inserted, with any neccessary space already
        having been made for it.

        A ``target`` of ``None`` indicates that ``node`` should be
        the last root node.

        If ``commit`` is ``True``, ``node``'s ``save()`` method will be
        called before it is returned.
        """
        if node.pk:
            raise ValueError(_('Cannot insert a node which has already been saved.'))

        if target is None:
            setattr(node, self.left_attr, 1)
            setattr(node, self.right_attr, 2)
            setattr(node, self.level_attr, 0)
            setattr(node, self.tree_id_attr, self._get_next_tree_id())
            setattr(node, self.parent_attr, None)
        elif target.is_root_node() and position in ['left', 'right']:
            target_tree_id = getattr(target, self.tree_id_attr)
            if position == 'left':
                tree_id = target_tree_id
                space_target = target_tree_id - 1
            else:
                tree_id = target_tree_id + 1
                space_target = target_tree_id

            self._create_tree_space(space_target)

            setattr(node, self.left_attr, 1)
            setattr(node, self.right_attr, 2)
            setattr(node, self.level_attr, 0)
            setattr(node, self.tree_id_attr, tree_id)
            setattr(node, self.parent_attr, None)
        else:
            setattr(node, self.left_attr, 0)
            setattr(node, self.level_attr, 0)

            space_target, level, left, parent = \
                self._calculate_inter_tree_move_values(node, target, position)
            tree_id = getattr(parent, self.tree_id_attr)

            self._create_space(2, space_target, tree_id)

            setattr(node, self.left_attr, -left)
            setattr(node, self.right_attr, -left + 1)
            setattr(node, self.level_attr, -level)
            setattr(node, self.tree_id_attr, tree_id)
            setattr(node, self.parent_attr, parent)

        if commit:
            node.save()
        return node

    def move_node(self, node, target, position='last-child'):
        """
        Moves ``node`` relative to a given ``target`` node as specified
        by ``position`` (when appropriate), by examining both nodes and
        calling the appropriate method to perform the move.

        A ``target`` of ``None`` indicates that ``node`` should be
        turned into a root node.

        Valid values for ``position`` are ``'first-child'``,
        ``'last-child'``, ``'left'`` or ``'right'``.

        ``node`` will be modified to reflect its new tree state in the
        database.

        This method explicitly checks for ``node`` being made a sibling
        of a root node, as this is a special case due to our use of tree
        ids to order root nodes.
        """
        if target is None:
            if node.is_child_node():
                self._make_child_root_node(node)
        elif target.is_root_node() and position in ['left', 'right']:
            self._make_sibling_of_root_node(node, target, position)
        else:
            if node.is_root_node():
                self._move_root_node(node, target, position)
            else:
                self._move_child_node(node, target, position)
        transaction.commit_unless_managed()

    def root_node(self, tree_id):
        """
        Returns the root node of the tree with the given id.
        """
        return self.get(**{
            self.tree_id_attr: tree_id,
            '%s__isnull' % self.parent_attr: True,
        })

    def root_nodes(self):
        """
        Creates a ``QuerySet`` containing root nodes.
        """
        return self.filter(**{'%s__isnull' % self.parent_attr: True})

    def rebuild(self):
        """
        Rebuilds whole tree in database using `parent` link.
        """
        opts = self.model._meta

        cursor = connection.cursor()
        cursor.execute('UPDATE %(table)s SET %(left)s = 0, %(right)s = 0, %(level)s = 0, %(tree_id)s = 0' % {
            'table': qn(opts.db_table),
            'left': qn(opts.get_field(self.left_attr).column),
            'right': qn(opts.get_field(self.right_attr).column),
            'level': qn(opts.get_field(self.level_attr).column),
            'tree_id': qn(opts.get_field(self.tree_id_attr).column)
        })

        cursor.execute('SELECT %(id_col)s FROM %(table)s WHERE %(parent_col)s is NULL %(orderby)s' % {
            'id_col': qn(opts.pk.column),
            'table': qn(opts.db_table),
            'parent_col': qn(opts.get_field(self.parent_attr).column),
            'orderby': 'ORDER BY ' + ', '.join([qn(field) for field in opts.order_insertion_by]) if opts.order_insertion_by else ''
        })

