Return the SqlNode associated with the query path.

def get_node_at_path(query_path, context):
    """Return the SqlNode associated with the query path."""
    if query_path not in context.query_path_to_node:
        raise AssertionError(
            u'Unable to find SqlNode for query path {} with context {}.'.format(
                query_path, context))
    node = context.query_path_to_node[query_path]
    return node