Rails3: conversion of RoR sqlite to postgresql for Heroku

I'm not familiar with postgresql and I have this code working on my local dev machine with SQLite, but it fails in my production Heroku environment which uses postgresql

>  @categories = @user.posts.includes(:category).where(
>          ['`categories`.name = ?', params[:category]])

Error in heroku logs:

 ActiveRecord::StatementInvalid (PG::SyntaxError: ERROR:  syntax error at or near "."
2015-02-04T11:38:35.977789+00:00 app[web.1]: LINE 1: ..._id" WHERE ("posts".user_id = 2) AND (`categories`.name = 'm...
2015-02-04T11:38:35.977791+00:00 app[web.1]:                                                              ^