        idx = 0
        for (pk, ) in cursor.fetchall():
            idx += 1
            self._rebuild_helper(pk, 1, idx)
        transaction.commit_unless_managed()

    def _rebuild_helper(self, pk, left, tree_id, level=0):
        opts = self.model._meta
        right = left + 1

        cursor = connection.cursor()
        cursor.execute('SELECT %(id_col)s FROM %(table)s WHERE %(parent_col)s = %(parent)d %(orderby)s' % {
            'id_col': qn(opts.pk.column),
            'table': qn(opts.db_table),
            'parent_col': qn(opts.get_field(self.parent_attr).column),
            'parent': pk,
            'orderby': 'ORDER BY ' + ', '.join([qn(field) for field in opts.order_insertion_by]) if opts.order_insertion_by else ''
        })

        for (child_id, ) in cursor.fetchall():
            right = self._rebuild_helper(child_id, right, tree_id, level+1)

        cursor.execute("""
            UPDATE %(table)s
            SET
                %(left_col)s = %(left)d,
                %(right_col)s = %(right)d,
                %(level_col)s = %(level)d,
                %(tree_id_col)s = %(tree_id)d
            WHERE
                %(pk_col)s = %(pk)s
        """ % {
            'table': qn(opts.db_table),
            'pk_col': qn(opts.pk.column),
            'left_col': qn(opts.get_field(self.left_attr).column),
            'right_col': qn(opts.get_field(self.right_attr).column),
            'level_col': qn(opts.get_field(self.level_attr).column),
            'tree_id_col': qn(opts.get_field(self.tree_id_attr).column),
            'pk': pk,
            'left': left,
            'right': right,
            'level': level,
            'tree_id': tree_id
        })

        return right + 1


    def _calculate_inter_tree_move_values(self, node, target, position):
        """
        Calculates values required when moving ``node`` relative to
        ``target`` as specified by ``position``.
        """
        left = getattr(node, self.left_attr)
        level = getattr(node, self.level_attr)
        target_left = getattr(target, self.left_attr)
        target_right = getattr(target, self.right_attr)
        target_level = getattr(target, self.level_attr)

        if position == 'last-child' or position == 'first-child':
            if position == 'last-child':
                space_target = target_right - 1
            else:
                space_target = target_left
            level_change = level - target_level - 1
            parent = target
        elif position == 'left' or position == 'right':
            if position == 'left':
                space_target = target_left - 1
            else:
                space_target = target_right
            level_change = level - target_level
            parent = getattr(target, self.parent_attr)
        else:
            raise ValueError(_('An invalid position was given: %s.') % position)

        left_right_change = left - space_target - 1
        return space_target, level_change, left_right_change, parent

    def _close_gap(self, size, target, tree_id):
        """
        Closes a gap of a certain ``size`` after the given ``target``
        point in the tree identified by ``tree_id``.
        """
        self._manage_space(-size, target, tree_id)

    def _create_space(self, size, target, tree_id):
        """
        Creates a space of a certain ``size`` after the given ``target``
        point in the tree identified by ``tree_id``.
        """
        self._manage_space(size, target, tree_id)

    def _create_tree_space(self, target_tree_id):
        """
        Creates space for a new tree by incrementing all tree ids
        greater than ``target_tree_id``.
        """
        opts = self.model._meta
        cursor = connection.cursor()
        cursor.execute("""
        UPDATE %(table)s
        SET %(tree_id)s = %(tree_id)s + 1
        WHERE %(tree_id)s > %%s""" % {
            'table': qn(opts.db_table),
            'tree_id': qn(opts.get_field(self.tree_id_attr).column),
        }, [target_tree_id])

    def _get_next_tree_id(self):
        """
        Determines the next largest unused tree id for the tree managed
        by this manager.
        """
        opts = self.model._meta
        cursor = connection.cursor()
        cursor.execute('SELECT MAX(%s) FROM %s' % (
            qn(opts.get_field(self.tree_id_attr).column),
            qn(opts.db_table)))
        row = cursor.fetchone()
        return row[0] and (row[0] + 1) or 1

    def _inter_tree_move_and_close_gap(self, node, level_change,
            left_right_change, new_tree_id, parent_pk=None):
        """
        Removes ``node`` from its current tree, with the given set of
        changes being applied to ``node`` and its descendants, closing
        the gap left by moving ``node`` as it does so.

