SA like query chaining for web.py db

sa.py

import web

class SelectQuery:
	def __init__(self, tables):
		if isinstance(tables, basestring):
			tables = [tables]
		self._tables = tables
		self._columns = "*"
		self._where = web.SQLQuery("")
		self._order = []
		self._limit = None
		self._offset = None

	def _copy(self):
		q = SelectQuery(self._tables)
		q._columns = self._columns
		q._where = self._where
		q._order = list(self._order) # make a copy
		q._limit = self._limit
		q._offset = self._offset
		return q

	def filter(self, condition, vars={}):
		q = self._copy()
		q._where = self._where + " AND " + web.reparam(condition, vars)
		return q

	def join(self, table, condition, vars={}):
		q = self._copy()
		q._tables.append(table)
		return q.filter(condition, vars)

	def order_by(self, column):
		self._order.append(column)

	def limit(self, n):
		self._limit = n

	def execute(self, db):
		if self.order:
			order = ",".join(self._order)
		else:
			order = None

		return db.select(self._tables, 
			what=self._columns, 
			where=self._where,
			order=self._order,
			limit=self._limit,
			offset=self._offset)

xgetposts.py

def getposts(basequery=None, pinned=False, showall=False, statuses=None):
    if not statuses:
        statuses = POSTSTATUS.LISTED

    if basequery is None:
        basequery = SelectQuery("jobpost")

    query = basequery.filter("jobpost.status IN $statues", vars=locals())

    if showall:
        query = query.filter("jobpost.datetime > $d", vars=dict(d=datetime.utcnow() - agelimit))
    else:
        query = query.filter("(" + 
        	"(jobpost.pinned = $t AND jobpost.datetime > $d1) OR" + 
        	"(jobpost.pinned = $f AND jobpost.datetime > $d2)",
        	vars=dict(t=True, 
        			  f=False, 
        			  d1=datetime.utcnow() - agelimit, 
        			  d2=datetime.utcnow() - newlimit))
    if g.board:
    	query = query.join("board_jobpost", "board_jobpost.jobpost=jobpost.id")
    	query = query.filter("board_jobpost.board_id = $g.board", vars=dict(g=g))        

    if pinned:
        if g.board:
        	query = query.order_by("board_jobpost.pinned DESC")
        else:
        	query = query.order_by("jobpost.pinned DESC")

    return query.order_by("jobpost.datetime DESC")