        If ``parent_pk`` is ``None``, this indicates that ``node`` is
        being moved to a brand new tree as its root node, and will thus
        have its parent field set to ``NULL``. Otherwise, ``node`` will
        have ``parent_pk`` set for its parent field.
        """
        opts = self.model._meta
        inter_tree_move_query = """
        UPDATE %(table)s
        SET %(level)s = CASE
                WHEN %(left)s >= %%s AND %(left)s <= %%s
                    THEN %(level)s - %%s
                ELSE %(level)s END,
            %(tree_id)s = CASE
                WHEN %(left)s >= %%s AND %(left)s <= %%s
                    THEN %%s
                ELSE %(tree_id)s END,
            %(left)s = CASE
                WHEN %(left)s >= %%s AND %(left)s <= %%s
                    THEN %(left)s - %%s
                WHEN %(left)s > %%s
                    THEN %(left)s - %%s
                ELSE %(left)s END,
            %(right)s = CASE
                WHEN %(right)s >= %%s AND %(right)s <= %%s
                    THEN %(right)s - %%s
                WHEN %(right)s > %%s
                    THEN %(right)s - %%s
                ELSE %(right)s END,
            %(parent)s = CASE
                WHEN %(pk)s = %%s
                    THEN %(new_parent)s
                ELSE %(parent)s END
        WHERE %(tree_id)s = %%s""" % {
            'table': qn(opts.db_table),
            'level': qn(opts.get_field(self.level_attr).column),
            'left': qn(opts.get_field(self.left_attr).column),
            'tree_id': qn(opts.get_field(self.tree_id_attr).column),
            'right': qn(opts.get_field(self.right_attr).column),
            'parent': qn(opts.get_field(self.parent_attr).column),
            'pk': qn(opts.pk.column),
            'new_parent': parent_pk is None and 'NULL' or '%s',
        }

        left = getattr(node, self.left_attr)
        right = getattr(node, self.right_attr)
        gap_size = right - left + 1
        gap_target_left = left - 1
        params = [
            left, right, level_change,
            left, right, new_tree_id,
            left, right, left_right_change,
            gap_target_left, gap_size,
            left, right, left_right_change,
            gap_target_left, gap_size,
            node.pk,
            getattr(node, self.tree_id_attr)
        ]
        if parent_pk is not None:
            params.insert(-1, parent_pk)
        cursor = connection.cursor()
        cursor.execute(inter_tree_move_query, params)

    def _make_child_root_node(self, node, new_tree_id=None):
        """
        Removes ``node`` from its tree, making it the root node of a new
        tree.

        If ``new_tree_id`` is not specified a new tree id will be
        generated.

        ``node`` will be modified to reflect its new tree state in the
        database.
        """
        left = getattr(node, self.left_attr)
        right = getattr(node, self.right_attr)
        level = getattr(node, self.level_attr)
        tree_id = getattr(node, self.tree_id_attr)
        if not new_tree_id:
            new_tree_id = self._get_next_tree_id()
        left_right_change = left - 1

        self._inter_tree_move_and_close_gap(node, level, left_right_change,
                                            new_tree_id)

        # Update the node to be consistent with the updated
        # tree in the database.
        setattr(node, self.left_attr, left - left_right_change)
        setattr(node, self.right_attr, right - left_right_change)
        setattr(node, self.level_attr, 0)
        setattr(node, self.tree_id_attr, new_tree_id)
        setattr(node, self.parent_attr, None)

    def _make_sibling_of_root_node(self, node, target, position):
        """
        Moves ``node``, making it a sibling of the given ``target`` root
        node as specified by ``position``.

        ``node`` will be modified to reflect its new tree state in the
        database.

        Since we use tree ids to reduce the number of rows affected by
        tree mangement during insertion and deletion, root nodes are not
        true siblings; thus, making an item a sibling of a root node is
        a special case which involves shuffling tree ids around.
        """
        if node == target:
            raise InvalidMove(_('A node may not be made a sibling of itself.'))

        opts = self.model._meta
        tree_id = getattr(node, self.tree_id_attr)
        target_tree_id = getattr(target, self.tree_id_attr)

        if node.is_child_node():
            if position == 'left':
                space_target = target_tree_id - 1
                new_tree_id = target_tree_id
            elif position == 'right':
                space_target = target_tree_id
                new_tree_id = target_tree_id + 1
            else:
                raise ValueError(_('An invalid position was given: %s.') % position)

            self._create_tree_space(space_target)
            if tree_id > space_target:
                # The node's tree id has been incremented in the
                # database - this change must be reflected in the node
                # object for the method call below to operate on the
                # correct tree.
                setattr(node, self.tree_id_attr, tree_id + 1)
            self._make_child_root_node(node, new_tree_id)
        else:
            if position == 'left':
                if target_tree_id > tree_id:
                    left_sibling = target.get_previous_sibling()
                    if node == left_sibling:
                        return
                    new_tree_id = getattr(left_sibling, self.tree_id_attr)
                    lower_bound, upper_bound = tree_id, new_tree_id
                    shift = -1
                else:
                    new_tree_id = target_tree_id
                    lower_bound, upper_bound = new_tree_id, tree_id
                    shift = 1
            elif position == 'right':
                if target_tree_id > tree_id:
                    new_tree_id = target_tree_id
                    lower_bound, upper_bound = tree_id, target_tree_id
                    shift = -1
                else:
                    right_sibling = target.get_next_sibling()
                    if node == right_sibling:
                        return
                    new_tree_id = getattr(right_sibling, self.tree_id_attr)
                    lower_bound, upper_bound = new_tree_id, tree_id
                    shift = 1
            else:
                raise ValueError(_('An invalid position was given: %s.') % position)

            root_sibling_query = """
            UPDATE %(table)s
            SET %(tree_id)s = CASE
                WHEN %(tree_id)s = %%s
                    THEN %%s
                ELSE %(tree_id)s + %%s END
            WHERE %(tree_id)s >= %%s AND %(tree_id)s <= %%s""" % {
                'table': qn(opts.db_table),
                'tree_id': qn(opts.get_field(self.tree_id_attr).column),
            }
            cursor = connection.cursor()
            cursor.execute(root_sibling_query, [tree_id, new_tree_id, shift,
                                                lower_bound, upper_bound])
            setattr(node, self.tree_id_attr, new_tree_id)

    def _manage_space(self, size, target, tree_id):
        """
        Manages spaces in the tree identified by ``tree_id`` by changing
        the values of the left and right columns by ``size`` after the
        given ``target`` point.
        """
        opts = self.model._meta
        space_query = """
        UPDATE %(table)s
        SET %(left)s = CASE
                WHEN %(left)s > %%s
                    THEN %(left)s + %%s
                ELSE %(left)s END,
            %(right)s = CASE
                WHEN %(right)s > %%s
                    THEN %(right)s + %%s
                ELSE %(right)s END
        WHERE %(tree_id)s = %%s
          AND (%(left)s > %%s OR %(right)s > %%s)""" % {
            'table': qn(opts.db_table),
            'left': qn(opts.get_field(self.left_attr).column),
            'right': qn(opts.get_field(self.right_attr).column),
            'tree_id': qn(opts.get_field(self.tree_id_attr).column),
        }
        cursor = connection.cursor()
        cursor.execute(space_query, [target, size, target, size, tree_id,
                                     target, target])

    def _move_child_node(self, node, target, position):
        """
        Calls the appropriate method to move child node ``node``
        relative to the given ``target`` node as specified by
        ``position``.
        """
        tree_id = getattr(node, self.tree_id_attr)
        target_tree_id = getattr(target, self.tree_id_attr)

        if (getattr(node, self.tree_id_attr) ==
            getattr(target, self.tree_id_attr)):
            self._move_child_within_tree(node, target, position)
        else:
            self._move_child_to_new_tree(node, target, position)

    def _move_child_to_new_tree(self, node, target, position):
        """
        Moves child node ``node`` to a different tree, inserting it
        relative to the given ``target`` node in the new tree as
        specified by ``position``.

        ``node`` will be modified to reflect its new tree state in the
        database.
        """
        left = getattr(node, self.left_attr)
        right = getattr(node, self.right_attr)
        level = getattr(node, self.level_attr)
        target_left = getattr(target, self.left_attr)
        target_right = getattr(target, self.right_attr)
        target_level = getattr(target, self.level_attr)
        tree_id = getattr(node, self.tree_id_attr)
        new_tree_id = getattr(target, self.tree_id_attr)

        space_target, level_change, left_right_change, parent = \
            self._calculate_inter_tree_move_values(node, target, position)

        tree_width = right - left + 1

        # Make space for the subtree which will be moved
        self._create_space(tree_width, space_target, new_tree_id)
        # Move the subtree
        self._inter_tree_move_and_close_gap(node, level_change,
            left_right_change, new_tree_id, parent.pk)

        # Update the node to be consistent with the updated
        # tree in the database.
        setattr(node, self.left_attr, left - left_right_change)
        setattr(node, self.right_attr, right - left_right_change)
        setattr(node, self.level_attr, level - level_change)
        setattr(node, self.tree_id_attr, new_tree_id)
        setattr(node, self.parent_attr, parent)

    def _move_child_within_tree(self, node, target, position):
        """
        Moves child node ``node`` within its current tree relative to
        the given ``target`` node as specified by ``position``.

        ``node`` will be modified to reflect its new tree state in the
        database.
        """
        left = getattr(node, self.left_attr)
        right = getattr(node, self.right_attr)
        level = getattr(node, self.level_attr)
        width = right - left + 1
        tree_id = getattr(node, self.tree_id_attr)
        target_left = getattr(target, self.left_attr)
        target_right = getattr(target, self.right_attr)
        target_level = getattr(target, self.level_attr)

        if position == 'last-child' or position == 'first-child':
            if node == target:
                raise InvalidMove(_('A node may not be made a child of itself.'))
            elif left < target_left < right:
                raise InvalidMove(_('A node may not be made a child of any of its descendants.'))
            if position == 'last-child':
                if target_right > right:
                    new_left = target_right - width
                    new_right = target_right - 1
                else:
                    new_left = target_right
                    new_right = target_right + width - 1
            else:
                if target_left > left:
                    new_left = target_left - width + 1
                    new_right = target_left
                else:
                    new_left = target_left + 1
                    new_right = target_left + width
            level_change = level - target_level - 1
            parent = target
        elif position == 'left' or position == 'right':
            if node == target:
                raise InvalidMove(_('A node may not be made a sibling of itself.'))
            elif left < target_left < right:
                raise InvalidMove(_('A node may not be made a sibling of any of its descendants.'))
            if position == 'left':
                if target_left > left:
                    new_left = target_left - width
                    new_right = target_left - 1
                else:
                    new_left = target_left
                    new_right = target_left + width - 1
            else:
                if target_right > right:
                    new_left = target_right - width + 1
                    new_right = target_right
                else:
                    new_left = target_right + 1
                    new_right = target_right + width
            level_change = level - target_level
            parent = getattr(target, self.parent_attr)
        else:
            raise ValueError(_('An invalid position was given: %s.') % position)

        left_boundary = min(left, new_left)
        right_boundary = max(right, new_right)
        left_right_change = new_left - left
        gap_size = width
        if left_right_change > 0:
            gap_size = -gap_size

        opts = self.model._meta
        # The level update must come before the left update to keep
        # MySQL happy - left seems to refer to the updated value
        # immediately after its update has been specified in the query
        # with MySQL, but not with SQLite or Postgres.
        move_subtree_query = """
        UPDATE %(table)s
        SET %(level)s = CASE
                WHEN %(left)s >= %%s AND %(left)s <= %%s
                  THEN %(level)s - %%s
                ELSE %(level)s END,
            %(left)s = CASE
                WHEN %(left)s >= %%s AND %(left)s <= %%s
                  THEN %(left)s + %%s
                WHEN %(left)s >= %%s AND %(left)s <= %%s
                  THEN %(left)s + %%s
                ELSE %(left)s END,
            %(right)s = CASE
                WHEN %(right)s >= %%s AND %(right)s <= %%s
                  THEN %(right)s + %%s
                WHEN %(right)s >= %%s AND %(right)s <= %%s
                  THEN %(right)s + %%s
                ELSE %(right)s END,
            %(parent)s = CASE
                WHEN %(pk)s = %%s
                  THEN %%s
                ELSE %(parent)s END
        WHERE %(tree_id)s = %%s""" % {
            'table': qn(opts.db_table),
            'level': qn(opts.get_field(self.level_attr).column),
            'left': qn(opts.get_field(self.left_attr).column),
            'right': qn(opts.get_field(self.right_attr).column),
            'parent': qn(opts.get_field(self.parent_attr).column),
            'pk': qn(opts.pk.column),
            'tree_id': qn(opts.get_field(self.tree_id_attr).column),
        }

        cursor = connection.cursor()
        cursor.execute(move_subtree_query, [
            left, right, level_change,
            left, right, left_right_change,
            left_boundary, right_boundary, gap_size,
            left, right, left_right_change,
            left_boundary, right_boundary, gap_size,
            node.pk, parent.pk,
            tree_id])

        # Update the node to be consistent with the updated
        # tree in the database.
        setattr(node, self.left_attr, new_left)
        setattr(node, self.right_attr, new_right)
        setattr(node, self.level_attr, level - level_change)
        setattr(node, self.parent_attr, parent)

    def _move_root_node(self, node, target, position):
        """
        Moves root node``node`` to a different tree, inserting it
        relative to the given ``target`` node as specified by
        ``position``.

        ``node`` will be modified to reflect its new tree state in the
        database.
        """
        left = getattr(node, self.left_attr)
        right = getattr(node, self.right_attr)
        level = getattr(node, self.level_attr)
        tree_id = getattr(node, self.tree_id_attr)
        new_tree_id = getattr(target, self.tree_id_attr)
        width = right - left + 1

        if node == target:
            raise InvalidMove(_('A node may not be made a child of itself.'))
        elif tree_id == new_tree_id:
            raise InvalidMove(_('A node may not be made a child of any of its descendants.'))

        space_target, level_change, left_right_change, parent = \
            self._calculate_inter_tree_move_values(node, target, position)

        # Create space for the tree which will be inserted
        self._create_space(width, space_target, new_tree_id)

        # Move the root node, making it a child node
        opts = self.model._meta
        move_tree_query = """
        UPDATE %(table)s
        SET %(level)s = %(level)s - %%s,
            %(left)s = %(left)s - %%s,
            %(right)s = %(right)s - %%s,
            %(tree_id)s = %%s,
            %(parent)s = CASE
                WHEN %(pk)s = %%s
                    THEN %%s
                ELSE %(parent)s END
        WHERE %(left)s >= %%s AND %(left)s <= %%s
          AND %(tree_id)s = %%s""" % {
            'table': qn(opts.db_table),
            'level': qn(opts.get_field(self.level_attr).column),
            'left': qn(opts.get_field(self.left_attr).column),
            'right': qn(opts.get_field(self.right_attr).column),
            'tree_id': qn(opts.get_field(self.tree_id_attr).column),
            'parent': qn(opts.get_field(self.parent_attr).column),
            'pk': qn(opts.pk.column),
        }
        cursor = connection.cursor()
        cursor.execute(move_tree_query, [level_change, left_right_change,
            left_right_change, new_tree_id, node.pk, parent.pk, left, right,
            tree_id])

        # Update the former root node to be consistent with the updated
        # tree in the database.
        setattr(node, self.left_attr, left - left_right_change)
        setattr(node, self.right_attr, right - left_right_change)
        setattr(node, self.level_attr, level - level_change)
        setattr(node, self.tree_id_attr, new_tree_id)
        setattr(node, self.parent_attr, parent)
